Understanding Electronics - 3rd Ed

Urzd_wEpnfli49 ELEETRONIES 3rd Edition

R, H. Warring, Edited bg E. Randg Slone

1 Units, Abbreviations,and Slmbols whir$definewhat Inelectronicsthereareshbasicnrjb to measurcquandties srbols usedas letter i" grhg ; h a cirurit. Herese thefirst three,with tbe abbrtriatims: VAx M - Aneasneofthepotential,enl(electromotivefotec)'ot ooltagena potentialdifiereooe,enf, udvoltage dl rcalty Ct"oii. i'. practicalpurposes, meanthe samething' Anls (,0-A neasureof the currealfrowingin a circuil iittt fW -A neasgred theroreldeveloBedbytheflowd cqnentthro*b a chqdt The othertlree referto the efiectof co4ments in tbe circuit: in a circuit olzs (o)-A measul€of.rhe nsisi4,n/-or individualresistances (dc). whenthe currentflw is &ect producedby Far& (F)-AmeasrFe 6the @itancepresent in a circ{it or i.e., capacitors. individualcomponents; proftced bf n*r* fn-ineasure 4-6iln&mncepresent in a circuitor suchascoils. individualcomponents 'I\m otherinporbnt quantities,bothof wtricharemeasured in ohms,are: or individtlalresistanoes Imgqlane (A - A measnedtbeeftectiveresistance (ac)' U i circuitwhenthe currentflowis dternating in an ac andcapacitance R;*fur* (x)_Tbcombined efiectof inductance cLoit

Capital letters are also used as abbreviations for voltage and current. Strictly speaking E (for emf) is the correct symbolfor a voltage source, with V (for volts) in other parts of the circuit. V, can be used instead of E for a source voltage. The capital letter I is used for current. In some circuits lowercase letters are used to indicate voltages and currents flowing in different parts of a circuit; e.g., v and i, respectively. These may have a reference annotation attached, particularly in the case of transistor circuits; e.g., v,, describiig emitter voltage. The relationship between units is explained in Chapter 3. There are also various other units employed in electronics, the use and meaning of which will be made clear in appropriate chapters. In practical circuits, numerical values of these units may be very large, or very small. Resistance values, for example, may run to millions of ohms. Capacitor values may be in millionths or even million-millionths of a farad. To avoid writing out such values in full, prefixes are used to designate the number associated with the particular value involved. Normally, the symbol rather than the full prefix is

used, mega (M)-X 1,000,000 kilo (k)-X 1,000 milli (m)-divided by 1,000 (or 1/1,00Oth) micro (p)-divided by 1,000,000 (or 1/1,000,000th) nano (n)-divided by 1,000,000,000 (or 1/1,000,000,000th) pic0 (p) -divided by 1,000,000,000,000 (or 1/1,000,000,000,000) For example, instead of writing out 22,000,000 ohms, it is shown as 22 Mohms, or 22 MQ, using symbols both for the prefix and basic unit. Similarly a capacitor value of 0.000,000,000,220 farads is shown as 220 picoF, or, more usually 220 pF. The multipliers (M and k) are most commonly associated with values of resistors, and also for specifying radio frequencies. The lowest divisor (m) is most usually associated with the values of current typical of transistor circuits, etc. It is also used to specify most practical values of inductances. The larger divisors (p, n, and p) are most commonly associated with capacitor values. S i e capital letter abbreviations are also used for components. The main ones are:

C- capacitors D -diodes L- coils R- resistors These are all standard and universally accepted abbreviations. With other components this is not always the case. Thus transistors may be designed T, TR, Tr, VT or even Q on circuitsoriginatingfrom different sources. The use of TR, Tr, or Q is preferred, leaving the letter T as the abbreviationfor transformers. Note the abbreviation FET (or fet) is used for a field-effect transistor in text, although it may be "Q" in diagrams. In practical circuits, more than one of the same type of components are

YYr

---f-

Fhed ValueResistm

-J-- -t-

+F

Electrolytic Capacitr

Dode

lnudspeaker

I

VarbbleResistor

:

-,yE-

-+F

(

Earth Ground

Chagsis Ground

- y t-l ' T

rl

TrimmerCapacitc

-\.0..0.,1- -lF---lF coil

Variable Capacitor

),

Rteo**

-\JJrr"

-#-n+ Jr- -lrlr Cell

Battery

Fig. 1-1.Symbolsfor basiccircuit comPonents. Othcrsxntblsaregiwn in later chapters.

normallyused.Individualcomponents of the sametypearethendesignated by numbers(usuallyreading fromleft to right acrossthecircuit)associatedwith the gomrynentsymbol(Fig. 1-1).SeealsoChapterB.Thusresistorsaredesignated Rl, R2, R3; capacitorsCL,C2, C3;andso on.Thereis no correctot specific sequence in whichsuchnumbersareallocated. Theyarethereonlyto identifya particularcomponent. Here are someotler generalabbreviations that are widelyused,although againtheymaybeshownin variousdifferentways:capitalletters,or lowercase letters in uprightor italic, with or withoutperiods.Thusthe abbreviation of alternatingcurrentmayappearin five differentways: AC a.c. a.c. ac ac The generalpreferenceis that all suchabbreviations shouldbe in lowercase withoutperiods,andsothe followingabbreviations areshownthat way: ac- alternatingcurrent af-audio frequency agc-automatic gaincontrol (or amplitudemodulation) am*-amplitude modulated dc-direct current eht-extra hightension fin*-frequency modulated (or frequencymodulation) hf-high frequency ht-high tension * Thereisagoodreasonforretainingcapitallettersforthese abbreviations, sinceAJUandFMradios ugethernthiqv/ay.

3

ic -integrated circuits if -intermediate frequency(alsoi-f) if-low frequencY rf-radio frequencY uhf -ultra high frequencY vhf-very high frequencY

DC and AC A basicdirect current (dc) circuit is simple enoughto understand.A sogrceof electricalforce (suchasa battery) is connectedviawirestovarious components with a return path to the source. Cprrent then flows through the circuit in a particular direction. Figure 2-1 shows a very elementary circuit of tlis type where a battery is connectedto a dcelectric motor andis comparedwitha similar closedloop hydraulic motor in a simple recirculating system. It is obviouswhat happensin the hydraulic circuit. The pump is a sourceof pressurizedwater which impingeson tlte vanesof the hydraulicmotor to drive it. There is a flow of water aroundthe system.At the sametime, tlere is someloss of pressureenergydueto the fiction of the water flowing through the pipesand t}1emotor. This is the resistancein the circuit. But most of the pressureenergy deliveredby the pump is convertedinto power by the hydraulic motor. In the electrical circuit counterpart, the battery is a source of. electrical pres{ilre (which in simpleterms we designateooltage),This forces an electrical currentto flow through the circuit, opposd by ttre resistanceoffered by the wiringandtheelectricmotorcoils.Again,mostofthe originalelectricalenergyin the battery is convertedinto power by the electric motor. Providedthe battery voltagedoesnot change,a constantvalueof current will flow tlrough the circuit alwaysin the samedirection, and the electric motor will continue to nrn at a constantsped. Conventionally,dc current flow is regardedas beingfrom the positive to the negativeterminalsof a battery or any otler dc source(suchasa dynamo).It is a

Battery Electrical Circuit

HydraulicCircuit

circuitis similartoa hydrauliccircuil Fig.2-1.An electrieel streamflow,just like the waterflow in the hydrauliccircuit,but the streamis of sub.atomicparticlesor electrons.Unfortunately,after actuallycomposed thepositiveto negativeflowdefinition,it wasfound badestablished convention that tlis electronstreamflowwasactrullyfuon negatioetoPositioe.Thisdoes of howtransis. butfor anunderstanding not rnatterfor mostpracticalpurposes, this "reto appreciate tors andother solid-statedeviceswork it is necessary verse"working. Positivedwaysseemsstrongerthan negative,so it is difrcult to think of currentasflowingotherthanfrom positiveto negative.TVecanrelatetlis to negativeto positivebythinkingofelectronsasprticlesof electronsflowingfrom theyrepresenta reverseflow, negativeelectricity.Being"weaker"(negative), of current-from positiw to negatiae. settingup conditionsfor a Positioeflow andworkonthepracticalfacttbat* and Otlerwise,simplyforgetthedifference - are onlytermsof convenience in a circuit usedto ensuretlnt components upcorrecdy.Thisapplies whichhavepositiveandnegativesidesareconnected mainlyto batteries,transistors,diodes,andelectrolyticcapacitors. of atomsin whichthere is a stablebalanceof All materialsare composed The elements). positiveandnegativecharges(exceptin theatomsofradioactive from the of an electricalpressurecauseselectronsto be displaced application atom,leavingit with aneffectivepositivecharge.It is thenin a stateto attract anystrayelectrons.Sincethereis electricalpressurepresent,this meansthat electronsalongthechainofatomscomprisingthewiring thereis a movementof tlat constireturncircuit.It is thismovement in thecomplete andcomponent(s) tutestheelectriccurrentflowingtlrough thecircuit,thestrengthof thecurrent ontlte numberof electronspassinganyparticularpointin the beingdependent circuitin a giventime.Breaktle circuitandthe pressureis broken,socurrent So,in fact,the analogywith a hydrauliccircuitis not reallyvalidin flowceases. thisinstance(thehydraulicpumpstill deliverswaterunderpressureif its circuit is brokenuntil it hasemptiedthe fluidin the circuitbetweentle breakandthe punp). Atomsof materialslike metalswill giveupelectronsreadilywhensubiectto electricalpres$re, andsomakegoodconductorsof.electricity.Atomsof most includingplastics, arereluctanttogiveupelectronsevenunderhigh non-metals, 6

I I

Switch On

Time --.+>

Switch

off

Fig.2-2.Directcurrentfowwith constant circuit resistance. electricalpres$Ee,andso are essentiallynonconductors. If materialsare extremelyresistantto givingup atoms,they areclassified as insulators. sunmarizing,then,a dc circuitwhenconnected or switchedon providesa constantflowof currentin onedirectionthroughthecircuitasin Fig.i-2, unless somethingchangesin the circuit (e.g.,sourcevoltagechanges,or a circuit resistance valuealters).The ralueof this curent is determined by the souroe voltageandthetotalresistance in thecircuit(seeChapter3).Currentflowisalso regardedaspositive(or positivecurren$. In the caseof an ac circuit, tre sourceof electricalpressurecontinually reversesin a periodicmanner.Thismeansthatcurent flowsthroughthecircuit first Ir onedirection(positive)andthenthe other(negative). In o6er words,a simplegraphof currentflow with time will lookrikeFig. 2-3.The swingfrom naximumpositiveto maximumnegativeis knownasthe amptitudeoiaaac ctnrent.Alsoonecompleteperiodfromzeroto maximum positive,backto zero, doumto naximumnegativeandbackto zeroagainis knownasa cycle.These cyclesmayoccuratvaryingratesfrom afewtimesa second tomillionsoftimesa secondanddefnethefrequencloftheaccurrent,frequencybeingequalto the numberof cyclesper second.In the caseof the domesticmainsiupply (in Britain),for example,tbe frequencyis s0 cycles-per-second, or 60 cycles-persecondin the u.s. But "cycles-per-second" is anobsoleteterm.It is nowcalled hertz(abbreviated I{z). Thusstandardmainsfrequencyis S0c 60 Hz.

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FiS. 24. Alteraatitg currentfuu b in cycbsof positiu and regatioecurnnt.

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ru

z

Peak: + Peakto Peak RMS:0.707X Peak

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Fig.24. Peah-ta-peah, dcfinedand conpared. fuah, ond raotmcansquaneaoltages Apart from the fact that ac is continuallyswingng from positive to negative current flow, the other differenceis that the actualcurrent valuepresentis also "average" value which can be changingall the time. It does,however,have an definedin variousways.The usualoneis the RootMeanSquare(or zzs), whichis equal to 0,707L times the ma:rimumcycle values for sine wave ac such as normally generatedby an alternator, Fig. 24. Alternating current may, however, be generatedwith other types of waveform. Anotlrer characteristicof ac is that both the voltage and qrrent are continually changingin similar cycles.Only rarely, however,do the voltageandcurrent both attain maximum andzerc valuesat exactly the sametime. In otlrcr words the current (waveform) curve is displacedrelative to the voltage (waveform) curve, Fig. 2-5. This displacementis known asa phav diference.ltis nomlally orpressedin terms of t}reratio of the actualdisplacementto a full cyclelength on t]1ezero line, multipliedby 360 (sincea full cycle represents360 degreesof ac "peak" after the working). This is calledthe plase angle.Usuallythe current will voltage (i.e., be displacedto the right of the diagram),whereuponthe current is said to be leggng and the phaseangleis referred to as angle of lag.

I

q) I

q)

db q)

z

+ Currenttageng by ] Cycleor 90 Ilegreee

Fig.2-5. Current uaul$ lagsbhind aoltagein an altcrnating curnnt circuit.

at first. It is reallyamatter Theuseof theterm "angle"canbeabit conftrsing involving calculations usefulin morecomplicated convenience, of mathematical of acit isbetterto thinkofangleas For ageneralunderstanding vectordiagrams. onefull cycle meaninga particular"numberpoint" ona linelengthrepresenting angleof30degreescanbeunderstooda Thusaphase 360divisions. dividedinto a point30/360thsalongthat line. Phasedifierence(phaseangle)canbe animportantfactorin the desigtand whena currentlags(or workingof manyalternatingcurrentcircuitsbecause are affected. of a circuit leads)the voltage,the timingaspects

3 Basic Circuits and Circuit Laws Asnotedin Ctnpter2,thecurrentwhichflowsin asimpledccircuitis dependent in thecircuit.Voltagecanbemeasured ontheappliedvoltage andtheresistance direcdyby a aoltmeterplaced acrossthe battery(or dc source)terminals;and currentbyan ammeterconnected n seiesn the circuit,asin Fig. 3-1.This in symbolicform. diagramalsoshowsthe circuitcomponents OUM'S LAW (R) is given Therelationship betweenvoltage(E),current(I), andresistance by Ohm'slaw:

.t : FE In plain language: amps:

volts resistance in ohms

or the formula can be rewritten:

volts: ampsX ohms 10

Battery

rl

Voltmetcr

L' -- ---- rl 1 v-

I

;---J

\-/ FA. 9.1.fusic dc circuitdnun in ttm ua1s,uith nwterslor ncaarhg cttrat end odtryc indicated

volts . onms-This is oneof the mostbasicandmostusefirllawsof electronics andis equally applicable to accircuitswhicharepurelyresistive(i.e.,do not haveadditional resistance effectsproduced bythecunentbeingdternatingratherthansteady). Ohn'slawmakesit possible (andthrs designltheperformance to calculate ofa simpledccircuit.Forexample, youneedacurrentof200miliamps(mA) suppose to flowin apartictlarcirorit to bepoweredbya &volt battery.Usingotrn'staw, tle correspoading circuitresistance requiredto givethis cunent caneasityle

wortecln

.

volts

tias:-:..:-

6

amps 0.200 - 30ohms

components areconnected bywires,buttheresistance ofwiringisqnatteoough to benegligible. Thusin a simpledccircuitit is theeffective totalof all the resistorraluesandothercomponents whichofrerresistance.Just whatthistotal valueis depends onhowthevarious resistors whichmaybepresentareconnected(seeChapter 4). In somecasesit is easyto caletatethe reeistance of a typicarload"Fc example, aflashlightbulbisusuallyratedbyvoltsandtheqrrentitdraws. ohm,s lawcanthenbeusedtofind itsnominalresistance. Forexample, if abulbbrated at 6 voltsand50nA" fromObm'slaw:

Resistance-#- uo obns.

1l

There is just onesnagto this methodof estimatingloadresistance.Withfilament bulbs, for example,the specifiedcurrent drawn refers to the bulb in working conditionswith the filament heatedup. Its actual resistanceinitially when the filament is cold can be considerablylower, drawing more current through the bulb. This may, or may not, be a disadvantagein a particular circuit. Also, tlere are other types of loads,like dc electric motors, where the effective resistance variescondiserablywith the speedat which the motor is running.Initially, sucha motor will havea very low resistance;its effectiveresistancethen increaseswith speed. Two other basicrelationshipsalso apply in a simple dc circuit: 1. The currentvalue is tlte samethrough every part of the circuit, unlessa part of the circuit involves parallel-connectedpaths. Thus, in a circuit (A) of Fig. 3-2, all the resistorsin the circuit are connectedin seriesso that the sirmecurrent will flow through eachresistor. In circuit (B) of Fig. 3-2, the resistors are connectedin parallel. ln this case eachresistor representsa separatepath for the current andtlte valueof current flowing through eachleg dependson tlre value of tint resistor. These current valuescan be calculatedfrom Ohm's law:

throughresistort, current: throughresistorZ,currentthroughresistor3, current:

ft S $

Thecurrentflowingthroughthewiringpartof thecircuitis thesumof these tlree crrrents;Le., RT

B ResistorsinParallel A Resi$orsin Serbs Fig. &2. Current has tlrc sne oalu thrwgh all r*is&lrs cotutebd in cries, htt is difcmrt throaghcrch nslttor conncctedi; fulallcl

u

E , E E 1 . , 1 I orE Rl R2'R3 Rl-R2-R 2. Thevoltagethroughouta simpledccircuitis not constantbut suffersa drop acrosseachresistor. This canbe illustratedby the circuitshownin Fig. 3-8,wherethe voltages acrossthe individualresistorsarecalculated (or measured with a voltmeter)as VL,VZ andV3. The total resistance in the circuitis Rl * RZ+ R3. The cur:ent(whichis the samethroughoutthe circui| is givenby: .^:Rl]ft.!![ E Wethenhavethe conditions: Vl, measured acrossRl : currentX resistance :IXRl V2, measured acros{rR2 : I X R2 V3, measured acrossR3 : I X R3 Eachof thesevoltagesis lessthanE. Comparison with ahydrauliccircuitagain(seeChapter2)canhelpunderstand how a resistorworksas a voltagedropper.In a hydrauliccircuit,pressureis analogous to voltagein anelectroniccircuit.Theequivalent to aresistoris some devicerestrictingfluid flow-say a partiallyclosedvalve.Flow throughthis resistorproducesa pressuredrop.similarly,tle flow of electricitythrougha resistorproducesa voltagedrop.

,--@_,

Q

Fig. 3-3. R*istors dropooltagein a dc circuit. 13

VOLTAGE.DROPPER CIRCUIT Theaboveisnowreworkedasapracticalemmple.To ponera&volt electricd (saya &volt transistorradio)froma l2-volt battery.In this case,tlte appliance asa resistanceload.To dropthe voltagefrom 12 to 6 is considered appliance acrossthisload,adropperresistor,R,is requiredin thecircuitshownin Fig.3'4. butto dot}is it It remainsto calculatea suitablevaluefor thisdroppingresistor, of the load.(If this is not knownit to knowtlre effectiveresistance is necessary it is 100ohms. with anohmmeter.)Suppcse canbemeasured UsingOhm'slawagain,if this loadis to have6 voltsappliedacrorxlit, andits is 100ohms,the currentrequiredto 6owthroughthe circuitis: resistance 1 - : 6 100 :0.06 amps(60 millianls) the This samecurrentflowsthroughthe rest of the circuit. This, considering circuitfrom the l2-volt end: required: total resistance

# = 200 ohms

The loadalreadycontributed100 ohms,so the rnalueof droppingresistorre' quiredmustbe 200- 100: 100ohns.Afirthercalculationshowstbevoltage dropacrossthis resiston V- 0.06X 100 - 6 volts drop nrleconcerning anothersimple Thisparticularexamplealsodemonstrates ping resistors.If the voltageis to be halved,then the rnalueof the dropping resistorrequiredis the sameast}at of the load.

12 Volts I

r-O-r R€sistorto "Ilrop" 6 Volts

Fig. 34 Prutical a!flicotiut of d drofuing tesisbn

tl

POWER IN THE CIRCUIT The power developedina circuit byvirtue of theelectricalpressure(volts)and resulting current flow (amps)is given by the product of these two values,and measuredin araffs.Thus: power: watts: volts X arnps This samedefinition appliesboth to dc and ac circuits. Power is used up in producing a usr;fulresurt rn makingthe crcuit work (whether this be operatinga radio, driving an electric motor, heatingan electric element, etc.). But all componentswhich have resistance absorb a certain amountof power which is waste power normally dissipatedin the form of heat. No practical device can work without someresistancein the circuit, and thus somepower lossis inevitable.More important, the heatingeffect must not be so great that the componentis damaged.Thus componentsnormally havea power rating which shouldnot be exceeded.In specificcases,even when operating within their power rating, provisionmay haveto be madeto conductheat away from tlre component-as in the caseof heatsinksusedwith power transistors. Referringto the exampleof the droppingresistor, t}is definitelywastespower to the tune of 6 (volts) X 0.06 (amps): 0.36 watts. To be on the safi side, trerefore, tre resistor chosenwould needto havea power rating of at lqstvz watt, and would alsohave to be placedin a position where it receivesadequate ventilation to prevent heat build-upin the surroundingair. The majority of transistor circuits work on low voltages, with low current values,and so componentswith quite moderatepower ratings are usuallyade. quate.circuits carrying higher voltagesandcurrents demandthe useof components with correspondinglyhigher power ratings, and often need even more attention to-ventilation.Thus, the actualvalueof a componentis only part of its specification.Its power rating can be equallyimportant. Note that since V: IR, power can also be calculatedas: watts - (current)2X resistance P: I?R This is oftena moreconvenientforrrulaforcalculatingpowerinaparticularpart of a circuit. SHUNT CIRCTIITS A shunt circuit is usedto drop a current flowing through a particular oompG nent. It normally comprisestwo resistancesin parallel,one resistancebeingthe componentresistanceandthe other the shuntresistance.The appropriatevalue of the shunt resistanceis againcalculateddirectly from Ohm's law. A typical exampleof the use of a shunt resistanceis to adapt an ammeter movementto measuredifferent cur:mt ranges(asin a multimeter). In this case

15

is thatof thecoiloftheammeter,whichisinitiallydesignedto theloadresistance with a particularcurrentflowingthroughit (callthis deflection ruu-r.ae En". anyhighercurrentthanI, sincetlis would ir). tt " instrumentcannotmeasnre sihpry tendto carrythe pointerpastits full deflection,andvery likely cause to handlethe lowestcurrentrangereor.ag". Thus,the meteris desigrred quird, anda shuntresistor(or a seriesof shuntresistors)addedwhicttcanbe orirctt"o into the meter circuit to extendtlre range.Figure3-5 showsthis for switchingintothecircuit. withjustoneshuntresistorconnected arrangement qfient, 12, If the shuntresistoris to extendthe ammeterrangeto a higher follows resistor thentle requiredvalueof tle shunt deflection, Euingruu-sc"ru from: 1. current whichhasto flow throughthe shuntis I2-I.. This meansthat a (unless currentgreaterthanI, will neverflowtlrough themetermovement I)' exceeds meter the the actualcrrrent appliedto of the 2. Voltagedrops".t*. thuteter - I, X R- (whereR- is theresistance meter). 3. Shuntresishncerequiredis therefore: voltagedropacrossinstrument: Irt" S Iz - Ir currmt flow througbshunt Again,tlere is a simplerule to followif the currentrangeof the meteris to be dJubbd.In this casethe shuntresistancerequiredis the sameasthat of tlte meter. AMMETER INTO VOLTMETER current,canalsobe An ammeter,whichis aninstnrmentusedfor measuring a resistorin selt?swiththe meter-Fig. voltsby connecting madeto measure ofavoltagedropper.Again,if thema:rimum 3-6.This,infact,is anotlrerexample whichmustbein is 11, the fofalresistance metercurrentfor full-scaledeflection circtit is:

totalR: Ir1 whereV is the voltagerangeit is desiredto measure. (Rx) Met€trResistance

SimpleAmmeter

ShutResi*c Fig. 3-5. Ertcnding thc range of a milliamnutcr.

16

The valueof the seriesresistorrequiredis this total resistancelassthe resistance ofthemeter(thelattermaybenegligible in comparisonwith thevalue of seriesresistorrequiredandits likelytolerance-see Chapter4). Again,severalseriesresistorsmaybeused,switchedintothecircuitindividually to providedifferentvoltage-measuring rangeson ttremetermovement, as shownin the right handdiagramof Fig. 3-6. DTVIDERS A voltagedivideris yet anotherexampleof the practicalapplicationof a voltagedropper.Thebasiccircuitis shownin Fig.3-7,andsincethecurrentflow throughRl andR2 is the same,tle followingvoltagevaluesapply: Vl : sourcevoltage(e.g.,batteryvoltage) V2-VlXRl

vs:ffixnz l-..

vl

---\

is tlte currentflowingthroughRt andRZ/. \Note R-t + Rt It followsthatbysuitableselectionofvaluesfor Rl andR2,virtuallyanylower voltagethanVl canbetappedfrompointsA andB, or B andC (or both).It also hasthe advantage that it is not necessary to knowthe loadresistancebefore suitabledropperresistances canbecalculated. It couldthusbea morepractical alternativefor theorampledescribed in Fig.3-4,butconnection to aloaddoes,of course,resultin a further dropin voltage. If theresistance of theloadisknovm,tlrcnthereisnopartictlarproblemwitha fixedresistorvoltagedivider.Calculate thevalueof R2(Fig.3-Z)onthebasisof noloadresistance, tlen subtractthe actualvalue oftheloadresistancefromthis to arriveat therequiredvaluefor R2.(In thecomplete tap@ circuit,R2andttre loadresistance is effectivelyin series.)

Anmeter Moqeoeot

snlle Voltnetcr

Voltmeter with Five Ranges

Fis.34.C.onoertingan dmmcteriilo a ooltmcbr. 17

R1

B

'Droeeea' Voluge Ortput

Fig.&7. tusic fuobntialdiilder circtit.

BASIC AC CIRCUITS in Chapter2, the voltageandcurent flow bothalternatein ac As e:rplained circuits,with the possibilityof oneleadingor laggingthe other.Also,it was offeredbyresistancecomponentsrnaybe intimatedthat theeffectiveresistance by reactiveeffects.Theseeffectsbecomeincreas. modified(usuallyincreased) are inglymarkedas thefrequencyof theacincleases,andat radiofrequencies thanpureresistance. morepronounced It is possibleto obtainanac circuitwhichis purelyresistive,particularlyat in whichcaseOhm'slawisequallyvdid for suchcircuitsasit is lowerfrequencies, for dc circuits.Ohm'slaw canalsobe appliedto ac circuitsin whichreactive effectsare present,but in slightlymodifiedform. Thesereactiveeffectsare as reactanceandimfedance, describedspecifrcally loadingeffectproducedby cafocitors andin&rc' is circuit the Reactance by thesymbolL Its actqal in ohmsanddesignated is measqred trt tances(atrs). the frequencyof tlte ac. value and component tlrc both on is dependent value (usually designated&) is reactance capacitive capacitors, of the case In givenby:

\:L^ in farads,aadTt: 3.L4L2. wheref is theacfrequencyin llz, Cis thecapacitance )&,) is ind*ctioe reactatre(usuallydesignated In the caseof inductances, givenby: Xa:ZnfL in hen4n whereL is the inductance 18

If the ac circuit contains only reactance(i.e., does not have any separate resistance),then x takes the placeof resistance(R) in the ohm's law forrrula:

-I : Ex In practice,reactancepresentis alsousuallyassociated with resistance,the resultingcombination representingthe impedance (Z) of.the circuit. If reactance andresistancearein series:

2:lfrpl If reactanceand resistanceare in parallel:

"-

[r

! RtR

Again, impedance(Z) takes the placeof resistancein the Ohm's law formula:

I:2 Theseare the basicformulasfor accircuitcalculations. POWER FACTOR Powerfactoris something specificto accircuits,althoughit is onlytheresistancein suchcircuitsthat actuallyconsume power.Thispowerconsumed canbe calculatedas tle productof the squareof the current flowingthroughthe resistance andthevalueof theresistance; i.e.,IzRwatts.Theapparentpowerin the circuitis the productof acvoltageandcurrent,correctlyspecifiedasvoltamps. The ratio of tlre powerconsumed to the apparentpoweris calledthepower asa percentage. If the circuitis purelyresistive,then factor,usuallye:rpressed thepowerfactoris 100percent(sincealltheapparentpoweris consumed in the resistance). Reactance power,soin a purelyreactivecircuit doesnot consume tlte powerfactoris zero.TVhen a circuitcontainsbothresistance andimpedance (i.e.,reactance), thent]tepowerfactoris alwayslessthan100percent,its ralue present. depending on tlte resistance DC and AC in the SameCircuit It is quitepossibleto havebothdcandacflowingin the samecircuit.In fact, this is theprincipleonwhichmostradioandsimilarcircuitswork.Thedcis the basicsourceof electricalsupply,onwhichrrarious accurrentsaresuperimposed. Theoneessential differenceisthatdccanonlyflowthrougha continuous circuit,

ac+dc ac+dc

+

*l

ac+dc

€

c

Fig. 3{.. Flow of ac and dc in a circuit.

suchascapacitors whereasac canpassthroughcomponents whichpresenta breakinthecircuittodc.Theseeffectscanbeusedtoadvantagetoisolatestages in a circuit. In the type of circuitshownin Fig. 3-8,for example,aninputcomprisinga mixtureof dcandacis appliedto the left-handsideof ttrecircuit.If onlythe ac component of thesignalis required,thedccontentcanbeblockedby acapacitor (C1).Meantime,thenextpart of the circuitwhichhasto dealwith that signalis poweredby dcfromthesourcesupply(saya battery),probablyviaresistorsRl andR2actingasdividersto getthevoltagescorrectfor thatstage(otler stages mayneeddifferentworkingdcvoltages,all comingfromthesamesource).The outputsigrralfromthisstagetlen consists of amixtureof dcandac.If onlytheac (C2)isagainusedasa contentis wantedfor passing to thenextstage,acapacitor blockfor dc.

n

Supply Voltage

Resistors Resistors,as the nameimplies,are designedto providesomedesirable,or necessary, amountof,resistance to currentflow in a circuit.Theycanalsobe usedto dropvoltages,as explainedin chapterB. As such,t.heyare the nain elements usedincircuitdesignto arriveat thedesiredcurreotflowsandvoltages tlat work the circuit.Resistorsdo not generateelectricalenergy,but merely absorbit. This energyis dissipated in tbe form of heat.The perforrnance of a resistorisnotaffectedbyfrequency, soit behaves in thesamewayinbothdcand ac circuits.(Thereareexceptions, asnotedlater.) Resistorsare specifiedby (a) resistanceralue in ohms;(b) toleranceas a percentage of the nominalvalue;and(c) powerratingin watts.Theyare also categorized by the typeof construction.

COLOR CODE Resistance ralueandtolerancearenormallyindicated by a colorcodeconsisting offour coloredrings,startingat, or closeto, oneend(FiS.4-1).Theseare readasfollows: lst ring givesfust digit 2ndring givesseconddigit 3rd ring givesnumberof zerosto put afterfrst two digits

21

i ---

Black-0 Brown-l Red-z Orange-3

Yellow-'l Green-S Blue-6

Violet-7 GreY-8 White-g

Fig. Ll. Stsndarit tesistor color cdc marhing.

The fourth coloredring gives t}re tolerance: silver- 10% toleranceeither side of the nominalvalue gold-5% toleranceeither side of the nominalvalue toleranceeither side of the nominalvalue iA-ZW brown-1% toleranceeither side of the nominalvalue BramPle;Resistor color coderead as brown, blue, orange'

Valuereadas

Brown 1

Blue 6

Orange 000

i.e.,16,000Q or 16 kQ (kilohms). Absnce of a fourth ring implies a toleranceof 20 percent' Certain types of modirn resistors of larger physicalsizemay haveletters and on tle body instead of colored rings. With this coding, tle numbers "*f"a numbers indicate the numerical value and the following letter tle multiplier, where: E:Xl K: X 1,000 (or kilohms) M - X 1,000,000(or megohms) A secondletter then gives the tolerance: M: 20% toleranceeittrer side of the nominalvalue K - 10% toleranceeither side of the nominalvalue J - 5% toleranceeither side of ttre nominalvalue H:2.5oti toleranceeither sided the nominalvalue G:2% toleranceeither side of tle nominalvalue F : 1% toleranceeither side of the nominalvalue The actual range of (nominal)resistancevaluesto which resistors are madeis basedon stepsthat give an approximatelyconstantPercentageclnnge in resistancefrom oneralue to the ne:rt-not simplearithmeticalstepslike 1, 2, 3, etc. These are basedon tle preferred numbers:

22

t, 1.2,1.5,1.E, 2,2r2,7,,3.3, 3.9,4.?,5.6,6.E,E.Z, 10,12,15,lE,etc. Thus,for example,a typicalrangeof resistorvalueswouldbe: L0,12,15,19,22,27,33,39, 47,56,69,92, and100ohms 120,150,180,220,270,330,390,470, 560,680and820ohms L,1.2,1.5,1.8,2.2,2.7,3.3, 4.7,5.6,6.8,and8.2kilohms). L0,12,etc.kilohms 1, 12,etc.megohms As a generalrule, resistorswith a l0 percenttoleranceare suitablefor averagecircuituse.Theactualresistance valueof,say,a 1kilohmresistorwould thenbeanythingbetween900and1,100ohms.For morecriticalwork,suchas radiocircuits,resistorswith a 5 percenttolerancearepreferred.Closertolerancesarenot normallyrequired,exceptfor very criticatcircuits. POWER RATING The physicalsize(or strape)of a resistorprovidesno clueto its resistance value,butcanbearoughguidetoitspowerrating.physicalsizes (Fig.4-2)range fromabout4 mmlongby 1 mmdiameterupto abouts0 mmlongand6 mmor morediameter.Theformerwouldprobablyhaveapowerratngof.Ll2}watt and the latter possibly10 watts.More specifically, however,the powerratingis relatedto typeaswell assize.A generalrule that doesapplyto powerrating, however,is that while this figurenominal$representsa safemaximumthe resistorcantoleratewithoutdamage, it is usuallybestto operatea resistorwell belowits powerrating-say at b0 percent-particularly if components are crowdedonacircuitorthe circuitisenclosedin acasewithlittle ornoventilation. VOLTAGE RATING Maximumoperatingvoltagealsocanbespecified for resistors,butsincetlis is usuallyof theorderof 250voltsor more,thisparameteris not importantwhen choosingresistorsfor batterycirctits. Resistorsusedon mainscircuitsmust, however,havea suitablevoltagerating. Tpes of Construction Resistorfy'asclassified by construction follow.

^----

-

-

l-l

-

Fig, tL2. Eramplesof mdern resistoroutlinx (actualsie).

23

Carbon Resistors (alsocalledcarbon-composition,molded-carbon,andcarbonrod). Carbonresistorsare in the form of a smallrod moldedfrom carbonand a binder, with wire connectionsat eachend.The rod is usuallyprotected with a paper or ceramic sleeve,or a lacquer coatin$. Carbon resistors are the most common(andcheapest)type of resistor, andare availablein valuesfrom 10 ohms to 22 megohms.Standardtypesare usuallyavailableinl/a,Vt,Yz, L,and?watt ratings. It is a generalcharacteristicof carbonresistorsthat their valueremainsstable at normal temperatures,but above60"C their resistanceincreasesrapidly with increasingtemperature. Carbon-film Resistors (alsoknown as high-stability carbonresistors). To makea carbon-filmresistor, a thin film of carbonis depositedon a smallceramic rod. The rod is fitted with metal end caps,to which wire leadsare attached.The body of the resistor is usually protected by a varnish, paint, or siliconeresin coating, but some types may be encasedin a ceramic, plastic, or glass outer coating. Carbon-filmresistorsare little affectedby temperaturechanges(theirstability is usuallybetter than 1 percent)andare alsocharacterizcdby low noise.They are availablein sub-miniaturesizes(U20 and U10 watt power rating, andin larger sizesup to 1 watt power rating. They are a preferred type for radio circuits, particularly as they have excellenthigh-frequencycharacteristics. Metal-film Resistors. To makea metal-filmresistor, a metallicfilm (usually nickel-chromium)is depositedon a glassor ceramicrod. A helical track is then Metallicendcatsarethen cutin the filmtoproducethe requiredresistancevalue. protected is by a lacquer,paint, or wire leads, the body fitted, carrying the and plasticcoating.Stability characteristicsare similar to thoseof carbon-filmresistors, but they are more e:rpensive.They are generally producedin miniature sizeswith power ratings from 1/10 watt upwards. Metaloxide Film Resistore. Constructionis similar to that of a metal-film resistor except that the coating used is a metallic oxide (usually tin oxide), subsequentlycovered with a heat-resistant coating. This type of resistor is virtually imperviousto accidentaloverheating(e.g.,when makingsolderedconnections)andis alsonot affectedby dirmpness.Stability is very high (better than 1 percent), and the power ratings are higb for their physicalsize. Metal-glaze Resistors. In this type, tlre resistive film depositedon the rod is a cermet (metal-ceramic);otfierwise, construction is similar to metal-film resistors. Film-resistors also can be classifiedas thich-film or thin-rth. As a general rule, individualresistors of this type are thick-fiIm. Thick-film resistors are also madein groupson a smallsubstrateandencapsulatedinintegradedcircuitchips. Thin-film resistorsare madein a similar way, but on a considerablysmallerscale for use in the manufactureof integrated circuits. 24

Efiect of Age All resistorscanbeexpected to undergoa changein resistance is mostmarkedin thlgse of carbon-colnporiu*resistors, with age.This wtrerethe change mightbeasmuchas2O-percent in a yearor so.In the caseof carbon-firm and met'llic-fiknresistors,trrechanges"ido. will be,ooi" tu- " ril pur."nt. Efiect of High Frequencies The generaleffectof increasingfrequencyin ac circuitsis to decrease the apparentvalueof ther:y:torland thehighertheresistor ,.ru" ii"-i""ter this changeis likelyto be.This effectis mosimarked with carbon+omposition and wire-roundresistors.carbon-filmanametar-nr,n-resiriil,iirr"""it'ure high_ frequencycharacteristics. WIRE.WOUNDRESISTORS A wire-roundresistorismadebywrappingarengthof resistance wirearounda ceramiccoil.The wholeis thencoveredwilh a protectivecoatingor film. The offeredby wireresistorsarethat a wiaerange';varues :p".i6g advantages can (tyeicallyfrom i ohmto sookitohms)*ith po;;;;; eroduced !: from 1 to 50 watts (or up to 225 watts-in"power" types)and tolerancesas closeas 1 percent.Theyalsohaveexgelelt stability andlow noise.Thea disadvantages arethat trey aremostcostryandalso*.uit"ut" for usein,. "i.JL carrying highfrequencies because their effectivevaruechanges. physicary,theyneedbe no biggerthanfilm-typeresistorsfor the samepower rating. VARIABLE RESISTTVEDEVICES Themostcommontlpe ofvariabreresistive deviceusedinmodernerectronics il calleda potentiomefar. A potentiomet"iisa three.leadresistivedeviceconsistingof a fixedresistiveerementtrratcanbesweptby a wiperarm.The6xed resistiveelement,or trach,may b" circurar r".u"uv r zi6J"g* arc) or a straightline,circulartypesbeingthe morecommon. Theresistiveelementmaybe-wire-wound, carbon-composition, carbon-fiIm, or metallic-fiIm.The former tlpe is knownas a wire.wbundpotenuometer. carbon-trackpotentiom€ters. are the cheapest(with the samelimitationsas carbon-composition resistors),but are availableonly with ,nJ"ot" power ratings-e.g.,Yr wattforlowresistance ralues-reducingwith higherresistancevalues.wire-woundpotentiometers usualyhavehigG po","i'oting, .no arealsoavailablein lowerresistance varuesttrancarbonrtraclpotentiometers. Tolerances areusuallyontheorderof r0 percentof 20percent,futo'"v u",*r, closerwith precisionpotentiometers. connections shouldbeobviousfromFig.4-3.Thus,with connections to endr the.tryckandthe wiper,length1 to ck the resistivetrack 9f is in the circuit. Actualcircuit resistancethus can be variedby moving the wiper towards3 (increasing resistance), or toward1 (decreasing resistance). The changein resistancecal occurpropoiionanyto the actuarrengtrrof track involved,or rogarithmeticafu,*n*u trrereis a logarithmic increasein

Potentiometetr

Wipet (Turnedby lhob)

rr"*

Fig. H. Potentiometershown in vhematic form (lefr). Correspondingtcrminal positions are sluun on thc symbols(ight).

moretrack(similarto tlte "stegs" uncovering resistance with wipermovement adoptedfor standardresistorvalues).Theformeris knownasa linearpotenttPotentiometers canalsohavecharacometerandthelattera logpotentiometer. teristicsbetweentlre two. Note that linear in this descriptionhasquite a To avoidconfirdifferentmeaningto a linearphysicalshapeof potentiometer. sion,it is bestto refer to the latter asa slide-typepotentiometer. Thereis alsoaclassofvariableresistorsintendedtobeadjustedtoapartiodar Theseareknownas trim Potentiresistance settingandthenleft undisturbed. ometers, or just trim pots.Theyaresmallin sizeandmorelimitedin maximum resistancevalue-typically from 100 ohmsto 1 megohm.They are usually by a screwdriverappliedto the centralscrew. for adjustment designed Anothertypeofvariableresistivedeviceis calleda rheostat.A rheostatis a endof canbecreatedby simplybrpingone twoleadvariableresistor.Rheostats its associatedtap. Thus,thepotentiomofapotentiometerto thefixedresistance resistor.Otler tlpes of rheqstatsareawila two'leadadjustable eter becomes Theyconsistof a able,whichare usuallyintendedfor highpowerdissipation. powerresistorwith atrackoftheresistiveelementexposed. An fixedwirewound lockedat any ring makescontactwith this trackandis permanently adjustable valuealongtlre traclc desiredresistance in a circuitin whichit is necessary to be Potentiometers areusedspecifically ableto adjustresistance.A typicalexampleis the volumecontrolin a radio sothat at oneendof the canbedesigned circuit.In this casethe potentiometer track the wiperrunsright off the track to breakthe circuit.Thusthe volume up to work asanon-offswitch,usingtlis ortra controlcanalsobe connected facilityprovided. of fixedresistorsin a voltage Anotherpracticalexampleis the replacement potentiometer make circuit variablein performance. to the by a single divider Thus,t}re circuit previouslydescribedin Fig. 3-7 (Chapter3) alwaysgivesa

26

Fig. tU. A Practicaladjustablepotentialdioider.

predetermined voltageat the tappingpoints(providedthe supplyvoltagere(Fig.4-4), mainsconstant). Replacing resistorsRl andR2with a potentiometer wittr the tappingpointtakenfromoneendof tle potentiometer andthe wiper, willgiveatappedvoltagethatisfullyvariablefromthefullsupplyvoltagedownt zero,depending on tle positionof the wiper. In practice,in a variable'voltage circuitof this type,it maybe necessary to leavea fixedresistorin serieswith thepotentiometer to limit theorrent being drawnin ttreeventthat the potmtiometerhasbeenadiusted to zeroresistance andthe tappedcircuit is brokenor switchedoff with the originalsupplystill connected. Witlout thefixedresistor,thesupplyis shorted.Thevalueof afixed resistoris calculated to limit the currentdrawnin sucha caseto a safelevel Withafi:redresistorinserieswiththepotentiometer, of course,tie maximum voltagethatcanbetappedfromtle potentiometerisequal tothe supplyvoltage lessthe voltagedroppedby the fixedresistor. The mainthing to watchin sucha circuit is that the powerrating of the potentiometer is adequate to accommodate thevoltageandcurrentdrainin the tappedcircuit.But it hasonefurther advantage overa 6xedresistorpotential divider:Whena loadis addedto the tappedcircuit,it addsresistancein that circuit,causing afurthervoltagedrop.Unlessthisisallowedfor in calculating the valuesfor thefi:redresistorsin a potentialdivider,theloadwill receivelessthan the designvoltage.With a potentiometer replacingthe two fixedresistors,its positioncanbeadjusted (Fig. to bringtheloadvoltage backto therequiredfigure 4.5).Thisconsiderably simplifiesthedesignof a potentialdividerwheretheload resistance is knownonlyapproximately, or not at all

Fig. +5. Tltcfotzntiometcrcan b adjustztltogiaerequiredooltageacrosstlu load.

27

R4

R3

R2

RT

- Rl * R2+ R3+ R4 TotalResistance in vriesFig.tt6. Resrsfors CIRCUIT RULES FOR RESISTORS '' in in series(Fig.4-6),thetotalresistance connected Ifr thecaseof resistances circuitwill be tle sumof tle variousresistorvalues;i.e., :Rl * R2+ R3+''' totalresistance In the caseof resistorsconnectedin parallel(Fig. 4-7)' the total effective resistanceis givenby: 1

1

1

1

n-Rt-R2-85T"' whereR is the total resistance. In tle caseof two dissimilarresistors: K - -

Rl R2

Rl+R2

as: or remembered total resistance

productof resistorvalues sumof resistorvdues

I _ 1 .' 1 * ' l totA nesistance Rl R2 R3

in Parallel. Fig.+7. Resistors

%

F

o

Capacitors A capacitoris basicallya devicewhich storesan eJytric charge.physicary, it consistsof two metarplates-ora*troa.. ."p"oted by aninsurati'gmateriaror dielectric.Apprication of.. a. r"rig" .-"Jirrg napacitorprodu.ur-, a"fi.iun.y of electronson tre oositiygpr.r" ria "-ilt'of erectronson tre negativeprate (Fig. 5-l). This diffLrenua...rrut"tioi}"r".t onsrepresentsan erectricar up

ffii,ft"Yr1*rt$:i

acertain tuvet tJepenoing "" t[";;il6

andthen

As far asdc is concerned, the insulatoractsasa bluhing deienfor current flow(atthoughtrrereis a certaintt-.luni.rr.rging current which stops assoon asthe capacitoris frrtv charged).rn trr"-.iJorac appliedto thecapacitor, trre upduringone r,"[.y.r"u"."i".

reversed

lfo-ry:so that effectively ontrrcsecond harfoftre cycle' the capacitor "onou.6 current through it as if the f$y.

didnotexist.fhus,..i;;;.;;;cerned,

"."p."ito?i.acoupring

Therearescarcelyanyelectroniccircuits carryingacwhichdonotincorporate oneor more capacitors,ei_ther for coupringor shapingtrr" o'.on-i"qu"n"y responseof the netwolk. In the tattei?i,-a capacitoris associated with a resistorto form an RC cornbination GJtiiapter.6). The charge/discharge phenomenon associated_wittr *pr.i-t-")" *.],"* be usedin otfr", typ"* or circuits(e'g', trrephoto-graphic "r*troni"il'.r, is-operatedby the chargeand subsequent dischargeof a capacitorttiggurJat the appropriatemoment).

a ?

to a dc ktikls up d chargewlpn connected Fig.5-1.Illustratinghowa capacitor ttochingcurrentfow. ooltage, to havefixedvaluesor berrariable maybedesigrred Likeresistors,capacitors arethe mainbuildingblocksof a circuit(together in capacity.Fixedcapacitors aremainlyusedfor adjustingtunedcircuits. witl resistors).Variablecapacitors FD(ED CAPACITORS and non-polarizdcapacitors, fall into two maincategories: Fixedcapacitors thetypeof polarized Themainthingwhichdetermines or electrolyticcagacitors. capacitoris the dielectricmaterialused. Non-polarized capacitorsconsist,basically,of metallicfoil interleaveswitlt sheetsof soliddielectricmaterial,or equivalentconstruction.The important it As a consequence, thingis that the dielectricis readymadebeforeassembly. positive plate in The capacitor works or negative. is made doesnotmatterwhich just thesameway,whicheverwayit is connected in a circuit,hencethedescrip "non-polarized." butthisformof construction is convenient, This obviously tion in a single accommodated which can be doeslimit ttre amountof capacitance physicalsize.Up to about0.1 microfarads, the package packageof reasonable tle valuesmuchaboveI microfarad, canbemadequitesmall,butfor capacitance large in physicalsizeof a non-polarized become excessively capacitortendsto in the same circuit. likelyto be used with othercomponents comparison This limitationdoesnot applyin the caseof anelectrolyticcapacitor.Here, by a thin film of elecfuo separated initialconstruction consistsof two electrodes a voltageis appliedacrossthe electrodes bte. Fs a finalstageof manufacture, producing metallicoxide averythinfilmof nonconducting whichhastheeffectof plate of a to formthe dielectric.Thefactthat capacitance onthe surfaceof one much capacitorincreasesthe thinnerthe dielectricis mademeansthat very canbe producedin smallerphysicalsizes.The onlydisadhighercapacitances vantageis that an electrolyticcapacitormadein this waywill havea polarity to theoriginalpolaritywith whichthedielectricwasformed,tlis corresponding tbe correctpolaritybeingmarkedon tlre bodyof the capacitor.If connected wrongwayin a circuit,the reversedpolaritycandestroythedielectricflm and permanently ruin the capacitor. whichappliesto anelectrolyticcapaciThereis alsooneothercharacteristic electrolyteremainsafterits initialforming.This tor. A certainamountof unused 30

acts as a conductorand canmakethe capacitorquite leakyas far as dc is concerned. This mayor maynot beacceptable in particularcircuits. NONPOLARZED CAPACITOR TI"ES varioustypesof constructionareusedfor nonpolarized capacitors, mostof whichareeasilyidentifiedby theshapeof thecapacitor-seeFig. s-2.Thereis noneedto gointodetailabouttheactualconstructions. Theirspecificcharacteristicsareimportant,though,astlese candeterminethe besttpe to usefor a particularapplication. Paper Dielectric capacitors. Generallyrecognizable by their tubular form,arethelestexpensive but generallybulky,valuefor value,compared with moremoderntypes.Theirothermainlimitationis that tley arenot suitablefor useat frequencies muchabove1 MHz,whichvirtuallyrestrictstheirapplication to audiocircuits.Theyaregenerallyavailable in varuesfrom0.0bpF upto I or 2 pF, withworkingvoltagesfrom200to 1,000volts.Plastic-impregnated paper dielectriccapacitors mayhavemuchhigherworkingvoltages. ceramic capacitors. Nowwidelyusedin miniaturized audioandrf circuits. Theyarerelativelyinexpensive andareavailable in a widerangeofvaluesfrom 1 pFto 1 pF with highworkingvoltages,andalsocharacterized byhighleakage resistance. Theyareproduced in bothdiscsandtubularshapes andasmetallized ceramicplates. silver-mica capacitors. Moreorpensivethanceramiccapacitors buthave excellenthigh-frequency response andmuchsmallertolerances, soaregenerally

H ^mmf/ "fr N s n -r'x c ' \l-M f t

A-Ceramic Dsc B-Gramic Ptate C-SilveredMca D-Polystyrene E-Polycarbonate

H

F

I

F-Tantalum Bead G-Electrolytic(Polarizcd) H-Non-PolarizedElectrolytic J-Plug-InType(Sienens)

Fig, &2. Erample^sof modern capacitors

31

regardedas superior for critical applications.They can be madewith very high working voltages. Polystyrene Capacitors. Madefrom metallicfoil interleavedwitl polystyrene film, usuallywith a fusedpolystyreneenclosureto ensurehigh insulation resistgrce. They are noted for their low lossesat high frequencies(i.e., low induptafce and low seriesresistance),goodstability andreliability. Valuesmay range from l0 pF to 100,000 pF, but working voltage generallyfalls substantially with increasingcapacitance(e.g., as low as 60 volts for a 100,000 pF polystyrenecapacitor). Polycarbonate Capacitors. Usually producedin tlte form of rectangular slabswith wire end connectionsdesignedto plug into a printed circuit board. They offer high values(up to I pF) in very smallsizes,with the characteristicsof low lossesand low inductance.Like polystyrene capacitors,working voltages becomemore restricted with increasingvalue. Polyester Fitm Capacitors. Also designedfor use with printed circuit boards,with valuesfrom 0.01 1B up to 2.2 pF. Valuefor valuethey are generally larger in physicalsizethan polycarbonatecapacitors.Their low inherent inductancemakesthem particularly suitablefor couplinganddecouplingapplications. Valuesof polyesterfilm capacitorsare indicatedby a color codeconsistingof five color bands(seeFig. F3). Mylar Film Capacitors. A general-purposefilm type, usually availablein valuesfrom 0.00f pF up to 0.22 1$, with a working voltageup to 100 volts dc. ELECTROLYTIC CAPACITORS The original material used for electrolytic capacitorswas aluminium foil, together with a pasteelectrolyte, wound into a tubular form with an aluminum outer cover, characterizedby dimpledrings at one or both ends.The modern form of aluminum electrolytic capacitor is basedon etched-foil construction, enablinghigher capacitancevaluesto be achievedin smaller can sizes.Values availablefrom I pF up to 4,700 lE (or even larger, if required). Working voltagesare generallylow, but may range from 10 volts up to 250 or 500 volts dc, lstDigit

-

ffi_2dDisit Multiplier W Tobrance rc vort"g" n"tiog l:'l\

l

-

Black*OR-20% whiter oR- lolo

E:fr?'rKB

l FA. 5-?.C;olorcodefor poly.stercafuibr*

z

Capacitancein pF

1QOLOR Black Brown Red Orange Yellow Green Blue Violet Grey White

Ring

Ring

1 2 3 4

0 1 2 3 4

O

D

6

6

t

t

8 9

8 9

Working Voltage

3rd Ring dc and Multiplier

x 1.00 x

Color

Voltage

White Yellow Black Green Grey Pink

3 6.3 10 16 25 35

x 0.01 x 0.10

Fig. 54, C,olorcdc for tantalumbad capacitors. dependingon valueand construction.A singleleademergesfrom eachend,but singleendedtypes (both leadsemergingfrom oneend), andcan-t1peswith rigid leadsinone endfor plugginginto a socketare alsoavailable.single-endedtypes are preferred for mounting on printed-circuit boards. The other main types of electrolytic is the tantalum capacitor. This is produced both in cylindrical configurationwith ddal leads, or n tantalumhead configuration.Both (anathe latter type particularly) can offer very high capacitance valuesin small physicalsizes,within tlre range 0.1 to 100pF. voltage ratings are generallylow; from 35 volts down to less than 10 volts dc. All electrolytic capacitorsnormally have their value marked on tlre body or case,togetler with a polarity marking (* indicatingthe positive lead). Tantalum-beadcapacitors,however, are sometimescolor codedinstead of marked witr values.This color codingis shownin Fi g. s-4,while other codeswhich may be found on otfier types ofnon-polarizedcapacitorsare given in Fig. F-b.

' : MultiplierTolerance

2ndDgit Multiptier \ rst Digit

Murtiprie.53{'"**

q"/ iTffi: w

}}J1

Toterancell tr*'r*u .^,.-..-l I lst Digit 2nd Digit Multiplier i

i

l

l

|

|

Fig. 5-5. Aher codingsysbmsuvd on capacitars.

3:)

Tolerance of Fixed Capacitore As a generalrule, only silver-micacapacitorsare madeto closetolerances is (plusor minus1 percentis usrul).Thetoleranceonothertypesof capacitors (as as 50 much higher percent even may be and 20 10 and usuallybetween Because of thewidetolerances percent)in thecaseof alurrinumfoilelectrolytics. is seldom criticat value actual choice of norrralwith electrolytics, VARIABLE CAPACITORS setsof metalplates,onesetbeing arebasedoninterleaved Variablecapacitors by a dielectricwhichmay Theplatesareseparated fixedandtheothermovable. area onesetofplatesdterstheeffective Movementof beairorasoliddielectric. present. plates, ralue capacitance the of and thus the of for fre' Thereis alsoa generaldistinctionbetweentuning capaa;i/airsv*d quentadjtstment(e.g., to tute a radio receiverto a partiorlar station)and of a hmedcircuit.Tuningcapacifor initialadiustment trimmer caprciforsused generallyof airdielectrictype. and robust in constnrction more larger, tors are dielectricwitha smaller mica orfi}n ona based are usrully Trimmercapacitors a ceotralscrewto rary trrning plates, by adjusted being capacitance numberof Becausetheyaresmallerinsize,however, thepressurebetweenplatesandmica. onasubminiabeusedasahmingcapacitor a trimmercapacitormaysometimes are made,decapacitors hning miniature ture radiocircuit, althougbspecial printed*ircuit board" signedto mountdirectlyon a themanner determines theshapeof tbernanes In thecaseof tuningcapacitors, cbaracteristics Tbese movemeot" with spindle charryes in whichcapacitance usully fall underoneof the followingdescriptions: anequalchangein e Linear-where eachdegreeof spindlerotationproduces Tbis is the mostusualtne chosenfor radioreceivers. capacitance. poducesa constant o Logaithmic-where eachdegreeof spindlemovement circuil a hmed of n change frequncy fercentage -Ewn p&rcs m equal o freqtuncyI whseeachdegreeof spindlemovement circ'uit. in a tuned frequency in change ispropodionalto thequare of' o Squirelaw- wherethechangeintapacitance movemeot. spindle of the angle BASIC CIRCUIT RI'LES FOR CAPACITORS of capacitorsin series andin paralleltare The rules for total capacitance connectednseria (Fig.5-6)' the For capacitors resistors. for to that opposite given (C) by: is capacitance totaleffective

-l a

HHI-Fig.ffi. &Pcitors itsriss

I

-i-'1,-A ' '

TotalCapacitance Cl

C2

Ci!

- C1+ C2+ Ci| TotalCapacitance

FA. e7. Cafacitorsin parallel.

'c2 lc: : + c ll + l +' c 3 or, in tle caseof hro dissimilarcapacitors clcz A ar:f,ffi In words : total capacitance '

product-ofcapacitances sumofcapacitances

Fc capacitorsconnected in parallet(FiS.$Z):

C - C l + C 2 + C 3 +. . . Thiscapacitance efrect,of course,isonlyapparentin anaccircuiLIn adccircuita capacitorsimplybuildsup a chargewithoutpassingcurrent.In a practicalac circuit,a capacitoralsoorhibits reactance(seeChapter2), andbecarceof its ao$itruction,mayalsoexhibita certainamountof.indrctance(seeCbapterZ).

ffi

6

capacitorand RC Circuits capable ofpassing Oneofthe principalusesofa capacitoris asa couplingdevice practical In any circuit, tlere will be some but acting as a block to dc. ac in serieswith the capacitor(e.g.,tle resistiveloadof the resistance connected circuit beingcoupled),This resistancelimits the currentflow andleadsto a certaindelaybetweentle applicationof a voltageto the capacitorandthe build-upof chargeon tle capacitorequivalentto tlnt voltage.It is this charge of of dc. At the sametime,tbe combination voltagewhichblocksthe passage generallyabbreviated to RC,actsasafilterapprbb resistance with capacitance, onthechargedischarge timeof thecapaciof passing acfrequencies, depending tor, or the time consfuntoftheRCcombination

TIME CONSTANT the time constant(t) is quitesimple: The formulafor calculatirrg

t -R C where: t : time constantin seconds in ohms R - resistance in farads C: capacitance fi

raluefor T is givenif R is in ohmsandC 0t canbenotedthatthesamenumerical in farads,but megohmsandmicrofaradsare usully muchmore convoient units.) The time constantis actuallythetime for thevoltageacrossthecapacitorin an RCcombination to reach63 percentof the appliedvolt4ge(this63 percent is chosen as mathernatical a ThevoltageacK)ssthe capaciconvenience). foure tor goeson buildingup to almost(butneverquite)100percentof the applied roltage,as shonmin Fig. Gl. T\e time+onstantfadorrcfersto the durationof timein termsof the time ftctor; e.g., at I (whichrs the time factorof the RCcombinations) 63 percent full voltagehasbeenbuilt up, in a time equalto 2 timesthe time consbnt,80 percentfull rcltage;andso on. After a time constantof 5 the full (almost100 percenDvoltagehasbeenbuilt up acrossthe capacitor. The dischargecharacteristics of a capacitortakeplacein essentially the b yercemantrer;e.g.,aftera periodof timeequalto tle timeconstanttbevoltage acrossthecapacitorbasdropped100- 63 - 37 percentof thefull volt4geand so orl" In theory,at least,a capacitorneverchargesup to full appliedvoltage;nor doesit fully discharge. In p'ractice,full charge,or completedischarge, canbe considered as beingachievedin a periodof time equalto five time constants. Thus,in the circuit identifiedwith Fig. G2, closingswitctr1 producesa fdl chargeon ttre capacitorin 5 timestime constantseconds. If $ritch 1 is now opened, thecapacitorthenremainsina conditionof storingavoltageequiralent to the originalappliedvoltage,holdingthis chargeindefnitelyif there is no internalleakage.In practice,it very slowlylosesits charge,as no practical capacitoris perfect,but for someconsiderable time it remainseffectivelyasa potentialsourceof full*tnrge voltage.If the capacitoris part of a high-voltage circuit,foroanple, itisreadilycapable of givinganelectricshoc.kif touc.hedfor sometime afterthe circuithasbeenswitchedoff. To completethecycleof chargedischarge asshownin theseconddiagramof Fig.&1, switch2 is closed, whenthecapacitor discharges throughtheassociated resistance, tahng a finiteamountof time to completeits discharge.

0

1 2 3 { 5 0 1 2 3 4 5 Tim Coostatrt Fact6 Tiuc CosteatFactc FA. eL. Penentageodtageaclor*calacitor nlated to tittu ubn bing cluryed(W anddialurge (ighil.

tr

TimeConstantFactor

TLncCortrnt Frctr

Fig.62 Closingwihh 1allouscaluitor b clwrgcigltt upin a tisu c.qulb 6 ritu a similarpriod todinluryefil$ throrgha ld rcifur tfrcs cmstantsIt thcntahos witch2isclovd. Figure&3 showsavery simplecircuitthatworksonthisFinciple.It coosists ddcvoltage.Ar seriestoasource of aresistor(R)andcapacitor(C)connectedin a visualindicationof ttre workingof the circtit, a neonlampis connectedin parallelwith thecapacitor.Thelamprepresentsavirtully opencirctit until its thresholdvoltageis applied,whenit imnediatelyconductscurrentlike a low resistance andglows(seeChapter10for moreaboutneons).Tbevoltagesouroe for this currentmustthereforebe abovetbat of the neontum-onvolt4ga ata Whenthiscircuitis switchedon,the capacitorstartstobuiH upa c.harge on tle time constantof R andC. Tbe lampis fed by voltage rate dependfuig acr$s the capacitor.Oncethis reachesthe turnon voltqged ttre developed tlrowb the hmp, the lampswitcheson andcausesthe capacitorto disc.harge nom6€ qrrrent neongas,causingit to glow.Orcethecapacitorhasdischarged, flowsthroughthelampandsoit switcbesoffagainutil thecapacitorhasbuiltup throwh the to theturnon voltage,whenit dischargea anotherchargeequivalent by lamp,andsoon.In otherwords,theneonlampwill flashat a ratedetermined R and C. the time constantof valuesshown,tbe time constaotfor tle cirqdt iss Usingthe component

x 0.1(niaoftrads) t - 5 (megohms) - 0.5seconds

Fig. ffi. Sinpleneonfoshcr circuit (notctheqnbl lor a ,wrr lanfu). Ttu oalucEof the rcsisbr (R) and ca!rcitor (C) detemiw thcfoshing rate. I

theactualflashingrateof thecircuit.It maytakeaperiod Thisisnot necessarily of morethanonetime constant(or less)for the capacitorvoltageto buildupto the neonturn-onvoltage-more if the turn'on voltageis greaterthan 63 percentof thesupplyvoltage;lessif theturn'onvoltageislessthan63percentof the supplyvoltage. It atsoiollowsthattheflashingratecanbealteredby alteringthevdueof R or C, either by substitutingdifferentvaluescalculatedto give a differenttime asimilar resistoror capacitor.Connecting constant,oi wittra paraUit-connected valueresistorin parallelwith R, for example,woulddoublethe flashingrate Connecting (sinceparalleting iimitar resistorvalueshalvesthetotalresistance). rate. flashing the parallel halves c with in capacitor value a similar This type of ctcuit is knownas a relaration osillator. Usinga variable for aspecificflashingrate.It candsobeextended resistorfor R it canbeadiusted aseriesofRCcircuits,each in theformof anoveltylightingsystembyconnecting with a differenttimeconstant eachRCcombination with a neonlampin cascade, neonsin tle completecircuit. -Fig. &4. Thi; produces randomflashingof t11e CAPACITORSIN AC CIRCUITS thefactthattheappliedvoltageis alternatingmeans Asfar asacis concerned, that duringone half cyclethe capacitoris effectivelybeingchargedanddischargedwith onedirectionof voltage;andduringthesecondhalfof t]1eaccycle, with oppositedirectionof voltage.Thus,in effect,ac ctrarieaanddischarged maybe restrictedonlybysuchlimitationsas uottaie.pass*noughttrecapacitor, proportion applied of the what determines appffr UvtfreRCtimeconstantwhich the time, At the same t}rough the capacitor. niit"g" is Uuittup anddischarged (see reactance passage through ac of to the ."p.Jitot offersa certainopposition power.Its maininfluenceis Ciiipt* 3), althoughthisdoesnot actuallyconsume of RCcircuits. on frequencyresponse SIMPLE COUPLING Couplingonestageof a radioreceiverto the next stagevia a capacitoris onitsown, isapparentlyused Althoughthecapacitance commondesignpractice. loadofthe by the represented with aneffectiveseriesresistance it is associated

thiswayand will fash in Fig. &4. Randomcasadefla.slprcircuit. Any numfurof lampscanbeconnecbdin rardomorfur.

stagebeingfed-Fig. &5. This, togetherwith the capacitor,forms an RC whichhasa particulartimeconstant.It is importanttlat this time combination from of theacsignalfrequenclbing passed therequirements constantmatctres to the other. onestage In the caseof AM radiostage,the ma:rimumsignallikely to be presentis How10 kIIz. The cycletime of sucha signalis U10'000- 0.1 milliseconds. funcpass two charge/disc;harge cycle represents frequency each this ever,to positive negative. one andone concerned, tionsasfarasthecouplingcapacitoris functionis 0.05milliseconds. Thusthetimeperiodfor a singlecharge/discharge this application mustbea to accommodate The RCtime constantnecessary preferably percent passing 63 of the appliedacvoltage-and valuecapableof moretlnn 63 percentof the appliedvoltage. Thesefigres cangiveyou a clueas to the optimumralue of the coupling capacitorto use.For example,the typicalinput resistanceof a low power transistoris of the orderof 1,000ohms.The time constantof a matchingRC (seeabove),givingthe requirements: couplingwouldbe 0.05milliseconds f

t:RC.'.C:= t(

-9W

1X ld :0.05 x 10-5 :0.05 pF (or higtrer,sincethis wouldensuremorethan63 percentvoltagepassed). wlue wouldnormallybeused;evenas In practice,a muchhighercapacitance higb as lpF or more This usuallygivesbetter results,at the expenseof (Anapparentcontradiction, but it efficiencyof ac(in tlis caserf) transrnission. the loadis reactiteratherthanpurely happensto work out that waybecause reistive.) Whatthe simplecalculationdoesshowis that capacitivecoupling frequencyof acsignalwhen lessefrcientwith decreasing increasingly becomes usedfor couplingduties. with practicalvaluesof capacitors associated FILTER CIRCUITS usedasafllarcirczitis shownin Fig. &6. Fromthe A basicRCcombination reutance,witha a resistorin serieswith a capacitive inputside,thisrepresents

acSigld Gcocntcd

SuScf

EficcrivcRceicrncc

Copling C.Fcit6

is b W o signalsad tlu* dc Fig.ffi. fusicf*tution of a couflingcapacitor signals dc uwlulating dgnals.It ab lases 4)

FiS.6$. fusiefilter circuit.It Huhsrcfnquncieshighcrtlvn theilbtfnq.cnct of ofR andC, thccoabination If the reactanceof the capacitor()b) is voltagedropacrosseachcomponent. acrossthecapacitorand muchgreaterthanR,mostof tle inputvoltageappears is the input voltagein value.Reactance tlus the outputvoltageapproaches to frequency, however,andsowith increasing frequency inverselyproportional andso doesthe outputvoltage(an the reactanceof the capacitordecreases, hcreasingproportionof tbe inputvoltagebeingdroppedby the resistor). tlere is a criticalfrequeocyat As far aseffectivepassage of acis concerned, in valuetlat suchacircuit component becomes sodegraded wbichthereactance Le.,tle ratioofvoltqd/volt% st rts to become blockingratherthanconductive; formin Fig. C7. startsto fall rapidly.This is shom in simplifieddiagrammatic point, point knownastheroll-off Thecritical or catotrfreqaenq(4),is givenby: f ..

I

2nRC

where: R is in ohms C is in farads z- 3.1416 0dB

gd

\

-t* ! -uorE

\

I

g

-r* .E I -o*F

\

I

g

\

g

\

0.rE

0.125 0.09 h0

a

-$dB! \ I

-"*$ -21 dB>

t/to -rfirqu4t

[

1&f,

FA.e7. Sinllifieddiagramoflnw thcratioof ooltsinl ooltsnt dnhsralidb u the cntoffteWenqofafiltq b e,rualcd. All Ebrrole Uorr tb cubt Inryenq an lM sitho*t attenuatio*

previously, ButRC,asnoted isegual tothetimecmskntdtheRCombinatio. Ilence:

\. : mt whereT irsthe time constant,in seconds. Theperforrnance of suchafilter is definedby its cutofrfrequencyandtherate at whichthe volts5/volts"nratio fallsabovethe cutofffrequency.Thelatter ic normallyquotedas(somany)dBperoctave(or eachdoublingdfrequency)(see Fig. e8) whichshowstherelationship betweendBandvoltqbfrcttsdratio,and alsothe tnre form of thefrequencyresponse ctree. IOW.PASS FILTDRS Circuitsd this type are ealledlowlass filtersfuu* they passsg siguts belowthe @crrt-off frequencywith little or no loss or atbntutios e Jgnal streryth" with signls abovethe cutoftfrequency,thereis increasingatteotntim. suitablecomponent rnalues are readilycalcnlated" For examplga typical sratch filbrassociatd with a recordplayeror amplifierwonHbedesigned to attmuatefrequencies above,say,l0 kHz-Fig. Gg. Thisraluerepresenrts the oilofr frequencyrequired;i.e.,Anycombination d R (in obns)ad C (inhrads) giyingthis prodrctrnalue couldbe r$ed. EIGtr.PASS FILTERS High-passfilters work the other way around.They attenuatefrequencies belowthe ortoff frequency,but passfrequencies at andabovethe cutofrfre

>.

0dB -3 dB -6 dB .9 E

\

I Ro[offpointor cutofftrequency \

-edB

& $ -udB

\ \

-15 dB

\

-18 dB -2r dB

Vl00

V10 Frequency

t

\ r0 x i

Fig.ffi Theoctr.olrollof fodntot ttu fnqrercy nsfurt* crtw of a fiItet is mt *r-tV defr,ten"Theilbt fnqunq b nallXa nninaifigun andgeturalgtthtn as tlelrcwcaq at uhichtlun iso J biM loss ora cr/lts inl ,dltsmt itu ofi.mz rhiEie qtiulest b a 50pncnt las offuwt

c

(R) Resistor

low-PassFilter

Fig.&9. simplcvrahhfiltcrcitcuit.Anycombination ofcomfionent giainga aarues lraductofRC- 7ffi0will worh. quencywith no attenuation. To achievethis modeof working,the two compo-' (Fig.6-10). interchanged nentsin the circuitare This typeof filter is againcommonlyassociated with recordplayercircuits, incorrporated to eliminatelow-frequencynoiseor "rumble" which may be present.The desigltcutofffrequency mustbe lowenoughnot to interferewith bassresponse, andso the valuechosenis usuallyof the orderof 15 to 20llz. Exactlythe sameformulais usedto determinethe cutofffrequency,hence, usinga desigrralueof 20llzz

,r-#'.,Ji RC: 125 Again,anycombination of R (in ohms)andC (in farads)grvinsa productof.l'25 wouldwork. Capocitor(C)

High-Pass Filtctr

FA. 6n. Rteh[ High-tdssor ntmblefilters cut of lowlreqrenciesfut passhigh A typicalaalrcfor thelroduct RC in thiscav wouldb 125. frequencies.

ln practical circuits, such filters are normally inserted in the preamplifier stage,or in tlre amplifierimmediatelyin front of the tone control circuit. For hi-fi systems,the type of filter circuits usedare considerablymore complicatedthan the onesdescribed. For more advancedfilter designs,seethe appendix.

tU

Coils and Inductances magneticfield.Thiscreationof magneticenergyrepresents a powerlossduring thetimethatfieldis beingcreated,whichismeasurable in termsofavoltagedrop to thevoltagedropproduced 9r bachemf,Thisis quitedifferent(andadditional) by the resistance of the conductor,anddisappears oncestabli conditionshave beenreached.Thus,in a dc circuit,the backemftendsto preventthe curent risingrapidlywhenthecircuitis switchedon.oncea constantmagneticfieldhas beenestablished, tre backemf disappears sinceno further "n*gy is being extractedfrom the circuitandtransferredto the magneticfield. In thecaseof anaccircuit,thecurrentis continuallychanging, creatinga back emf whichis alsochangrngat a similarrate. The valueof the backemf is dependentboth on tlre rate of changeof current(frequency)andto a factor dependent on the form of tle conductorwhichgovernsits inductance.Inductanceis tlus anotherformof resistance to ac,generated in additionto the pure resistance. Every conductorhasinductance whencarryingac, althoughin the caseof straightwiresthis is usuallynegligible(exceptat very highfrequencies). If the wireiswoundin theformofacoil,however, itsinductanceisgreatlyincreased.If thecoilisfittedwith anironcore,thenits inductance is evenhigheifor tlresame numberof turnsandcoil size. TViththe acflowingthrought}recoil,tlre "resistive"conditionestablished is notasdrasticasmayappearat first sight.Thepolarityof thebackemfis always

suchasto opposeanychangein current.Thuswhilethe currentis increasing, workis beingdoneagainstthebackemfby storingenergyinthernagnetic field. On the next part of the currentcyclewhenthe curent is falling,tle stored energyin tlte magneticfield returnsto the circuit, tlrus tendingto keepthe current flowing(seeFig. 7-1). An inductance,in fact, may be a very good conductorof ac,especially whencombined with a capacitorrn a tunedcircuit (seelater).Onthe otler hand,it maybedesigned to workasa resistivecomponentot chohe. Theinductance ofasingle-layer coil,woundwith spacebetweenadjacent turns canbecalculated from the formula: L-

R2 N2

9R + 10L

where: L is the inductance in microhenrys R is the radiusof the coilin inches N is the numberof turns L is the lengthof the coilin inches. Written as a solutionfor the numberof turns requiredto producea given inductance with R andL predetermined:

' - : 1 @lo[,)xL -lThisformulaappliesregardless of the actualdiameterof the wire used(also,it doesnot matterwhetler barewire or insulatedwire is used),providedttrecoil diameteris muchlargerthanthewire diameter. Forpracticalsizesof wiresused for coilwinding,this meansa minimumcoildiameterof at least1 inch(25 mm).

+

I E

a (J

switchoo

Time -*

SwitchOn

Time*

Fig. 7'1. fuch emfinducedin a dccircuit onxtitching on eristson\t momcntarig.In an accircuit, thcfuchemfis continuallychanging.

/to

For smallerdiametercoils,thewiresizehasanincreasing modifyingeffecton the actualinductance, andeventhe lengthof leadsat the endsof the coil can upsetthe calculation. Thussuchcoilsarenormallydesigned on empiricallines (i.e.,basedona specifiednumberof turnsofagivensize ofwireknowntoproduce a giveninductance whenwoundona specificforrr diameter). In practice,smallcoilsarenormallywoundona formintendedto takeaniron core. The positionof this core is adjustable, relativeto the woundcoil, by screwingin or out.Thust}e actnalvalueof inductance canberariedfor tuning (Fig. 7-2, left). purposes Alternativelya pot coremaybe usedwherethe coil is woundon a form or bobbin,subsequently in aniron housing.Providedthesiecific inilucenclosed tanceof.the pot coreis known(it is usuallyspecified by the manufacturer), the numberof turns (p) to be usedfor the windingcanbe calculatedwith good accuracy fromthe formula N:

t=

-/3

YA"

whereL is the inductance required,andA" is the quotedspecificinductance of . the pot corein the sameunitsasL. Practicalvaluesof inductanceusedin electroniccircuitsmay rangefrom microhenrys(in mediumand high frequencycircuits)to millihenrys(in low frequencycircuits),up to severalhenrysfor chokesin powersupplycircuits. Normally,aninductance will bewoundfromthelargestdiameterenameled wire it is convenient to use(andstill gettherequirednumberof turnsontlte formor bobbin),because this will minimizeobmicresistance andtlus improvetle efficimcy or &factarof.the coil RESONANTCIRCUITS A coil(inductance) anda capacitorconnected in seriesacrossanacsupplyhas theveryimportantcharacteristic that it is possible for thereactiveeffectof one to cancelouttle reactiveeffectsof theother.Thusin thedemonstration circuit shownin Fig. 7-3,L is the inductance, andC the capacitorconnected acrossa sourceof ac,the frequencyof whichcanbe varied.A resistor(R) is shownin serieswith L andC, asaninevitablepart of a practicalcircuit. If theacsupplyis adiusted to a lowfrequency, thecapacitive reactance will be verymuchlargerthanR, andttreinductivereactance will bemuchlowerthanR

E

g

ffi

g E ffi

Fig. 7-2 Coilform (lcfi) and tot con^c(rieht)47

PracticalEqrdnkrt

Th€oreticalCirqrit

Fig. 74. Components whichmahea resnantcircait. (andthusalsoverymuchlowerthanthecapacitive reactance). SeeCbapter3 for the formulasfor capacitiveandinductivereactance,andhowtlese valuesare on frequency. dependent If the ac supplyis adjustedto a highfrequencythe oppositeconditionswill lower reactance muchlargerthanR,andcapacitive apply-inductive reactance thanR andL. Somewhere betweenthesetwo e:rtremesthereis anacfrequency areequal,andthis is andinductance of the capacitance at whichthe reactances the really interestingpoint.I[hen inductivereactance(X") equalscapcitive (Xc),thevoltagedropsactosstiese two components will beequalbut reactance 7& degrees out of phav. This meansthe two voltagedropscanceleachotler to currentflow.In out,with theresultthat onlyR is effectiveastotalresistance onlybythe currentflowsthroughthecircuit,determined otler words,maximum valueof R andthe appliedacvoltage. ![orking undertheseconditions, thecircuitis saidto be resnant.Obviously, resonance occursonlyat a specificfrequency,whichis thuscalledthe resonant frequency.Itsvalueis givenby the simpleformula: . l ':ffiLC where: f: resonantfrequencyin IIz L - inductance in henrys C - capacitance in farads. formulato useis: A moreconvenient

'.

? : -

106

2nJfr,

wbere: f - resonantfrequencyin kilohertz(kIIz) tt8

L - inductance in microhenrys(pH) C: capacitance in picofarads(pF). Notethattlreformulafor resonant frequency isnotaffectedbyLy resistance (R) in thecircuit.Thepresence of resistance does,however,affecttheeualityfactor or 0 of the circuit.Thisis a measureof how sharfgthecircuit canbetunedto resonan@, thehighertlle valueof Qthebetter,in thisrespect.Theactualvalue of Q is givenby:

o:* whereX is thereactance in ohmsof eitherthe inductance or capacitance at the resonantfrequency(theyareboththe same,soit doesnot matterwhichoneis taken)andR is the valueof the seriesresistance in ohms. The practicalresonantcircuit (or tuned circuit) is basedon just two - aninductance components andacapacitor.Someresistance isalwayspresent, however.At lowto moderately highfrequencies, mostof tltis resistance comes fromthewirefromwhichthecoiliswound.At verymuchhigherfrequencies, the najority of the resistancemay comefrom the frequencyenergylossin the capacitor. TUNED CIRCUITS The combination of an inductance andcapacitance in seriesis the standard Iormof.tunedcircuitu#nalmost everyradioreceiver.Figurez-4illustratesa tunedcircuit with the inductorandcapacitorin a parallelconfiguration. The (Z) of thiscircuitis oppositein effectto thecircuitshownin Fig.Z-3. impedance As previouslydiscussed, whentlte resonantfrequencyis appliedto ttre circuit shownin Fig.7-3,thecircuitcurrentis at its highest(meaning its impedance isat minimum).Thecircuitillustratedin Fig.7-4presentsits highestimpedance at its pointof resonance.

TheoreticalCircuit

PracticalEquivdent

Fig. 74 TheoreticaQ, on$ a capacitor and indrctance are inoobed in a resnant circuit. In practice, snc resistanceis alway frexnt as uell.

To make the circuit tunable over a range of resonant frequencies,either componentcan be a variable type. The usualchoicefor antennacircuits is to maketlre capacitorvariable.In practice,tlte coil may alsohavevariablecharacteristics. It is usudly wound on a sleevefitted on a ferrite rod, and capableof being moved up and down the rod, providing a meansof varying tlre effective inductance.Oncean optimumpositionhasbeenfoundfor the coil, it is cemented to the rod. ln other words,the variablecharacteristicsof the coil areusedonlyfor initial adjustment.After that, all adjustmentof resonantfrequency,or tuning, is doneby the variablecapacitor. To assist in selecting suitable componentvalues, the resonant frequency formula can be rewritten:

LC:

1012 4n2 f2

where L is in mictohenrys,C is in picofarads,and f is the frequencyin kIIz. Maximum valuesof variablecapacitorusedare normally 300 pF or 500 pF. The working formula for calculatinga matchinginductancevalue is:

L(microhenry n: # thetunedcircuitistobedesignedto coverthemedium Asanexample, suppose from500to 1,600kHz;anda 500pFtuningcapacitor waveband, or frequencies is to be used.It followsfrom the resonantfrequencyformulathat ma:rimum will correspondto the lowestresonantfrequency(with a fixed capacitance inductance), whichin thiscaseis 500kHz.lnsertingtlese valuesin tle working formula:

L(microhenrysr:Z;#,

*

:200 the capacitoris turnedto its miniNowcheckthe resonantfrequencywhen (which pF, associated probably will be about 50 with this valueof mumvalue inductance:

FM 4nzX200X50

: 1,600kIIz

This showsthat a 50-500 pF variablecapacitorwill tune the circuitfrom 50

Fig. 7-5.A series-rennantcircuit. Tlv impedance acrosstlrctcrminab of tlu circuit b wry lou at thefrequencyof rennance;at thcconnection btween tlv capacitorand inducta4 the imfodanceis wry higrL Thc cafrcitor is usual$ theoariablcelcmentin thisanangement

T

1500kHz (the highestfrequency),downto 500kIIz satisfactorily.In other words,it coversttrewholeof the mediumwavebroadcast band. If the finalresultsachievedin the circuitdo not providequitetlre coverage required(for example, a stationnearoneendof the bandis not pickedup)then thereis stillthe possibilityofshiftingthefreguencyooverage inonedirectionor (i.e.,slidingthecoilupor downtheferrite the otherby adjustingtheinductance rd). Thereareotler typeof tunedcircuitswhichnormallyrequireadjustment only wheninitiallysettingup.Thesenormallyemployatunableinductance (e.g.,acoil powdered-iron woundona formwith anadjustable core).Suchcircuitsmayalso be tunedby a trimmer capacitor,or both a trimmer capacitorand tunable inductance. providesdoubletuniqg. The latter combination

SERIES.RESONANTCIRCIIITS Anottrerarrangement of the coil-capacitor combination is to connectthemin series(Fig.7-5).Thisproducesaseries-resonantcraitwherein thereactances of the coil andcapacitorare againequalbut opposite.The differenceis that it presentsa lowimpedance at the terminalsof the circuit(topandbottom).This lowimpedance hastheeffectof shuntingtheacfrequency of resonance outofthe circuit.Frequencies otherthanthe resonantonearenot affectedby the tuned circuit,asthe off-resonance impedance is undisturbed. A common usefor thistypeofcircuitis to remove,or reducein amplitude, any unwantedsignals,whileallowingall othersto pass.A popularapplication of the series-resonant circuitisin theantenna or rf-amplifierstagesofreceivers,where it is oftencalleda waoe-trap.Itcanalsobeusedquiteeffectivelyin transmitter power-amplifier (harmonics) stagesto trapunwantedmultiples oftlefundamental frequencyof operation. RADIO.FREQUENCYCHOKES A radiofrequencychoke(rfc) is a coilor inductance sodesigned that it hasa relativelylowohmicresistance buta veryhighreactance at radiofrequencies. It

Fig. 74. ?ypicalapfuaranceof clnhx woundon a ferrite core. can thus passdc but blockshigh frequencyac when tlte two are present in the samecircuit (Fig. 7-6). ln other words,it really works the oppositeto a capacitor as a circuit element in this respect. The characteristicsof any rfc vary with frequency.At high frequenciesit has characteristicssimilar to that of a parallel-resonantcircuit; and at low frequencies characteristicssimilar to that of a series-resonantcircuit. At intermediate frequencies,it has intermediate characteristics.The actual characteristicsare relatively unimportantwhen an rfc is usedfor seriesfeedbecausethe rf voltage acrqss the choke is negligible. If used for parallel feed (where the choke is shuntedacros{ra tank circuit), it must have sufficienfly high impedanceat the lowest frequenciesand no series-resonancecharacteristicsat the higher frequenciesin order to reduce power absorption to a suitable level. Otherwise, there is a danger of the chokebeing overloadedand burned out. Chokesdesigred to maintain at least a critical value of inductanceover tlre likely range of qrrent likely to flow through them are calledswinging chohes. They are usedas input filters on power suppliesto reduce riiple, or residualac content. Chokesdesignedspecificallyfor smoothingripple, and having a sub stantially constantinductance,independentofcbangesin current, are known as snmthing ch&es.

52

Transformers A transformerconsistsof two coilssopositionedthat theyhavemutualinductance.Thismagrretic couplingeffect canbefurtherenhanced bywindingthetwo coilson a commoniron core(seeFig. &1). The coilwhichis connected to tlrc sourceof supplyis calledthey'rimary,mdtheothercoilis calledthewcondary. In orderto transferelectricalenergrfromprimaryto secondary, the magretic fieldmustbe continuallychangrng; i.e.,the supplymustbe ac. Oneof the mostusefulcharacteristics of a transformeris its abilityto step down(orstepup)acvoltages.Thestepdown(orstepup)ratioisproportional to the numberof turnsin eachcoil: V,=NNrx% where: V. : secondary voltage N, : numberofturns on secondary \ : numberof turnsonprimary V, -prinaryvoltage. Thecurrentsffowingin theprinaryandsecondaryfollow a similarrelationship, but in oppositeratio:

L:Nn/N,x t 53

Iron Core

".,.71IF:.'** Secondary

Fig. 8-1. Thesimpleiron+oredtransformcr. where: l: secondarycurrent - nrimaw current | ln other words, a stepdown in voltage producesa step-upin current, and vice versa. In practice,there are alwayssomelossesdueto the resistanceof tlte coilsand energy lost in hysteresisand eddy currents in the core (in the caseof an ironcoredtransformer),andalsofrom reactancecausedby a leak of inductancefrom both coils.Thus, the powerwhich canbe takenfrom the secondaryis alwaysless than the power put intothe primary, the ratio of the two powersbeingameasure of.the eftciencyof the transformer. Typically, efficiencymay rangefrom 60 percentupwards,but is not necessarily constant.A transformer is usuallydesignedto haveits maximumefficiencyat its rated power output. Its actual efficiency figure decreasesif the output is higherorlower.This lossofpowerappearsintheformofheat.Thus,overloading a transformer can both reduce its efficiency and increasethe heating effect. Operatingat reducedoutput hasno harmfuleffect, exceptforreducing efficiency becausethe actual power loss (and thus heating effect) is lowered. TRANSFORMERS AS POWER SUPPLIES By selecting a suitable turns ratio, a transformer can be used directly to convert an ac supply voltage into a lower (or higher) ac output voltage at efficiencieswhich may be as high as 90 percent lpig. 8-2). There are also applicationswhere a I : 1 turns ratio transformer is used,providingthe sameac output voltage astlte ac input voltage,where it is desirableto isolatetlte supply from the output circuit. All transformersdo, ofcourse, provide physicalseparation of input and output circuits, but the degreeof isolationsafety is very muc.h dependenton the actual constructionof tlte transformer. The more usualpower-supplyapplicationof a transformer is to stepdown an ac voltage into some lower dc voltage output. The transformer only provides voltage conversion.Additional componentsare neededin the output circuit to transform the convertedac voltage into a dc voltage. Two basiccircuits for doingthis are shownin Fig. 8-3. The first usesa single diodeandprovideshalf-waaeratification,passing onehalf of eachaccycleasdc 51

Power loss 10 Percent or More

ac ln

StepDownTransformer

StepUp Trandormer

Fig.&2 stct4owrr ard step-uptransformersdefined,In fractice, transfotztcrsare oftcn drawn in slmbolic fottl uith bth coilsof tlv vme length,regardless of actual turns ratio.

andsuppressing theotherhalfcycle.Thepurposeof thecapacitoris to maintain the dc voltageoutputasfar aspossibleby discharging on eachsuppressed half cycle,andfor this a largevaluecapacitoris required.Althougha very simple circuit,it hasthe inherentdisadvantage of generatingttrgh peahvolkgesand currents,especiallyif ahighcurrentisdrawnfromtheoutput.Also,thedcoutput is far from smooth.It hasa rippleat tlre acfrequency. Muchcanbedoneto smooththeoutputbyaddinganinductance or chokeanda secondcapacitor,asshownin theseconddiagrarn.Thesetwo components work as a filter (seealsochapter 6). The designof the chokehasto be specially matchedto tlte requirements, offeringlow resistanceto dc withoutbecoming saturated, whichcouldreduceits inductance. In particularcircuitstheinductance maybea swingingchoke,whenit is possibleto eliminatethereservoircapacitor

c1.

Thefirst diagramof Fig.84 showsa sinplefulr-wau rectifiercircuitaddedto tlte transformer(thesecondary of whichmustbecentertapped).For the same secondary voltageasthehalf-wave rectifier,thedcoutputvoltageis nowhalved, but the currentwhichcanbedrawnfor a givenrectifierratingis doubled.The reservoir capacitorchargesand dischargesalternately.This producesa smootherdcsupply,butrippleis still presentandin tlis caseisequalto twicethe acfrequency. Themoreusualformof full-waverecffier isthe bridgecircuit,shownin the seconddiagramofFig.8-4.Thisgivesapproximately thesameno-loadvoltageas a half-waverectifier with the advantage of full-waverectificationandbetter smootldng.

*l Vob r

2!l0 Volts ac

Fig. &3. Half-wau rectificationof ac.

55

Fig. 8.4. Full-waaenctifuation of u.

A practicalcircuit of this type is shownin Fig. &5. A singlehigh ralue electrolyticcapacitoris usedfor smoothing.Additionalsmoothingbetrreen stagesfedfrom sucha powersupplymaybeprovidedby a resistor,associated with a decoupling capacitorQikeFig.8-3).Theresistorrraluecandsobechosen to dropa specificamountof voltageif thepreviousstage(s) donotrequirethefrdl power-supply outputvoltage. TRANSFORMERSAS COUPLING DEVICES Transformersare very usefulcouplingelementsfor ac circuits.As well as providingcouplingtheycanstepupa voltageor current,andevenmoreimpormatching.By choosingthe properturnsratio,the impedtant for imPedance anceofafixedloadcanbetransfornedto anydesiredhigherorlowerimpedance, within practicallimits.This canbe a particularlyimportantrequirementwhen couplingtransistorradiostages. For impedance matching,the followingrelationshipapplies:

*-,8 where:

Altmat'w! Symbda f6 BridS! Rccdfa

@ Fig. &5. Practicalpower-svpfl1circuit. A hiSh wluc capacitoris usd. Thefur singlecom|oneil calleda bridgerectifer. 56

diodesdreboaght a 8 a

ac Output

FiS,e4. Theaubtransformer is a singlcfuil-lcrgthcoilwitha tallittg loint Znis the impedance of the tran$ormerlookinginto ttre primaryterminats z. is the impedance of the loadconnected to the secondary of tle trandormer For impedance matching,it is thereforenecesvry to designthe primaryto providethe requiredZo^d selectthe turnsratio to satisfythe equation AUTOTRANSFORMERS An autotransforzarisa one.winding coilwith anintermediate tappingpoinl The full lengthof the coil (usually)formsthe primary,andthe lengthof coil betweenthe tappingpointandoneendof tlre coilsenesastlre secondary (Fig. &6). It worksonoractlythesameprincipleasa conventional transformer,with the voltagedeveloped acrossthe outputproportionalto the turnsratio of this lengtl ofcoil to thefull lengthofcoil.Thebiggestdisadvantage ofanautotrans" formeris that it doesnot provideisolationbetweenthe primaryandsecondary windings.This canbe critical(or evendangerous) in somehigh-powerapplications.

57

9 Semiconductors Deare knownas passiu components. Resistors,capacitorsandinductances in circ'uitconditions by reactingto appliedsignals viceswhichproducechanges The majorityof activecomponents usedin are knownas actioecomponents. modernelectroniccircuitsue smiconductors,or morecorrectlyput, devices materials. basedon semiconductor materialis onewhichcanbegivena predomiVery simply,a semiconductor nanceof mobilenegativechargesor electrons,or positivechargesor holes. of bothelectronsand Currentcanflowthroughthematerialfromthemovement holes.This is quitedifferentfrom the behaviorof a normalconductor,where clrrrentflow is the resultof electronsthroughthe material(seeChapter1). propertiescanbe givento a strictly limitednumberof mateSemiconductor rials by dopingwith minutetracesof impurities.The two mainsemiconductor or "semimetals").Doping materialsaregermaniumand silicon(bothnon-metals of .fusitivecharges(holes) canproducea materialwith eithera predominance of l\>ive charges resultingn a P-tyfe material;or with a predominance (electrons),knownasm N-$pematerial. This doesnot becomeparticularlysignificantuntil a singlecrystal(of gerrnaniumor silicon)is treatedwith botha P-typedopeandanN-typedope.In this regionsareformed-a P-regionandanN-region.Sincetlese case,twoseparate regionshaveoppositechargesthereis a tendencyfor electronsto miratefrom theN-zoneto theP-zone,andholesto migratefromtheP-zoneintotheN-zone. of chargesin the regionof thejunctionof the P- and Theeffectis a cancellation N-zones,formingwhatis calleda dePtetionlayr(Fig.9-1). This layer,which 58

P-Dope

ffi.w

in theconstntction ofa xmiconductor dide, slnut in simplc lis. s't. Fourstages diagrammatic form.

containsnofree electronsorholes, tlenactsasabarrierbetweenthep-zoneand t.heN-zone,preventinganyfurther migrationof either erectronsor holes.In effect,the barrieror depletionlayersetsup a potentialdifferenceberweenthe two regionsandthe deviceremainsin a stablestateuntil anexternalvoltageis appliedto it. Figure9-2 showswhathappenswhenan qrternalvortageis appriedto tle device.In thefirst diagram,the * voltageis connected to the p-zone.provided this voltageis sufficientlyhighto overcomethepotentialdifferencesetupin the construction of thedevice(whichmaybeonlya fewtentls of a volt (it willrepel holesin theP-zonetowardstheN-zone,andattractelectronsin theN-zoneinto theP-zone.Effectively,thebarrieror depletionlayerwill disappear andcurrent will flowthroughthedevice.Voltageappliedthiswayis knownas/orward bias. If theexternalvoltageis appliedtheotherway,asin tre seconddiagram,t.he oppositeeffectiscreated;i.e.,thetlicknessofthedepletionlayerincreases, tlrus buildingupa higherpotentialin thedevice,opposingtheexternal voltage.The backvoltagedeveloped is equalto thatof theappliedvoltage,sonocurrentflows throughthe device.Voltageappliedthis wayis knownas negatioebias, Thedevicejust described is a semiconductor diode.lttnsthebasiccharacteristic of actingas a conductorwhenconnected to an externalvoltageonevxay Gorwardbias),andasaninsulatorwhenconnected theotherway(reversebias). Diodecharacteristics are describedin somedetaillater on, but tle same principlescanbe appliedto e:rplainthe worki4gof a transistan

Fonrard Bias

ReverseBias

Fig.92 Thctw mdes in whicha diodecan b operated. 59

TRANSISTORS Basically, atransistoris twodiodesplacedback-to-back with acommon middle layer,tlre middlelayerin thiscasebeingmuchthinnerthanthe otjrertwo.Two confgurationsare obviouslypossible,PNPor NPN (Fig. g-3).Thesedescrip tionsareusedto describethetwobasictypesof transistors. Because atransistor junctions,it is referredto as a bipolar containstwo seperatesemiconductor device,or bifolar transistor, A transistorhasthree elements,andto operatein a workingcircuit it is connectedwith two externalvoltagesor polarities.one externalvoltageis workingeffectivelyasa diode.A transistorwill, in fact,workasa diodeby using justthisconnection andforgettingaboutthetophalf.Anexample is ttresubstitution of a transistorfor a diodeasthe detectorin a simpleradio.It worksiust as well asa diodebecause it is workingasa diodein this case. The diodecircuitcanbegivenforwardor reversebias.connectedwith forward bias,as in the first diagramof Fig. 9-4, drawnfor the PNP transistor, currentflowsfromP to thebottomN. If asecond voltageis appliedto thetopand bottomsectionsof the transistor,with the samepolaityappliedto thebottom, the electronsalreadyflowingthroughthe bottomN sectionpromotea flow of currentthroughthe transistorbottom-to-top. By controllingthe degreeof dopingin the differentlayersof the transistor duringmanufacture, this abilityto conductcurrentthroughthe secondcircuit throughtle resistorcanbe very marked.Effectively,whenthe bottomhalfis forwardbiased,the bottomsectionactsasa generoussourceof freeelectrons (andbecauseit emitselectronsit is calledthe emitter).Theseare collected readilyby the tophalf,whichis consequently calledthe collector,butthe actual amountofcurrentwhichflows throughthisparticularcircuitiscontrolledbytle biasappliedat tle centerlayer,whichis calledthebase. Effectively,therefore,trere aretwoseparate workingcircuitswhenatransistor is workingwith correctlyconnected polarities(Fig. 9-S).Oneis the loop formedby the biasvoltagesupplyencompassing the emitterandbase.This is calledthe 04s,circuitor inlut arcwt Thesecondis the circuitformedby the

PNPTransistor

NPNTransistor

FA.A?.ConstnrctionofPNPand NPNtransistors,shownin simplediagrammatic form,

ffi

shoundiagrammaticalll taa PNPtransistor Fis,94. Biasandsvpfll connections (neht). (lcfi)andin symbol form ofthetransistor.Thisis calledthe supplyandallthreeelements collectorvoltage (Note: appliesonlywhenthe this description outputglrcJjlt collectoretrqitor as commonemittcr circuits-known to both is common emitter connection transistors,but of connecting used way widely most This is tlte configuration. andcommon base configurations-common alternative there are two other in eachcase.) the transistor principles of in the working apply collectonT\esame srnallbase relatively is that a circuit particular by this offered advantage The (or,more current collector much larger very instigate a and cgrrentcancontrol producing larger output much power a of is capable input correcdy,a small power).In otherwords,tle transistorworlrsasanamplifier. circuitis the inputside,andthe Withttrismodeof working,the base-emitter Althoughthesehavea outputside. to collectorcircuitisthe emitterthroughbase are effectivelysepatwo circuits path the and emitter, throughbase common tle base polarity is concerned, circuit the base of far as ratedby thefacttlat, as Hence biased diode. reve6e as a are connected andupperhalfof the transistor circuit. collector into the basic circuit thereis no currentflowfrom the For the circuitto work, of course,polaritiesof both the baseandcollector circuitshaveto be correct(forwardbiasappliedto the basecircuit, andthe collectorsupplyconnectedso tbat the polarityof the commonelement(the

O{put Cirsuit

FtS.g44,. Trtctw cparatc citeuitsinoolutt in olerating a transisbr. Ditz.ctianol curnnt fuw is for a PNP tlarrsis/lor..

61

emitter) is the samefrom both voltagesources).This alsomeansthat the polarity of the voltagesmust be correct for the type of transistor. In the caseof a PNP transistor, asdescribed,the emitter voltagemustbe positive.It followsthat both tlte baseandcollectorare negativelyconnectedwithrespecttothe emitter. The symbolfor PNP transistor hasan uurowon the emitter indicatingthe direction of.current flow;i.e.,alwaystowardsthe base.(P for pcsitive,witl a PNP transistor.) In the caseof anNPN transistor,exactlythe sameworking principlesapplybut the polaritiesof both suppliesarereversed,Fig. 9-6. The emitter is alwap made negative relative to baseand collector. (N for negative in the caseof an |rIPN transistor). This is also inferred by the reverse direction of the arrow on the emitter in tlre symbolfor an NPN transistor; i.e., cgrrent flow away from the base. PRACTICAL DIODES The typical appearanceof a semiconductordiode is shown in Fig. 9-7. The cathodeend is usuallymarked by a red dot or color band,or a * sign, and also usuallywith a type numberconsistingof oneormore lettersfollowedbyfigures. This identifiesthe diodeby manufacturerandspecificmodel.Specifict1ryenumbers are usuallyquotedfor specificcircuit designs,but many circuits are fairly noncritical as regardsthe type of diodeused. Diodesmay also be describedin more generalterrrs by the crystal material (germaniumor silicon), and by construction.Here, choicecan be more important. C"tt oiurn diodesstart conductingat lower voltagest}an silicon diodes (about 0.2 to 0.3 volts, as comparedwith 0.6 volts), but tend to have higber leakagecurrents when reverse biased,this leakagecurrent increasingfairly substantialwith increasingtemperature. Thus, the germaniumdiode is inherently less efficient as a rectifier than a silicon diode, especiallyif reverse bias ctrrent is high enoughto produceappreciableheatingeffect. On the other hand, a gerrnaniumdiode is preferred to a silicon diode where very low operating voltagesare involvedbecauseit starts to conductat a lower forward voltage. The constnrction of a diode governs both its curent{arrying capabilities

diqramnaticalll toan NPNtransistor,sltoum Fig. 94. Biasand wpp$ connections (lefi) and in syttrlu,lic form (right).

e

.r'.;.-i

Dodes AiF

@

m F

Volts ReverseBias Volts Forward Bias

-.....---

Forward Voltage Needed to Overcome

Fig.97. AlerationalcharactzrMics ofa ffiical vmiconductor diode. whenconducting, andits capacitance effect.Thelargertheiunction areaof.a diode,tlte highertlrecurrentit canpasswithoutoverheating-for example, tltis characteristic is desirable in high-power rectifiers.Ontlte otherhand,increasing thejunctionareaincreases the readiness with whicha diodewill passacdueto inherentcapacitance effects.To reducethiseffectto a minimum,a diodecanbe madefromasingledopedcrystal(usuallyN-type),onwhichthepointof apieceof springwire rests.The endof this wire is givenoppositedoping(P-type).This reducesthe junctionareato a minimum,suchas a diodebeingknownas a point+ontacftype.It is afavoredtlpe for usein circuitscarryinghighfrequency ac signals,andfor this reasonis sometimes calleda signal diode. Thetypicalcharacteristics of a diodearealsoshownin simplegraphicformin Fig.9-7.Biasis represented by thevoltageappliedto thepositiveside,referred to asanodewltage.Currentflowingthroughthe diodeis referredto asanode current. With forwardbias(positivevoltageappliedto the anodeendof the diode), ttrereis at first no anodecurrentuntil the inherentbarriervoltagehasbeen overcome(0.3 volts for a germaniumdiode,0.6 volts for a silicondioderegardlessof the constructionof either tne). Any further increasein anode voltageproduces asteepriseof anodecurrent.In practice,it isnecessary to limit this currentwitl a resistoror equivalentresistiveloadin thecircuitto prevent the diodebeingoverheated andthejunctiondestroyed. Withreversebias(negativevoltageappliedto theanodeendof thediode),tlre only currentflowingwill be a very smallleakagecurrentof the order of microampsonly, and normallyquite negligible.This leakagecurrent doesnot increase appreciably with risein (negative) anodevoltage,onceit hasreached its saturationvalue. It will beappreciated thatadiodewillworkin bothadcandanaccircuit.Inadc circuit,it will conductcurrentif connected with forwardbias.If connected the opposite way,it will actasastopforcurrentflow.Anexample ofthistypeofuseis wherea diodeis includedin a dc circuit-say the outputsideof a dc power

supply-to eliminateanypossibilityofreversepolarityvoltage surgesoccurring which could damagetransistors in the samecircuit (seeFig. 2&1). In an accircuit a diodewill "chop" the appliedac,passinghalf cycleswhich are positive with respect to the * end of the diode, and stoppingthose half cycles which are negativewith respect to * end of the diode.This is rectifieractton, widely usedin transforming an ac supply into a dc output. The sameaction is requiredof a detector in a radio circuit. Here the current appliedto the diodeis a mixture of dc andac.The diodedetector transformsthis mixedinput signalinto a varying dc output,the variationsfollowingthe form of ttre ac contentof the signal. BASIC TRANSISTOR CIRCUITS The transistor in common-emitter configuration works as an amplifier, as previouslyexplained.It needstwo separatesupplyvoltages-one for bias and the ot}rer for the collector-but tlese do not necessarilyhave to come from separatebatteries.Theycanbeprovidedbyasinglesupply(battery)takentothe commonconnection(the emitter) and the collector, and tapping t}te collector sideto applythe necessaryforward biasvoltageto the base,droppedthrough a biasresistor. A basicamplifiercircuit then lookslike Fig. 9-8. To makethe circuit do useful work, tie collector current hasto be fed through an output load, suchas a load resistor. These two diagramsalso show clearly input and output as separate entities, andcan clarify the point about amplification.The power derived in the output is far greater than that put into the input. This very simplemethodof supplyingboth collector andbiasvoltagesfrom a single source is known as current biasing. It needsonly one resistor, and it works.The resistor valueis chosento give a base-emittervoltageof the order of 0.1 to 0.2 volts for germaniumtransistors; and about 0.6 to 0.7 for silicon transistors.It is not asstableasit shouldbe for manycircuits, however,particularly if a germaniumtransistor is used,so voltage bias is often preferred (Fig.

e-9).

Fig. 98. Simfle current biascircuitsfor transistaroforation, fl

FA. 9-9. Voltage biascircuitsfor transistor oferation. Withvoltagebias,two resistors(Rl andR2)areusedto workasa divider.A resistor(R3)is alsoaddedin tlre emitterlineto provideemitterfeedback auto matically,to controlthe biasvoltageundervaryingworkingconditions.This latterresistorisalsousuallyparalleledwith acapacitortoprovidefurtherstabili(but zation this maybeomittedwitl silicontransistors). Determination of suitablecomponent valuesis nowmorecomplicated since tlree resistorsareinvolved.Theactualbasevoltagecanbecalculatedfromthe followingformula: basevoltage:

R2 X supplyvoltage (R1+ RA

The emittervoltageis equalto this lessthevoltagebetweenbaseandemitter (acrossthe transistor).ln mostcases,a voltagedropof I volt in the caseof germanium transistorsand3 voltswith silicontransistorsis thedesignaim.The emitter resistor(R3) alsoneedsto be quite largeso that there are minimal changesin emitter currentwith anyvariationin the supplyvoltage.This can causea little re-thinkingaboutsuitablevaluesfor R1 andR2, for tlre voltage developed acrossR3 mustbevery muchgreaterthanthe voltagedeveloped by tlte basecurrentacrosstlte sourceresistance formedbytheparallelcombination of Rl andR2. TRANSISTORCONSTRUCTION Theoriginaltransistorsweremadefromgermanium crystalswith point
Epitaxialsiliconplanartransistorshave itself(asin the normalplanarprocess). notablein rf andi-f for highfrequencyapplications, superiorcharacteristics circuitsfor superhetradios. Gemanium and Silicon Transistorg or siliconcrysJustlike diodes,transistorsaremadefromeithergermanium tals.Germanium transistorshavelowvoltagelossesbuttheircharacteristics are moreliableto varywith temperature, sot}rattlrespreadofcharacteristics under which they work in a circuit canbe quite wide. They are alsolimited to a maximumworkingtemperatureof about100'C. Silicontransistorsaregenerallymorestableandcanoperateat temperatures lossesandhighervoltageratings, upto I 50"Cor more.Theyhavelowerleakage andaregenerallyfar bettersuitedfor usein highfrequencycircuits. The Shapeof Transistors Transistorscomein dl sortsof shapesandsizes.However,the onlyproblem thethree wherea specified typeof transistoristo beusediscorrectlyidentifying leads.Thepositionof thesecanbeidentifiedbyreferenceto Fig.9-10.Themost leadconfiguration common isin line,with acircularcase,whentlreleadsfollowin logicalorder-collector, base,emitter, with the collectorlead beingmore widelyspacedfromthe middle(base)leadthanthe emitterlead,lookingat the

$

Collector

#T,h

TO18

2

l

-

l

l r 2 , 3 .

.330Max

J ---|

.440IUax Collecto to Case

.292itu

a

To4r

lHl

.SzsMax

TO92

G]l=fl7* \91 " -l#-

| I

.'ooJ '+1*hf B -"/L \ )c Caseto Collector

h

TO98

%## 3 A

Collectorto Case

Fig.910. Somecommontransistoroutlines(iliagramsbyElefirooalw, all dincnsionsin ircheg.

ffi

bottomof thetransistorfromwheretheleadsemerge.Thisdoesnotapplywhm the caseis partly circularwith oneflat side.Herethe threeleadsare equally spacedandwith tle flat sideto the left (andlookingat the bottom),the lead arrangement maybe bce,cbe,or ebc. Powertransistorsaremorereadilyidentifiedby their elongated bottomwith two mountingholes.In t}riscasetlere are only two leads-the emitter and base-and thesearenormallymarked.Thecollectoris connected internallyto thecan,andsoconnection to thecollectoris viaoneof themountingboltsor the bottomof the can. FIELD-EFFECT TRANSISTORS T\e Field-EfectTransistor(orFET)is reallya differenttypeof semiconductor devicetlan abipolartransistor,with characteristics morelikeavacuumtube thana bipolartransistor.Its correctdefinitionis a unipolarkansistor.Theway in whichit workscanbeunderstood bypresenting it in electronicpictureformas in Fig.9-11,whereit canbeseenthatit consists of a channelof.erfherP-typeor N-typesemiconductor materialwith a collaror gateof.opryite tlpe materialat junctionat this point.Oneendof the tlre center.This forrrs a semiconductor channelis calledthe source,andthe otherendthe drain. An FET is connected in a similarmannerto a bipolartransistor,with a bias voltageappliedbetweengateandsource,anda supplyvoltageappliedacrossthe centerof the channel(i.e.,betweensourceanddrain).The sourceis thusthe commonconnection bet'n'een the two circuits.Compared with a bipolartransistor, however,the biasvoltageis reversed.That is, the N-gatematerialof a PthannelFET is biasedwith positivevoltage,andthe P-gatematerialof an N-channelFET is biasedwiti negativevoltage(Fig. 9-12).This putsthe two systemvoltagesin opposition at thesource,whichis responsible for thecharacteristicallyhigh input resistanceof.FETs. The effectof this reversebiasis to form an enlargeddepletionlayerin the middleof the cbannel,producinga pinchingeffecton the flow of electrons tbrougbthechannelandconsequently onthecurrentflowin thesource-todrain

"."Gfl Cbannel Synbol

N+hannel FET

Symbolfor InsulatedGate FE"T P-channelFET

Fig.911. Consttution offield
P4hannel FET

N{hannel FET

Fig.912. fusic biasrequirenents for fieldcfecttransistars circuit.If enoughbiasvoltageis applied,the depletionlayerfills the wholegate ("shutsthe gate"),causingpinch of,when the source.todraincurrentfallsto znro(n practicenearlyto zero,for there is still someleakage).With no bias appliedto the gate,the gateis wideopenandsomaximumcurrentffows. ln effect,then,tlte amountof reversebiasappliedto the gategovernshow muchof thegateis effectivelyopenfor currentflow.A relativelysmallshanggin gatevoltagecanproducea largechangein source.to-drain current,andso tle deviceworksasanamplifier.In thisrespect,a P-channel FET worksverymuch like a PNPtransistor,andan N+hannelFET as an NPN transistor.Its main advantage is tlat it canbemadejustascompactinsize,butcancarrymuchmore power.ln thisrespect-and thefactthat it hasa highinputresistance, whereas a bipolartransistorhasa lowinputresistance-it is morelike a tubein characteristicsthana bipolartransistor.[t alsohasother advantages over a bipolar transistor,notablymuchlowerinherentnoise,makingit amorefavorablechoice for anamplifierin a highqualityradiocurrent. The type of field-effecttransistordescribedis correctlycalleda junction field-effect transistor,ofIFET. Thereareothertypesproduced bymodifuingthe construction. Theinsulated-gate field-effecttransistor,or TGFETis self-orplan(because atory.TheIGFEThasevenhigherinputresistance thegateisinsulated fromthechannel), andis alsomoreflexiblein application sinceeitherreverseor forwardpolaritycan beappliedto thegatefor bias.FETs,of eithertlrye,canalso bemadewith two gates.ln this casethe frst gatebecomesthesignalgab(to whichtlre inputsignalis applied)andthesecondgatebecomes the controlgatc, with similarworkingto a pentodetube(seeChapter12). FETsarealsoclassified by themodein whichtheywork.AJFETworksin the depletionmode;i.e., controlof the extentof the depletionlayer,andthusthe "gateopening"beingby the application of a biasvoltageto thegate.An IGFET canworkin thismode,or with oppositebiaspolarity,in whichcasetheeffectisto producean increasing"gate opening,"dth enhanced (increased) source.todraincurrent.This is calledthe enhancement mode. AnFETdesigned specifically to workin theenhancement modehasnochannel to start witl, onlya gate.Applicationof a gatevoltagecausesa channelto be formed. The basiccircuitof anFET amplifieris very simple,Fig. 9-13(with polarity drawnfor a P-channel FET). lnsteadof applyinga definitenegativebiasto the gate,a highvalueresistor(R1)is usedto maintaintle gateat substantially zero voltage.The valueof resistorR2 is thenselectedto adiustthe potentialof the sourceto therequiredamorntPositiaetothe gate.Theeffectis thentle sameas I

Fig. 913. fusicFETamplifiercircuit.Perfonrance isgenerally sufieriorb tlat ofa bipolartransisbramplifur. if negativebiaswereapplieddirectto tbe gate.This arrangement is alsoselfcompensating with variationsin source'todraincurrent.Thethird resistor,R3, is a loadresistorfor the FET to setthe designoperatingcurrent.CapacitorCl pathto removesignalcurrentsfrom the souroe. actsasa conductive junction-type (JFET)andinsulatedgateOGFET)field-effect Both transistors arewidelyused,thelatterhavingthewiderapplication, particularlyin integrated circuits.Themetaloxidesemiconductor FET, generallyreferredto asa MO$ FET, canbedesignatto workin eithermode;i.e.,asa depletionMOSFET,or enhancement MOSFET.TheformerisusuallyanN+hanneldeviceandthelatter a P-channel device.P-channel MOSFETS workingin tlte enhancement modeare by far the morepopular,mainlybecause they are easyto produce.In fact, an N
69

10 NeonLamps,LEDs, and Liquid Crystals NEON LAJI{PS Many circuits useneonlamps,LEDs, or liquid crystal displays.The neonlampis a glow lamp consistingof a glassenvelopefitted with two separatedelectrodes and filled with an inert gas (neonor argon). If connectedto a low voltage, the resistanceis so high that the neonprovidesvirtually an opencircuit, but, if the voltage is increased,there comesa point where ttre gas ionizesand becomes highly conductive,as well as giving off a glowing light locatedon tlre negative electrode.If the gasis neon,the glow is orangein color.Argon is sometimesused as the gas,in which casetlre glow is blue. The characteristicperformanceof a neon lamp is shown in Fig. 10-1. The voltage at which the neon starts to glow is calledthe initial breakdownvoltage. Once this has been reachedand the bulb triggered into firing (glowing), the voltage drop acrossthe lamp will remain virtually constant regardlessof any increasein current in the circuit. At the sametime, tlte area of glow increases with increasingcurrent, up to the point where the entire surfaceof the negative electrodeis coveredby glow. Any furttrer increasein current then pushesthe neoninto an arc condition,where the glow changesto a blue-whitepoint of light on the negativeelectrodeand results in the rapid destructionof the lamp. To operatea neonlampsuccessfully,tlerefore, it is necessaryto haveenough voltage for the neon to fire, and, after tlat, enoughresistancein the circuit to limitthe currenttothat whichwillensurethat thelampremainsoperatinginthe normal glow region. Becausethe resistanceof the neonitself is very low after 70

GlassBulb

I

I

€

Constant Voltage

ffir$

-_--_-.> Current Fig.10-1.Trpicalneonlampconstruction performance andclwrocbristic firing, this requirestlre useof a resistorin serieswith the lamp,knownas a ballastresistor.Typicallythe firing, or breakdown, voltagemaybe anything fromabout60to 100volts(in somecasesevenhigher).Thecontinuous current ratingis quitelow, usuallybetween0.1 and10 millianrps.The seriesresistor valueischosen accordingly, relatedto thevoltageof thesupplytowhichtheneon will beconnected. In thecaseof neonlampsto beoperatedoffa 250volt (mains) supply,a 2201<{2resistoris normallyadeqrute(seeFig. 10-2).With some commercial lamps,theresistormayactuallybebuiltintotle bodyof theassemblv. Lackinganyspecificinformationonthissubject,it canbeassumed t}at aneon lamphasnoresistance whenglowing,but drops80 volts.A suitablevaluefor a ballastresistorcanbecalculated onthisbasisrelatedto theactualvoltageofthe supplyto be used,andassuming a safecurrentof 0.2 milliamps,for example. For a 250-voltsupply,tlre resistorhasto drop 250- 80 - 170volts.The currentthroughresistorandneon(in series)is to be 0.2 mA. Therefore: volts KesFumce:amp.s L70

0.2x u1000 :850 kQ, or aboutI MQ Thisshouldbeplayingsafewith mostcommercial lamps.If theglowis notvery bright,thevalueof theballastresistorcanbedecreased to operatelampfarther

Fig. 1A2. In a practical circuit, a neonlamp is alwaysconnectcd in sries with a fullost resistorto limit currentfow.

Mitri.tw Non

ctrcutt' /4t2Wttel Attjustabteratc flashercircuit. FiS.nA. Adiustable be de' alongthe normalglow region.However,the resistanceshouldnever glow; this by is covered electrode negative the of o*irA * .uch ttrit ttrewtrole condiarc the approaching and overloaded is becoming indicatesthat the lamp tion. : Gotner pointaboutthe strengthof the glowlight is that it normallyappears glowmaybe O4iniri'yghtthan in dark.In fact, in completedarknessthe lampshavea it. Some start to voltage breakdown higher erritic and/oirequirea ionization gas stimulate to inert the gas to added minutetraceof iadioactiie unnoticeable. nrakingttrisparticulareffect of a neonlampundernormal characteristics of the constant-voltage Because device.Thus,in thecircuit stabilizing voltage a as grc* *nation , it canbeused of the lampis a sourceof side each from tapd iu"*" ir, Fig. ro2, the output normalglowregion' inthe lampremainsworking aslongasthe constantvoitage of the lamp' voltage breakdown nominal This voltageir O" ot" asthe circuithasalready oscillator relaxation a Theuseof a neonlarnpasafosherin in Fig' 10-3' is shown this on A variation (Fig. 6). beendescribed Gi, Chapter or four 45-volt two and resistor ballast the as usinga t megohmpoteotiomJter is adjusted potentiometer The supply. of iZ{nvonOty U.ttitio asthe source just *ttif tU" f.-ir lights.Thecontrolis thenturnedtle otherwayuntilthelamp go." o"t. fri"ittg thepotmtiometerin thisposition,thelampshouldthenflashat -iegular "An interrralsdeterminedby the valueof the capacitor' adaptionof this circuitis shownin Fig. 1G'1,wherethecircuitis switched acrossthe pointshownto listeninto bfa MorL key.Phonescanbeconnected a flashinglight. An ordinarybulb as visible ift"-tttot " .ig*lr, whichare also (andwith a muchlowervoltage indicator wouldwork iust as well as a visible clicks'TViththeneon signalswouldonlybeheardas r.quit"d),Uotinthiscasethe

FiS.101 Mors codcfoshercircuit'

n

Fig. 10-5.Simpletonegeneratorbasd on an NE-2miniature neonlamp.

circuit, tlte actual oscillation of the relaxation oscillator is heard. The time constantof the circuitis governedbythe rralueof the capacitorandthe settingof the ballastpotentiometer. A further extensionof the use of a neonlamp as an oscillator in a relaxation oscillatorcircuit is shownin Fig. 10-5. This is a true signalgeneratorcircuit, the output of which shouldbe audiblein headphonesor even a small loudspeaker, witl the tone adjustableby the potentiometer. Neonflasherscanbe madetoworkinrandomfashion(agarnseeChapter6), or sequentially.A circuit for a sequentialflasheris shownin Fig. 10-6. More stages can be addedto this circuit, if desired,taking the connectionof C3 to the last stage. Finally, an astable multioibrator circuit is shown in Fig. 1G7, using two lamps.Thesewill flashon andoff in sequenceat a rate determinedby Rl andR2 (which shouldbe equalin value)and Cl. As a generalguideto flashertiming, increasingthe valueof the ballastresistor or the capacitorin the relaxationoscillatorcircuit slowsthe rate of flashing;and vice versa. To preservetlre life of a typical lamp, however,the value of ballast resistor usedshouldnot be less than about 100 kQ; and best results in simple relaxation oscillator circuits can usually be achievedby keeping the capacitor value below 1 microfarad.

I

I ur*rn I

Cl, C2andC3 tuc All 0.5!F

Fig. 104. &quentialfoshcr rsing NE-Zminiature neonlamPs(or equiaalent). 73

1/F Crp.€l'ttr

Fig.1&7.Astablcmultiaibrator circuil erchwonfuhing in tun. LEDs LED is short for Light-EmittingDiode.This is essentiallya two element semiconductor devicewherethe energt producedby conductionin a specific directionisradiatedaslight.Theintensityof thelightis governed by thecurrent flowingthroughthediode.In theserespects,LEDsaresomewhat similarto neon lamps,but they light at very muchlower forwardvoltages(typically1.6 to 2 volts)andcangenerallydrawhigherforwardcurents without burningout (typically20mA).Originallythecolorof lightemittedbyIJDs wasred,butnow orange,yellowandgreenLEDsarealsoavailable. Againlikeneonlamps,anLEDisinrariablyassociatedwith aballastresistorin seriesto limit thevoltageappliedto theLEDandthecurrentflowingthrotUhit. The valueof resistorrequiredis:

p- v';vr l1

where: Vr:dc supplyvoltage Vr: ratedforwardvoltageof the LED 11: ratedforwardcurent of the IJD at specifiedforwardvoltage Thus,for operatingoBsay,a &voltsupply,atypicalvaluefortheballastresistor wouldbe(6 - 2)lQ0 x 10-1 : 200 ohms In the caseof anac supply,a diodeis connected in inverseparallelwith the LED andtheresistorraluerequiredis onebalfthatgivenby theaboveformula, seealsoFig. 10-8. LEDsareoftenusedin groups,suchasin calculators, or digitalinstnrments. ?he mostcommon point,seeFig. formis a seven-segment displayandassociated 1S9. Suctra displaycan light up truneralsfrom 0 to 9, dependingon the individualsegmetrts energized, with or withoutthe decimalpointligbt€d"Dach segment(or poin0is, d courre,anindividrulLED. Specificadvantages of LEDsaretbat theyrequireonlylowvoltages,arefast switchingandcanbeproduced in very snnll si2g6,if required.Themostwidely usedseven-segment displays,for example,give figureswbichare 0.3 io or 74

* Supply Voltage

Forward Volts

-Or Common EarthLine

Vr

in seiestndrofthesufufllwltqe in witha ballastresistnr Fig.10-8.LEDsareconnectcd thenqaindforlusnlwltrye.Notethcqnbolfor anLED (lfuN-ntittinsdiode). is relativelylow,but an8digtt seven-segment 0.5 in. high.Powerconsumption in excessof 2 watts (e.9., displaycouldhavea maximumpowerconsumption SXZX20mAat2volts). to displayspoweredbyminia' Thiscanplacerestrictionsontheir applications batterylife, tlte ture batteries,as in digitalwatches.To providea reasonable displayis normallyleft in opencircuitandonlyswitchedonfor the shodperiod whenit is requiredto readthe display. LIQTND CRYSTAL this particularpowerlimitationsinceit canbe T\e liquid crysialovercomes actiratedbyverymuchlowerpower(actuallyatiny xnotntof,heat,whichcanbe produced by anequallytiny amountof electricalenergy).Also,thedisplaycanbe powerlevels,sotlat it can mademuchlargerwhilestill workingat mictoscopic however.It is far beleft onallthe time.Theliquidcrystalhasits disadvantages, lessbrightthananLEDdisplay,andalsosuffersfromdarkeffectflikeaneonlamp in thisrespect).Thusto belegiblein dimlight,theliquidcrystaldisplayneedsto be illuminatedby a separatelight course. operatewithlowvoltageandlowcrulent.Curent drain Liquidcrystaldisplays tln canbe as little as 1 microamp(1419 per segpent.A later development, field+ffectliquidcrystal,canworkonevenlowervoltagesdrawingmicroscopic currents(of the orderof 300 nA), againmakingtheman attractivechoicefor Thefield-effectliquidcrystalalsohasbettercontrast, displays. battery-powered gving a blackimageon a light background. OneSideof Each LED Segment has a Common Connection (Maybe* Sideor - Side).Not AI Pins May beConnected

LED Seven-Segrnent

Ihcimal PointLED

Fig.I0-9. TyfubalLED rlisflay, as usedin calculators.ThceightLEDsarc intcntally unnuted h a amnon catWe or commonanodefin,

11 Other Components Othertypesof components likelyto bemetin electroniccircuitsaredescribed in this chapterfor easyreference.Manyare variationson standardcomponents previouslydescribed, butwith differentworkingcharacteristics. Thediodefamily, for example,is particularlynumerous. THE DIODE FATT{ILY Diodesareusedin awidevarietyofapplications. Seechapterg for information on generalpurposediodes. ?.enetDiodes The Zenerdiodeis a siliconjunctiondiode.Whenreversebiasvoltageis appliedandincreased, tlere comesa pointwherethe diodesuddenlyactsasa conductorratler thananinsulator.Thepointat whichthis occursis calledthe breahdown ooltage(orZenerpoint).oncereached,it remainsconstant,evenif tlte reversebiasvoltageis increased. In otherwords,oncereversebiasedto, or beyond,the breakdownvoltage,the voltagedrop acrossthe dioderemains constantat its breakdown voltagevalue,regardless of tle actualcurrentflowing throughthe diode. This importantcharacteristicmakeszener diodesparticularlyusefulas a sourceofconstartdcvoltage,or for stabilizing asupplyvoltage,usingthet1ryeof connection shownin Fig. 11-1.A seriesresistor(R) is necessary to limit the 76

VohageTappedAcrG Zeo6 D@ EqudsAner &eeldom Vdtry

FA.n-1. A Zenerdiode,worhingwithreoer&bias,breahs dowtat a Wifrc ?lu?a ooltage, Connated asshown, it canb usd asa autxeolconstant ooltqcsuf!L1.Nob thcqabolfor a Zencrdidc. amountof curent flowingthroughthediode;othenrise,it couldbeburnedout. Regardless of theralueof theinputvolts,thevoltagedroppedacrosstheZen€r dioderemainsconstant,soanyvariationsin tle inputvoltagedonot affectthe orhut voltagetappedfrom acrosstheTr,nerdiode.This voltageis the breakdownvoltageof thediode,whichmayrangefromabout2.7voltsupto 100volts or more,depending ontle construction of tle Zenerdiode.Iftheinputvoltage fallsbelowthebreakdown voltage,of @urse,theZenerdiodewillstopcondrrcting andbreakthe circuit. Performance of a Zenerdiodeasavoltage-stabilizingdevice is limitedontyby the powerrating,whichmaybequitelow-under 500 mWfor thesnrallZrner diodes,butupto 5 wattsormoreinlargersizes.ItsstabilityisalsoaffectedWthe beatingeffectof the actualcurrentflowingtlrough if causinga shift in the breakdown voltage,so the nominallyconstantvoltage€n wry with working temperature.If thisislikelytobetroublesome(thet}'peof Zenerdiodeusedbasa fairly higbtemperaturecoefrcientof resistance), thenconnecting two similar diodesinseriescangreatlyimprove thetenperaturecoefrcient.Also,thepower ratingis inaeased" Varicap Diodes Anotherspecialtypeof diodeis the aaricapor oaractonThesebehaveas (seeChapter6)whenbiasedinthe capacitorswithahighQ reversedirection,the actualcapacitance valuebeingdependent on the biasvoltageapplied.Typical applications are the automaticcontrolof tunedcircuits,"electronictuni4g," adirctingcapacityin the circuit, andthusresonantfrequency,in responseto in signalvoltage;automatic cnanges frequencycontrolof localoscillatorcircuits in superhetsandTV circuits;andalsoasfrequencydoublersandmultipliers Symbolsforararicapareshownin Fig. U-2.

7l

F;9. 11-2.Alternatiu symblsfor a oarbap. Tunnel Diodes The tunnel diodeis another type with specialcharacteristics,unlike that of any other semiconductordevice.It is constructedlike an ordinary diodebut the crystd is more heavily doped,resulting in an extremely thin barrier (potential hyer). As a consequence,electrons can tunnel through this barrier. This makesthe tunnel diodea goodconductorwith bothfonn'ardandreverse voltage.Behavior,however,is quite extraordinarywhen the forward voltage is increased,seeFig. l1-3. Forward current at first rises with increasingfonnrd voltage until it reachesa peak value. With increasingforward voltage, current then drops, to reach a minimum, or oalley value. After that it rises againwith further increasein forward voltage.T9orkedin the region from peakvoltage to valley voltage, the tunnel diode exhibits negatioe resistancecharacteristics. fuiother interesting feature is that anyforward cu:ent valuebetweenpeakand valley ralue is obtainablethree times (at three different forward voltages). Tunneldiodeshavea particularapplicationforvery high speedswitching,witl a particular applicationto pulseand digital circuitry, e.g., digital computers. Schottlry Diode T\e &hotthy diode is a metal semiconductordiode, formed by integrated circuit techniquesandgenerallyincorporatedin ICs asa clampbetweenbaseand emitter of a transistor to prevent saturation.Voltage drop acrosssucha diodeis

QQ. Q" Symbolr

E

(J


Valley Fo,nradVottr-|l

Fig. 113. $'rrrbls (W) and claracEristicfurfonnarcc of d tanul di&. fuun peah b wllq it e*hibib nqatiu nsistane.

n

Symbolfor SchottkyTransistor

Fig. 114. Tyfical cittuit for a *hotthy diodc(lefr)and tlv equioalentsingb confu nent,a *hottky trunsistor. less t}an that of a conventionalsemiconductordiode for the same forward current. Otherwise,its characteristicsare similar to that of a germaniumdiode. A gpical circuit employinga Schottlry diode is shownin Fig. 11-4. For circuits using a Schottky diode as a clamping device associatedwith a transistor, diodeandtransistor may be producedat tlte sametime in processing the transistor. This combinationdeviceis calleda &lnftfu transistar(w als

Fie.11-4). Photodiodes of semiconductor diodesthat if theyarereverse It is a generalcharacteristic is illuminated, reverse and thejunction the currentflowvariesin proporbiased plntodrodewhich This is nthe hasa to the amount of light. effect utilized tion light fall across the window tlrough which can on one side of the crystal and clear Pand N-zones. iunctionof the ttreamountof In effect,sucha diodeworksin a circuitasavariableresistance, ontlreamountof lightfallingon resistance offeredby thediodebeingdependent will havenormalreverseworkingcharacthediode.In thedark,thephotodiode provide infinitely highresistance with no currentflow.At almost teristics;i.e., proportionately resistance reduced, becomes increasinglevelsof illumination, flow The amount of currentto throughthediode. actual thusallowingincreasing provided proportionate is sufrcient there currentis to the illuminationonly, reversevoltage.In otherwords,oncepastthe "knee" of the [email protected]), increase substantiallywith thediodecurrentat anylevelofilluminationdoesnot increasingreversavoltage.

Symbol

Fig. 11-5. $mbl

ReverseVolts------.-+ (left); and claracteristic perfontance of a plntodidc.

79

$

Photodiodc

stgnalCirqdt

FiS.Il4. Prntdiodcust asa light saitcr,"Thorie in cunentuhenthcdido is illumhutedfiahasttenW gallin amlletingan e*tenal citttrit thrw4hthenlat contutc are extremelyusefulfor workingaslightoperatedswitchesas Photodiodes Theybavea tairlyhigbswitchingspeed,sotley candsobe in Fig. 11-6. shown eachintemrptionof a beamof light as a pulseof cormting usedas @unters, cunent. diodes:thephotovoltaicdiodeand Therearetwoothertlpesof light-sensitive photowltaic diodegenentevoltagewhen The thelight-emittingdiodeOED). produced cirodt being in anassociated current resulting bylight,the illuminated the constnrcThispropertyisutilizedin light. proportional to theintensityofthe photodiode produced can bevery by a ctnrent tionoflightmeters.Theamout of in sucha introduced may be emall,andso someamplifrcation the current of generally are photodiodes known as are cirflit Specialtypgsof Photuellsand moresuitablefor useaspracticallight meters. It T\e lightemitting diodcworksin an oppositemannerof a photodiode. dediodes are passed Ligbt€nitting it througb emitslight whena currentis scribedin Chapter10.

t--- r:j- - - -|

I

20 k Potentiometer Fig. 11-7.Practicaltight stlitchcircuit*sing a flntotrunsistor.Thenlay shottldb of a sasitiu W ail cdlittst' a n butt in at ab*t 2 miltiamla Thelotzntiomctcris a vnsititity control

&)

jbPhotocell Amplifier l___

J

F!e.11! fusicrhotodtaicdiodecittuit.$ornon the dghtis theWhl lor a furtotrtL TEE PHOTOTMNSISTOR Thephototransistor is muchmoresensitivethantle photodiode to changes in levelof illumination,tbusmakinga better switchingdevicewherefairly small changes inleveldilluminationarepresentandmustbedetected" Itworksboth as a photoconductive deviceand an amplifur of tbe current generat€dry incidentlight A simplecirorit employing aphototransistor isshoumin Fig. u-i. A phototransistor anda light-emittingdiode(seeChapten10) mayte conbinedin a singleenvelope asan oltoifitofor. In thiscase,the LEDproddestle sottrceof illuminationto whic;hthephototransistorreacts.It canbeusedintwo wcking modes-eitler as a photadide (Fig. u-9) with the emitter of the bansistorpart left disconnected, or 8s a phototransistol(Fig. 11-g).Itr both case8,workiqgis governedby the crrrent flowiqgthroWhthe LED sectim"

Solar Cells ?he photodiode is a photoooltarc cell.Light faling onits junctionproduces a voltage.Thisvoltagemeasured anopencircuit(e.g.,withavery-high-resistance voltmeterconnected acrossthe cell),is knownzathellntowlfuic \oterrt;atof thecell In thisrespectit is likea dry battery.Connected to anexternalload,the cellvoltagewillfalltosomelowervaluedependentontheresistrrceinthecircu (seeChapter18).

f,8. t!9 OlMar b?(Ntt).

GonbirudLED anil phototransisbr), oleratiry a.sa lhobdiodc(bfi) aut phototransis-

8T

Photovoltaiccellsdevelopa potential when illuminatedby any sourceof light. The photovoltaicpotential dependson the constructionof tlte cell, but for any given cell, is proportional to the intensity of the light. The solarcell is a photovoltaiccell (siliconphotodiode)designedto respondto sunliglrt. Modern photovoltaiccells (commonlycalled solar cells)are commercially availablein cell sizesup to 4 inchesin diameterwith anoutput of .45 volts at 1.5 ampseachin bright sunlight. To get higher voltagesand currents from a solar battery, a number of cells have to be used connectedin series-parallel.Series connection gives a cell voltageequalto tlre sumof the individualcell voltages.Parallelconnectiongivesa current equalto the sum of the individualcell currents. Suppose,for example,the solar battery was intended to operate a circuit requiring a nominal2 volts andgive a current of 15 milliampsthrough a 100 ohm load. From the voltage consideration,number of cells required:2 divided by 500 mV (voltage per cell) : 4 cells. From current considerations,number of cells required: 15 divided by 3 (current per cell): 15 cells. The solarbattery requiredwouldthus haveto consistof five rows eachof four cells, eachrow consistingof four cells connectedin series,and eachrow being connectedtogether in parallel @ig. 11-f0). A singlesolarcell canbe usedto measuresolarpower (the strength of sunlight at any time). The cell is simply mounted on a suitable panel, sensitive side (negative side) facing outwards, and the two cell leads connectedto a 0-500 milliammeter, Fig. 11-114. Directed towards the sun, tlre meter tlten gives a readingrepresentativeof the strength of the sunlight.To measuremaximumor peakradiation,point the cell directly towardsthe sun.To usethe instrument asa deviceforplottingsolarenergy(asaradiometcr)the panelshouldbepointeddue south and tilted upwardsat an angleapproximately10 degreesmore tlnn the locallatitude.Readingsare thentaken at intervalsthroughouttheday, indicating how much solar energy the panelis receiving. tlrc meter If the meter readingsare very low, add a shunt resistor acrGssr

\

--l tf

/**,,* sohrceu

Fig. 11-10.Connections for a 2qlt ular futtcry to gioe a current of 15 milliamis throryha 7N ohmlud p

o

@

- 1 Arrangeat anAngleof 10 DegreesMoreThan LocalIatitude

\

o

I

.t €n f I

J Mlliasuneter Fig.11-11. Aligningafhotocell tomeasure nlar enerpQeft).Thecircuitontheright is a comfuletc nla/ enelgtmeter,a 0-5Nmilliamptype. (shownin brokenlines)in Fig. 11-118.Thisneedsto bea verylowvalue(1 or 2 ohmsonly).Finda suitablevalueby trial anderror to givenearmaximum meter readingin the brightestsummersunlight. RECTIFIERS Theconventionaldiode is arectifier,itsmaximumforward currentcapabilities beingdeterminedmainlyby its junction:rea. For signalrectification,point contactdiodesareusuallypreferred(seeChapter9),whichmaylimit maximum fonnrard currentto 30 to 50 mA,depending on type.Wherehigherpowersare required,largerrectifier canbeused,withmaximumcurrentratingsupto diodes severalhundredamps. (Chapter26),fourdiodesin bridgeconfiguration In thecaseofpowersupplies arenormallyusedfor full-waverectification.Physically, thisdoesnotmeanthat four separatediodeshaveto be connected up (Fig. 8-5).Bridgerectifiersare available asintegralunits.Theaveragevoltageoutputfromsucha bridgeis 0.9 timestheroot-mean-square voltagedeveloped acrossthesecondary ofthetransformer,lessthe voltagedropacrossthe rectifieritself. For highvoltageapplications, semiconductor diodes(usuallysilicon)canbe placedin aseriesarrangement to increase theoverallPIV(peakinverse voltage) rating.Forexample, if youneeda diodewith a PWof 500volts,youcanconnect 5 diodesin series,eachhavinganindividualPIVof only100volts.Themaximum is the lowestmaximumcurrentratingof currentratingin suchanarrangement anyindividualdiodein the string.Selenium rectifiers,originallywidelyusedfor voltagesupto about100rms,havenowbeenvirtuallyreplaced bysilicondiodes. Siliconcontrolledrectifiersor SCRs(alsoknownas thyristors)are silicon diodeswith anadditional electrodeulled,agate.Ifa biasvoltageis appliedto the

gate to keep it at or near the samepotential as the cathodeof the diode, the thyristor behavesas if working with reverse voltage with both directions of appliedvoltage, so only a smallleakagecurrent flows. If the gate is biasedto be more positivethan the cathode,the thyristor behavesasa normaldiode.ln otler words, the gatecanbe usedto turn the rectifier on (by positivebiason the gate), thus enablingforward current to be controlled(e.g.,preventingforward current flowing over any required portion of a half cycle). A triac is a further variationon this principle,providingbidirectionalcontrol. It is virtually a double-endedthyristor which canbe triggered with either positive or negativegate pulses. Structurally, an SCRis a four-layerdiode,with connectionsto the inner layers. The terminal connectedto the P-regionnearestthe cathodeis the cathodegate, and the terminal connectedto the N-region nearestthe anodetlte anodegate, (Fig. 11-f2). Both gates are brought out in a triac. Only the cathodegate is brought outinathyristor. Bothdevicesare essentiallyacswitches.Thetlyristor is effective only on one half of an ac voltage, and the triac is effective on both balves. THERMISTORS A thermistor is designedspecificallyto exploit the characteristic of many semiconductormaterialsto showmarkedreductionin resistancewitl increasing temperature. This is the opposite effect exhibited by most metal conductors where resistanceincreaseswith increasingtemperature. The obvious value of a thermistor is to balancethe effect of changesin temperatureon componentcharacteristicsin a particular circuit, i.e., work as a compensatingdeviceby automaticadjustmentof its resistance,down(or up), as working temperaturesrise or fall and resistancesof other componentsrise or fall. Compensationfor temperature changesof as much as 100'C are po.*sible with thermistors-a typical applicationbeing shown in Fig. 11-13. Here, the

Anode

CathodeGate Cathode

scR Fig.11-12.Symbols for SCRandtrirc.

u

TRIAC

Fig. 11'13. Practical circuit incorforating a thermistor to counteractfluctuations in oalue of otlvr raistors in thecircuit duc b hcating efects or tcmperature changes.

tlermistor is used to stabilize the working valuesof the resistors in an audio amplifier circuit. Another use for a thermistor is to eliminate current surgeswhen a circuit is switchedon. Certain circuits offer relatively low resistancewhen first switched on, which could producea damagingsurge of high current. A thermistor in the supply line with a relatively high cold resistancelimits the initial peak current surge. Its resistancevalue then drops appreciablyas it warms up so that the voltagedroppedacrosstlre thermistor undernormalworking conditionsis neglieible. Thermistors are madein the form of rods, looking rather like a carbonrod, sintered from mixtures of metallic oxides. They are not madefrom the usual semiconductormaterials (germaniumand silicon) sincethe characteristicsof a thermistor madefrom these materialswould be too sensitiveto impurities.

85

L2 Tubes Tubes(vacuum-tubes) are distinctlyold-fashioned in thesedaysof transistors andothersemiconductor devices,yet they are still widelyusedin commercial circuits,especially wherehighpowerlevelsareinvolved. The basicform of a tube is an evacuatedglassenvelopecontainingtwo - acathode electrodes andanode. Thecathode isheated,causing electronsto be emittedwhichareattractedby the anode,thuscausingcurrentto flowthrough the tubein the basiccircuitshownin Fig. 12-1(6rst diagram). The originalform of heatingwasby a separatelow-voltagesupplyto a wire filamentformng the cathode.Thelaterformis a cathodein theformof a tube with aseparate heaterelementpassing throughit. Thisis knownasaninilirectly particuhradvantages heatedcathode, beingthat thereis novoltagedropacross the cathode(andthuselectronemissionis moreuniform),andalsothe heating filamentcanbe connectedto a separateac supply,if necessary, ratler than requiringa separatedc supply.otherwise,the workingof the tubeis identical. Bothrequirea filamentsupplyanda separatehighvoltagesupply. DIODES Thesimplestformof tubeshownin Fig. 12-1is calleda diode,because it has two internalelements.Its workingcharacteristics arethat whenthecathodeis heated,application of voltageacrosstheanodeandcathodewill causea current to flow,the cu:ent valueincreasingwith anodevoltageup to the saturation point (Fig. l2-2). 6

Fig. 12-1.t6de oacuumtub.suith directll heated filament(Iefi)and iulirectly (righ|. heatcdfilament A diodetubeonlyconductscurrentin onedirection.In ottrerwords,it is a rectifierjust like its solid-statecounterpart(seeChapter9). Note tlat in this workingcircuit a loadresistoris includedin the circuil Withoutanyorternalloadin circuit,allthepowerinputto atubewouldbeusedup in heatingthe anode.To dousefulwork,a tubemustworkwith a loadof some kindor another,sothatpoweris developed in theload.To workefficiently,most of the inputpowermustdo useftrlwork in tle load,ratler thanin heatingthe anode.Thusthe voltage&op acrossthe loadshouldbe muchhigberthrn the voltagedropacrossthe tube. TRIODES AND TETRODES If athirdelement,knownasagnd, isinsertedbetweentie cathode andanode, a negativebiasooltageeanbe appliedto this to controltheworkingof the tube andthusthe anodecurrent.Sucha tubeis knownasatriode.A basiccircuitis perforrrancecurves.The advantage shownin Fig. 12-3,with characteristic of (or voltageappliedto thismodeof workingis that a smallchangein Diasvoltage thegrid)isjust aseffectiveasalargechange in anodevoltagein b'ringingabouta cbangein anodecurrenl AnodeLoad

I c:

g 3

o

€tr

AnodeVolts+

Fig. 12-2.Tr|ical diodeclaracteistics (lefi). To do uvful worh,tlw ande cunent nust fow througha lud rcsistor.

t O

E

GridVolts+

Fig. EA. A bidc b a threc+lement tub. Ano& current is controllcdbl theSlid biasooltage.

The triode is a particularlyversatile t1ryeoftube whichcanreadilybe madeto work as an amplifier or an osillator (an oscillator is really only an amplifier working with excessivefeedbackproducingself-sustainedoscillation).It doe, however, have certain limitations which may be disadvantageousin certain circuits. Oneis that the inherent capacitancegeneratedbetweenthe anodeand grid can materially affect the pcrformance of an amplifier circuit where tlte presenceof this capacitanceis aggravatedby what is called"Mller effect." To overcometiis particularlimitation, a positivelybiasedsecondgrlrdcalleda*reen grid canbe inserted betweenthe grid and anode.This acts as an electrostatic shieldto prevent capacitivecouplingbetweenthe grid and anode.Sucha fourelement tube is known as a tetrode(Fig. 12-44). Even the tetrode is not witlout its faults. The cure for one limitation (interelectrode capacitance)has producedanotler fault. The screen grid tends to attract vcondary emission electrons bouncingoff the anodebecauseit has a positivebias,whereasin the triode the onlygrid presentis negativelybiasedand tends to repel secondary-emission electronsstraight back to the anode.

Anode SuppressorGrid ScreenGrid Control Grid Cathode

Fig. 124. Tetrofu (left) and penbde (/ight) tt&s slpwn in simplediagrammatic form.

88

PENTODES To overcome thiseffectin a tetrode,afifth elementcalleda aepressorgridis added,insertedbetweenthescreengridandtheanode(Fig.12aB).Thisactsas a shieldto preventsecondary emissionelectronsbeingattractedby the screen gnd. A five-element valveof this type is calleda pentade. THE CATHODE.RAY TUBE The cathode-ray tubehasthe samenumberof elementsasa triodeOeated catlode,anode,andgrid)but worksin anentirelydifferentrnanner.Insteadof electronsemittedby the cathodeflowingto the anode,they are ejectedin the form of a niurowstreamto impingeon tlre far endof the tube,whichis coated with aluminescent materialor phosphor, soproducing a pointof light.Thispoint of light canbestationaryor moving.Its directionafteremissionfrom thefar end of the tubeis influenced by the magneticfieldcreatedby two additionalsetsof electrodes or platespositioned at right angles(Fig.12-5).ThisplatesaredesignatedX and Y. Voltageappliedto the X plate displacesthe light spot in a horizontaldirection;voltageappliedto the Y platesdisplaces the light spotin a verticaldirection.(SeeChapter23for amoredetaileddescription of theworking of a cathode-ray tube.) A cathode-raytube canbeusedasa voltmeter,witlt tlte adrrantage thatit puts no loadon tlre circuit beingmeasured. Cathodeandanodeare connected to a separate supply,tltevoltageto bemeasured beingconnected to theY plates.A dc (orbelow)thecenterline voltagedisplaces thespotaproportionddistanceabove of thetube.If thevoltageappliedis ac,thelight spottravelsupanddownat the frequencyof the supply,whichis usuallytoofastfor the spotto beidentifiedas such,so it showsa traceof light in ttre form of a verticalline, Fig. 12-6.The lengthof this line is proportionalto the peak-to-peak voltageof tlre ac. Themoreusualapplication of thecathode.raytubeisin anoscilloscope, where a voltagewhichis increasing at a steadyrateis appliedto theX plates.Ifanother varyingvoltageistlen appliedto theY plates,tlrespotdrawsa timegrafhof.tltis voltage,or apictureof the waaefornof.tlnt voltage.TheX platevaryingvoltage supplyis usuallyarrangedsothatoncethespothassweptthewidthof thescrem it returnsto the start andrepeatsthe pictureoverandoveragain. Therateof repetitionis deterrrinedbythetimebaseof theX platecircuit,this beingone of the most importantfeaturesin oscilloscope designin order to

FA. U-5. Sinplified diagramof a catMe-ra1 tuh.

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13 Integrated Circuits An integratedcircuit,or IC, consistsof a singlecrystalchipof silicononwhich hasbeenformedresistors,capacitors, diodes,andtransistors(asrequired)to makea completecircuitwith all neoessary interconnections; the wholelot in micro-miniature form (Fig. 13-1).The cqstof an IC chipis surprisinglylow, howcomplicated it canbe.This is dueto the largequantitiespre considering cessedat a time. A l-inch squarewafer,for example,maybe dividedinto 50 individualIC chips.A singleLSI (arge scaleintegration)chipcancontainthouin anareasmallerthanthe top of a pencileraser. sandsof components Apartfromtlte convenience of havinga completecircuitin sucha srnallsize, ICsarevery reliablebecause all components arefabricatedsimultaneously and joints. tlere areno soldered Diodesandtransistorsin anIC chipareformedby exactlythe sameprocess usedfor producingindividualdiodesandtransistors,but in very muchreduced physical size.Integratedresistors aremuchsimpler. Theycanbe averytinyarea ofsheetmaterialproducedbydiffusioninthe crystal,orthinfilm(amillionthof an inchthick)deposited ontlre siliconlayer.Practicalresistorrangeswhichcanbe in an achieved are10obmsto 50kilohms,depending ontheactualconstmction, areatoo tiny to seewith the nakedeye. Capacitors area little moredifrcult.Theycaneitherbebasedona diode.tne formulation(diffusedjunctioncapacitor)or on t}rin-filmconstruction(MOScapacitor).Typicalcapacitorvaluesachieved arc0.2 pF perthousandth of aninch area.Usualma:dmumvaluesare 400 pF for diffusedjunctioncapacitors, and 800pF for MOScapacitors.

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MSI AND LSI MSI standsfor medium-scale integration;andLSI for large-scale integration, referringto tle componentfunsity achieved.For example,a densityof 50 per chipis typicalfor manycommercidlyarailableintegratedcircomponents cuits.Thesefall into the categoryof MSI chips,definedashavinga component perchip.LSI chips densityof morethan12 butnot moretlnn 100components per haveamuchhighercomponent density- asmanyas1,000,000components chip. Thisislargelydueto theconsiderable using savingin component sizespossible particularlyin the cased insteadof diffnsiontechniques, thin-filmtechniques transistors.For example,an MOStransistorcanbe one tenth the sizeof a can diftsed bipolartransistorfor thesameduty.Hencemanymorecomponents bepackedinto the samesizeof IC chip.

oP Alt{Ps amplifieror opamf is a typeof IC usedasthe basicbuilding Theoperational filters, blockfornumerous analogcircuitsandsystems-amplifiers,computers, voltage-tecurentor current-to-voltage converters,modulators, comparators, waveformgenerators, etc. It is a tlpical, almostcompletecircuit,usedin conjunctionwithafewexternalcomponents tocompletetheactualcircuitrequired. Threetpical circuitsusinga simpleop ampchipare shownin Fig. 13-2. DIGITAL SYSTEMSICS Digtal systemswork in discretesteps,or virtudly by countingin termsof binarynumbers.Basically,this callsfor the useof logic elementsor gates, generallycalled togetherwitl amemoryunitcapable of storingbinarynumbers, from gatesandflipflops.Intea flil-flot. Thus,a digitalsfntemis constructed gratd circuitscapable of performingthefunctionsof binaryaddition,counting, (dateselection),memoryandregister,digitaltoanalog multiplexing decoding, and analog-todigital conversion, are tlte basicbuildingblocksfor conversion, sptems. digital Thesegiveriseto a considerable numberof differentlogicfamilies,whichare a knowledge difficultto understand without of logrcitself. Most of tlem are (except NANDgatesbecause logic functions memory)canbeperformedby all gate, thefunctionof NANDbeinge:rplained verysimplywith referthis typeof gafe, inputsto the andY istheoutput. enceto Fig.13-3.A andB aretwoseparate Therewill beanoutputif thereis inputat eitherA or B, but zof whenthereis inputsimultaneously at A andB-NOT A AND B (NOT-ANDis simplifiedto NAND). The sameprincipleapplieswith moretlmn t'woinputs.Further,the NAND gateis easilymodifiedto form anyof the otherlogicfunctionsby negationor Thesefunctionsare(stillrestrictingdescrip inoersion,modffigtle response. tion to two inputs): AND-output whenA andB inputsignalsarebothpresent OR-output wheninputA orB is present

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amfulifers fosedontttcCA3035 IC,allgioingagainofabout100.All erternal caQacitors are70 Fig.13-2.Three mbroforad. (Thisis differentto a NANDgate,for wit} noinputat A or B thereis nooutput, but with a NANDgatethereis output.) NOR-NOT-OR Pursuingthesubjectof logiccouldfill therest of thechapter,or eventhewhole book,sowe will get backto digitalintegratedcircuits. in variousdifferenttamilies,identifiedbyletters. DigitallogicICsareproduced Theseletters are an abbreviation of the configuration of the gatecircuit employed.The mainfamiliesare: o TTL (transistor-transistor logic)-The mostpopularfamilywitl a capability for performinga largenumberof functions.TTL logicis basedon multiple NAI.IDgates. . DTL (diode-transistor logic)-Another majorfamily,andagainbasedupon multipleNANDgates. o RTL (resistor-transistor-logic)-Based uponmultipleNORgateswhichoccupyminimumspace. . DCTL (direct*oupled-transistor logic)-Based uponmultipleNOR gates similarto RTL but withoutbaseresistors. 91

I Fig. 13-3. The NAND gate slnwn in simple diagrammatic font.

. ECL (emitter-coupled logic)-May be baseduponmultipleOR or NAND gates. o MOS(metaloxide'semiconductor logic)-Also calleda CMOSsinceit uses complementary MOSdevices. Thesechipsareof LSIconstruction, witl avery highcomponent density.Some5,000MOSdevicescanbeaccommodated in a chipabout0.15in. cube.CMOSis usuallybaseduponmultipleNANDgate logic. Regardless of thefamilyused,thebasicAND,OR,NANDandNORgatesare combinedin oneintegratedchipof the samefamilyin variouscombinations of gatesandflipflopsto performspecificcircuitfunctions.Thesefunctionsrnayor maynot be compatible with otherfamilies(e.9.,TTL functionsarecompatible with DTL).Alsotheremaybedirectequivalents of tlrecompletechipin different families(e.9.,TTL, DTL andCMOS).Familydevelopment continuesandmore performedby yet moreandmore andmorefunctionsarecontinuallyappearing, ICsappearing on the market. Thecomputeris adigitallogicsystem.A computerconsistsoffourbasicparts: o The inputsection o The CPU(centralprocessing unit) r The memory r Theoutput In orderfor a digitalsystemto becalleda computer,it mustmeetfiveessential criteria: o It musthaveinputcapability o It musthavememoryto storedata. o It mustbe capableof makingcalculations. o It mustbe capableof makingdecisions. r It musthaveoutputcapability. A blockdiagramof a computeris shownin Fig. 134. Theinputsectionaccepts informationfroma selectedinputdeviceandconvertsit intodigitalinformation, whichcanbeunderstood by the centralprocessing unit. The CPUcontrolsthe timinganddataselectionpointsinvolvedwith acceptinginputsandproviding outputsby meansof theinput/outputaddress bus.TheCPUalsoperformsallof thearithmeticcalculations andmemorystorage/retrievaloperations. Thememory address busdefinesaspecificareain thememoryto beworkedupon,andthe memorydatabuseitherstoresor retrievesdatafromthat specificlocation.The outputsectionacceptstlte digitalinformationfromtheCPU,convertstheinformationinto a usableform,androutesit to tle appropriateoutputdevice.

Input/OutputAddressBuss

FA, 84 Blochdiagramill*;trating ttv fusicolerationofa conpttcr. The centralprocessing unit (CPU)of a computercanbefrrther dividedinto the arithmeticlogicunit (AtU) andthe readonlymemory(ROIU).The ALU controlsthelogicalstepsandorderfor performingaritlmeticfunctions.It interactswith the ROMfor instnrctionsfor performingredundantoperations. The ROMalsocontainsinstructionspertainingto start-upandpowerlossconditions, andinstnrctionsfor conversion of higher-levellanguages to machine-language programs(MLP).TheCPUusuallycontainsttrerealtimedoch(RTC).TheRTC is usedto cycletheCPUandtimethereal-timeprogramsaswrittenbytheuser. The popularityandsuccess of the computeris governedby the speedof its operations, nottheoperationcomplarity.Basically, acomputerisonlycapable of adding,subtracting,andaccumulating data.Because it is capable of perforrring thesesimpleoperationsatamaaaflyhighspeeds, complexmatlematicalcalculationscanbe brokendowninto simplestegswhichtle computercantlen calculate.For example, a computeractuallymultipliesby redundantadditionof tlre samenumber.Divisionis accomplished by redundantsubtraction. Thesmallestsingleoperationperformedby a computeris the machine cycle. This consistsof two stages:the fetchcycleandthe executecycle.Duringthe fetchcycle,theprocessor fetchesaninstructionfrommemory. Then,duringthe executecycle,the computerperformssomeactionbaseduponttre contentof that instruction.Theprocessor knowswhichinstructionto goto nextfromtle address storedintheprogramcounter.Italwayscontainstheaddress ofthenext instruction.TVhenprograminstructionsare written, tley are arrangedin a sequential order,andthe programcountersimplyincrementsby onefor eac.h machinecycle. ln

iscontainedin If thecentralprocessingunit asingleintegratedcbcuit,theICis A computerbasedona microprocessor chipis referredto asa microprocessor. calleda microcomputer. COMPUTERCONTROLOF ANALOG SIGNALS variablesignal(referredto TVhen a computerneedsto examine acontinuously asananalogsignal),theanalogsigralmustbeconvertedto a digitalwordbefore it. A logicsystemdesigned to dothis is calledan ttrecomputercanunderstand (commonly you converter symbolized bv4D). If, for example, analog-todigital wantthe computerto storean analogvoltagelevelof + 16.78volts,tle A/D converter would convert that voltage level to a digital word of Thenumerous andthe entire 0010100100011011. 0's and1'sarecalledDdfs abyte.Thtsoneword, orbyte,representsthe*16.T3voltagelevel wordiscalled to the computer. adigital-to-anaTo reconstructtheoriginalanalog signal,oranyanalogsignal, bv D/A)is required.TheD/A convertsdigitalwords log converter(symbolized fromthecomputerto analogvoltagesandoutputsthemat t}terateat whichthey wereoriginallycollectedby the A/D. In this manner,a computermayreador outputanyanalogsignal.Mostimportantly,it canalsoanalyzeandmanipulate analogsignalsin the sameunnner.

97

T4 IC Arrays IC chipscanbedividedbroadlyinto two maincategories-completecircuitsor andalays.Thehtter consistof anumber with internalconnections, subcircuits only connected to theexternalpinsoftheICchips,or, in ofindividualcomponents together components (or possiblymore)internallyconnected somecases,t*o inpairs transistors of Darlington example, For with individualcomponents. chip' the within cludedin anarraywouldbe connected Rn exampteof a simpleIC arrayis shownin Fig. 14-1.It consistsof three two typesof diodes,anda Zenerdiode. transistors(two interconnected), circuits are commonlydesigredaroundIC arrays,in this case'a voltage regulatorusingtwo of thetransistors,the SCRdiode,andtheZenerdiode'This to be utilizedwhichare ciriuit designis shownin Fig. l4-2. The components andZl. The outline, in the dashed Q/,Q},SCR, enclosed in theIC are contained Rl, R2, Resistors (Ql not required. are and chip in the Q4) othercomponents externally. connected components all discrete are C, R3,andR4anda capacitor Figure 14-3showsthis circuit redrawnas a physicaldiagram,relatingthe to thechip.Ptnsin thisdiagramareshownin of externalcomponents connection on the integratedcircuit. For easeof physical appear they order the actual in circlesrathertlrannumbered enclosed pins and numbered reading, areshown circledor not.For clarity, shown pin may be numbers tags.Oncircuitdrawings, tle externalcompoinproportionto ttreintegratedcircuitisdrawnmuchlarger nents.

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FA.&1. CA?$7E (5K3616)IC anal shematic. for completingtlre circuitof Fig. 14-3are: C,onnections . I€ads L,2, and3 are ignoredasQl is not used. o L€ad4 connectsonesideof the Zenerdiodeto the commongroundline. . l€ad 5 to Iead 13,connectstlre Zenerdiodeto tlre correctsideof the SCR. o L€ads11 and12 connecttogether(the SCRis usedasa simplediodein this is not required). circuit,andthe gateconnection

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Now to pick up the transistor connections;The baseof Q2 (15) connectsto the externalresistor Rl, andthe collectorlead(14) to the otlrer sideof Rl, which is also the input point for the circuit. The emitter lead (16) connectsto output. Q3 andQ4 in the chip are interconnected,but only oneof thesetransistorsis required.Connectinglead(6) to (9) shortsout Q4, whichis not wanted.Connecting the emitter lead(8) of Q3 to 11- 12 (alreadyjoined), the collectorlead(9) to (6), and the baselead (7) to tlre center tap of the external potentiometer R3 connectsQ3 into the circuit. It only remains for the external componentconnectionsto be completed. These are: o R2 to lead(14) andlead(12). o Capacitor C to lead (6) and ground point. Lead (10) on the IC is also the substrateor groundpointof the IC, soshouldalsobeconnectedtotle common ground line. o One end of the potentiometerR3 to the top (outpu0 line. o The other end of the potentiometerto R4. o The otler end of R4 to the bottom conmon ground line. 100

You'llnoticefromthis circuit,anda studyof others,that the numberof componentsin anarraymaynotbeusedin a particularcircuit,butthecostof thesingle IC canoftenbe lessthanthat of the equiralenttransistorsor diodesordered separatelyandusedindividuallyto completethesamecircuit.Acircuitusingtle IC is alsomorecompactandgenerallyeasierto constmct. A little studysometimes showswherefurther savingsare possible.Figure 14-5for example, input showsa voltagedoublercircuitfor a 1-kHzsquare-wave signal,basedonaCA3096EICarrayFig.14-5whichcontains5 transistors.Only threeof thesetransistorsareusedin this particularcircuit,leavingtwo spare. Thecircuitcallsfor two diodes,Dl andD2 (aswellasthreeresistorsandtwo capacitors), to beaddedasdiscretecomponents. Transistorscanalsobeusedas diodes(byneglecting thecollectorlead),andsot]tefunctionsof Dl andD2could be performedby the two "spare"traosistorsin the array(ttrusnsingup all its components). Alternatively,sincetlte currentneedsa squarewaveinput signal,the two sparetransistorscouldbeusedin a multivibratorcircuitto providethisinput,in for Dl andD2. Sincediodesare cheaper tltis caseusingdiscretecomponents wayof usingall the components in tlte tlan transistors,tlis is a moreeconomic originalarray. The fact that popularICsare quitecheapmeansthat is seldomworthwhile goingto elaboratemethodsof trying to useall the components availablein an array,unlesssuchutilizationis tairly obvious,asabove.Usingonlypart of an arraycanstill showsavingsoverthepurchase of individualcomponentsformany circuits.

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15 Transistor Characteristics A transistoris a three-electrode device,the connections beingto Da.s(designatedB orb), emitter(designated E or e), andcollector(designated C or c). Invariably(exceptwhena transistoris beingusedasa diode),oneelectrodeis commonto bothinputandoutputcircuits-usually the emitter(common emitthe base(commonbase).The common*ollector ter), or sometimes configuration is seldomused. DC PARAMETERS governingtle dcperformance Thefour mainparameters of a transistorare: o inputvoltage o inputcurrent o outputvoltage o outputcurrent Capitallettersareusedto designate averagevoltagesandcurrents,andlowerinstantaneous volumesof voltageor current: caselettersto designate o Vforaveragevoltages r v for instantaneous voltages o I for averagecurrents o i for instantaneous currents

103

For example:Vs, %, V" meansaveragevaluesof base,collector, and emitter currents respectively,andv6, vc, vc meansinstantaneousvaluesof base,collector, and emitter current, respectively.Is, I", I. meansaveragevaluesof base, collector, and emitter currents, respectively,and io, i", i" meansinstantaneous valuesof base,collector, and emitter currents respectively. TVhereinstantaneoustotal values of voltage or current are referred to, a capitalsubscriptis used,with a lowercaseletter designatingvoltageor current. Thus vs, v6, vB rn€aninstantaneoustotal valuesof base,collector and emitter voltagerespectively,or i3, is, iBmeaninstantaneoustotal valuesof base,collector and emitter current respectively. It is alsonecessaryto definethe electrodesbetweenwhich thesevoltagesor currents apply. This is done by using the appropriateletters in the subscript: V3p: v6": vsp: V6s : V"" : vg3: Vss : vs" : vss :

averagebase-emittervoltage instantaneousbase-emittervoltage instantaneoustotal baseemitter voltage av€rdgecollector-emitter voltage instantaneouscollector-emitter voltage instantaneoustotal collector-emitter voltage ?v€ragebase-collectorvoltage instantaneousbase-collectorvoltage instantaneoustotal basecollector voltage

Iogrcally, these shouldconform to the voltage direction (positive to negative), dependingon whether the transistor is a PNP or NPN tlTe, as in the following order: PNP transistor: EB, BC, BE (or eb, bc, be) NPN transistor: BE, CB, EB (or be, cb, eb) INPUT CHARACTERISTICS The input characteristicsof a transistor show the rariation of input current with input voltage.ln tlte common-emitterconfguration,input is to tle base,and basecurrent (Io) is plotted againsttlte basevoltagemeasuredbetweenthe base and emitter (VbJ. In the common-baseconfiguration,input is to ttre emitter, so the input characteristicsshowthe variation of emitter current (IJ againstemitter voltage measuredbetweenemitter and base(V"J, Theserelationshipsare given graphicallyin Fig. 15-1. The input resistancein eachcaseistltereciprocal of the slopeof the curve andisthereforelow. Because the input characteristicsare non-linear(shownby a curve rather tlan a straight line), input resistanceis not constantbut dependson the current at which it is measured.Becauseof tlte non-linearcharacteristicsof tlre input, a transistor is normally current biasedanddriven from a current ratler than a voltagesource. This is providedby usinganeffectivesourceresistancewhichislarge in comparison with the input resistance.If this resistance(or, correctly speaking,impedance)is not high enoughto swampthe varying resistance(impedance)of the transistor under drive, there will be considerabledistortion of ttre input signal. IM

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high.The valueof the collector resistanceof a transistoris characteristically crrrent, andto someextentthe slopeof tlte cun'e,alsodependson the base curvesarethusplottedfor (constant)basecurrents,seeFig. current.Separate with ahighsouroe 15-3.Again,notetlat aconstantbasecurent isonlyobtained in thetransfercharacterisin thebasecircuit.Also,anynon-lineariry resistance asunevenspacingof ticsof the transistorshowsuponthe inputcharacteristics the cuves for equalincrementsin inputcurrent. CTIRRENTAITTPLIFICATION A transistoris generallyusedasa cufrentamplifier.Here,the basecurrent circuit,althoughit mayonlybe a controlsthe currentin the emitter-collector of theemittercurrent.Currentcanbeaddedin theusualway. smallpercentage Thus,emitter currentequalscollectorcurrentplusbasecurrent;or, alternatively,basecurrentequalsemittercurrentminuscollectorcurrent. of thetransistor,collectorcurrentdividedby thecharacteristics he:cpressing a, andcollectorcurrentdividedby basecurrentis emittercurrentis designated o1. Knowingeitler a or cr, it is possibleto findthe currentsat tle designated since otler electrodes a:ar/(l*ar) qr:sl(L_a) Actualvaluesof o andcl canvary with frequencyandcurrentfor tle same isbased oftransistorcharacteristics simple,analysis transistor.Straighfforward, in directcqrrent.To Le.,smallchanges onlowsignalcurrentsat zerofrequency; 7o' / /

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Fig.15-3. Typicaltransistoroutputcharactcristics. 106

"o" used,ttresubsctript makethis clear,tlte symbolsan anddor aresometimes indicatingzerofrequency. (inputto configuration T\e currentamplificationfactorin cornmon-emitter (or given but ofcollectorvoltage, thebase)is byar a"). It islargelyindependent thevalued Specifically, is usuallymeasured at someconstantcollectorvoltage. al is directlyrelatedto tle slopeof the I.[6 curve,or tlte trandercharacteristics. It is alsocalledthesmall-signalgain of the transistor.Actualraluesmay rangefrom aslow as 10 up to severalhundreds. In the common-base with inputto emitter,tle currentamplificonfiguration, a (or co) andis equivalentto tlre slopeof the I"/I. cationfactoris designated curve.In thiscase,I" is alwaysa little lessthanI., sothevalueof a is alwaysless thanunity;typicallyof the orderof 0.98. DC CURRENT GAIN configuration andis Specifically, this is the ratio of I"[u in common-emitter generallyreferred to asf (strictlyspeakingf')or hre, thelatternormallybeing the parameterquotedby manufacturers. f nd hrs are not identical,andboth wry with collectorcurrent.The quotedhj3 valueis thereforenormallyasso ciatedwith a specificcollectorcurrent.Commonly available transistorsmayhave valuesof h6 varyingfrom about10 to 560 at collectorcurrmts rangingfrom I mA to 30 A Herearesomeexamples: o Lowandmediumpowergermanium transistors:typicalhp mnge30-200 at collectorcurrentsfrom I mA to 300 mA o Smallsignalhighfrequencygermanium transistors:tlpical hrBrange30- 100 at collectorcurrentsfrom 1 mA to 10 mA o Germanium powertransistors: typicalhp range40- 150at collectorcurrents from 500mA to 1A o Smallsignalsilicontransistors:tlpical fp range50- 500at collector@rrents from0.1rnAto 50 mA o Mediumpowerlowfrequencysilicontransistors:tlpical hst range90- 200at collectorcurrentsfrom 10 mA to 150EA o HiBhpowerlow frequencysilicontransistors:tjrpicalhp range25-100 at collectorcurrentsfrom 150rnAto 5 A . Smallsignalhighfrequencysilicontransistors:lrpical h6 range40- 100and collectorcurrentsfrom 1 mA to 25 nA MANUFACTURER'SSPECIFICATIONS providecun/esshowingthe static(dc)characteris" Manufacturers'normally baseconfiguticsof industrialtransistorsfor bothcommonemitterandcommon Sunmarized valuesasappropriate. rations,togetherwith othercharacteristic transistordatain catalogsor datasheetsshouldgiveat leastthe following: Vcro(ma:r): ruximum collectorto emitter voltageon opencurrent 107

76se(max): maximumcollectorto base voltageon opencurrent (max): maximumemitterto base Vsse voltageon opencurrent (max): nulximumcollectorcurrent I" (max): maximumtotal powerdissipation & (max)or Php: dc currentgain(usuallYquoted asa typicalfigureat a sPecific collectorcurrent) To assistselection,suchlistingsmaybearrangedin groupsof (roughly)similar is alsoshownfor each tlrpes.It is evenmorehelpfulwhenthe designapplication transistortype, suchas generalpurpose'switching,audioamp,etc' Hereis a andguide. usefulclassification C'ermaniumTransistofg o Small,medium-current, switchingandgeneralpurpcse o Medium+urrentswitching,lowpoweroutput o Small,medium-current amPlifiers o AF amplifiers,lowPoweroutput pairs o Complementary o Highpoweroutput(powertransistor) Silicon TransiBtorg r AF amplifiers,smallsigtal,generalpurpose o AF amplifiers,low level,low noise o Small-signal amplifiers o RF amplifiersandoscillators o Medium-current switching,low poweroutput o Highfrequency,mediumPowers o General-purpose switching o Powertransistors Anyreferenceto powergroupingis largelyarbitrarysincethereis nouniversal ontherangeof powerlevels(referringto thema:
componentvalues-such as resistors-then beingdeterall ttre associated of the particulartransistorspecified. minedwith respectto the characteristics No problemthere.Simplyusethe specifiedtransistor-unless,ascanhappen, youfind that it is unobtainable. threeoptions.Thefirst isto useanequioalent In tlnt case,tlere arebasically or typenumber,whichhasthesamecharactransistor,of differentmanufacture from whichto teristics.For this you needa historyof transistorequivalents selectan alternative.There are booksavailablewhichgive suchequivalent listings,or yourlocalhobbyshopor partsdistributormaybe ableto help. Tbeyaremore givenin suchlistingsareseldomexactequivalents. Equivalents to beusedin with sufrcientlyclosecharacteristics likelyto be near-equivalent radio circuits Simple not too critical. values are where component circuits basic are an example.In manycaseswith elementarycircuits,almostany t1rpeof or silicon),or betterstill,tlte same transistorof t]tesamebasict1rye(germanium work. will functionalSoup, group groupingisharderto comeby.Manufacturers Informationonfunctional stocks by type products only list their usually in way, but suppliers this their to check on manufacturers'catalogs helpful without is not very number,which you particular find goup TVhen can conforms. transistor a to which thefunctional transistortypeslistedunderfunctionalgroups,keepthis materialonfile. It can beaninraluableguidein selectingtransistorsfor a particulariob. IDENTIFYING TMNSISTORS BY SHAPE of differenttypes,tlre numberof Whiletransistorsare madein thousands produced limited andmoreor lessstandardized is more in whichtley are shapes (Transistor followed by a number. Outline) in a simplecode:TO TO1 is the originaltransistorshape-a cylindricalcanwith the threeleads emergingin triangularpatternfrom the bottom.Lookingat tlre bottom,the upperleadin thetriangleis tlreDas,tle oneto theright (markedbyacolorspot) thecollecbr,andtbe oneto the left theemitter.Thecollectorleadmayalsobe morewidelyspacedfrom the baseleadthanis the emitterlead. the threeleadsmayemergein similartriangularpattern In otherTO shapes, witl thesamepositionsfor base,collector,andemitter),or Out not necessarily in-line.Jrst to confusethe issue,there are alsosubtypesof the sameTO The TO92,for example,has numbershapeswith differentleaddesignations. tbreeleadsemergingin line parallelto a flat side,or anotherwisecircularcan, reading1, 2, 3 fromtopto bottom,with theflat sideto theright (ookingat the bottom). thingsfirrther,sometransistor rnayhaveonlytwo emerging To complicate to the caseinternally),andsometransistoroutline leads(the third connected arefoundwith morethanthreeleadsemergingfromthebase.These,in shapes fact, arc integratedcircuits 0Cs), packagedin the sameoutlineshapeas a transistor.More complexICsare packagdin quitedifferentform. Powertransistorsareeasilyidentifiedby $ape. Theyaremetalcasedwith an eloqgatedbottomand two mouting holes.In this case,there are only two

leads-the emitterandbase-and thesearenormallymarked-Thecollectoris tbe thecollectorisviaoneof to thecan,andsoconnectionto connectedinternally mountingboltsor bottomof the can and of transistoroutlineshapestogetherwith typicaldimensions Examples given in Fig. 9-10. are leadidentifrcation

110

16 Amplifiers An amplifiercanbe defnedasa deviceor circuitprovidiqgnultiplicationda dgnal;Le.,anoutputsignalgreaterthanthe inputsignal d Transistorswork asanamplifierof siglab.In the mostwidelyusedmode connectionof transistorsfor suchduties,with input andoutputcircuitsboth mode),tbedegreed amplificatimor connected to theemitter(common-enitter cnrrentgainis calledthe bta(fi of,tbtransistoa BASIC AMPLIFIER CIRCIIITS A basictransistoramplifiercircuitis verysimple,andis identicalfor a PNPor NPN transistor,exceptfor the batterypoladty(Fig. 1G1).Virtuallyanylow' poweraf transistorcanbeusedin thiscircuit Thebiasresistor(R) mrst havea rating thenaxinum specified valueprovidinga collectorcurrentnot exceeding also cir@it in collector current flowiqg tbe ttre actnal for the transistorused, voltage ofthe battery. by the beinginfluenced a suitablevaluefc R in Fig. 1&1 can Knowingtbe transistorcharacteristics, asfollows: be calculated . .- batteryroltage collectorflrrent

x-qartrA-

_L - batteryvoltage _oo^___F

t11

Fig. 16.1.BasicamPlifiercircuit. where I" is equalto, or preferably lessthan, the specificationfigure for I" ma:r. For example,a small signaltransistor has the vdues of I" max :250 mA, and hp : 30 to 90. Taking the maximumgain,and 175 mA asa safeworking figUre for the collector current: q

R:90 "

OfZS

- 4,630Q resistorwouldthusb 4-7 k. A suitable(preferred-value) af amplifrerof this circuitfor anelementary complete Figure1&2 showsthe (output)is The detector set. crystal a basic end of type, coupledto the front wouldbe which for value suitable C2, a coupledto the amplifiervia capacitot (0.001 pF) may notbe (20,25,30, pF): C1 Capacitor 50 40,or l0 pF or higher high-impedance formed by is circuit Theoutputloadin the collector neoessary. phones.

rI I I I I I

1 I I

Coupling Capacitor +tl

c2 c1

I

L

1L2

CrystalSetFront Eod Fig. 1&2. Crxstal vt with one stageof amillifuation.

NotethatthePolarityof.thebatteryusedto powertlfs circuitis importantas far asthetransistorconnections isconcerned andalsobecause oftheconnections to the diodeandC2(whichnormallyneedsto beanelectrolytictypeto provide thehigh*apacitance valuerequired).WithanNPNtransistor,thebatterypolarity is reversed,asarethe diodeandelectrolytic-capacitor connections. Thesametlryeof circuitcanbeusedtoprovideadditional amplification simply by addinganotheramplifierstage(Fig. 17-3).Thesecondamplifierstagecanbe identicalto the first, or basedon a higher-power transistortakingthe output presentandprovidingevengreatergain.Theloadin theoutput(collector)ofthe fust amplifierstageis providedby a resistor(R2) (whichshouldbe aboutthe samevalueasthephoneresistance, by 3.3to 4.7 k). Thetwo stagesarecoupled (C3).Thevalueof R3depends a capacitor onthesecond transistorused,andmay be anythingfrom 130to 1 k O. Capacitors C2 andC3 canbe 10pF or larger. A morecompacttwo-stageamplifiercircuitis shownin Fig. 174. Thismay provecapable of operatinga smallloudspeaker direct,althoughthecurrentdrain quite is high. Alternatively,tlree or four stagesof amplification usinglow-powertransistors,followingabasicfrontendcrystalset,shouldprovideenoughpowerto drive a smallloudspeakerat thefinaloutput,throughasuitable stepdowntransformer match. to provideanimpedance Simpleamplifiercircuitsof this tlpe haveanimportantlimitation:performanceof the transistor(s)tendsto vary with temperature.There is alsotle possibilityof thermalnrnawaydeveloping, whichcandestroythe transistors, increases because, asthe externaltemperature the collectorcurrentalsotends to increase, whichinturncauses afurtherincrease injunctiontemperature. The goes frombadto worse,evento tlte pointof ruiningtie effectis cumulative and It ispo.*sible transistorcompletely. to overcome thistroublebyarrangingforthe circuitto be self-biasing or dc stabilizedsothat a constantcollectorcurrentis

Fig. IN. Crystalst with tw stag* of amplifcation. Comfonentulucs matehing SK3007Aor nearequiodbnt for Ql and Q2 113

Fig.164 Anlliftcr for lM*akor urWL Corttior*nts R[- 6.8h Ql- SKifrM M- 33h Q2:5K3011 C8- T0nicmfards provided,regardlessof transistortype or t€mperahrrevariations.In other words,the workingpointof the collectorcircuitis stabiliz:d. of of a stabilizedcircuitis tbat it makesthe perforrnance Afrrther advantage individual transistors, characteristics d tle amplifierlessdependenton the evenfor tle sametype.Capacitoroupliqgbetween whichcandifferappreciably rather than stagesshouldbe used,sincethis nakes eachstageinOepeo0ent interdependenl To achievethis,the originalbiasresistoris split into two separateralues,Rl andR2(Fig. 1G5).A furtlrerbiasresistor(Ri|)is applieddirecdyto tbeemitter, in parallelwith a capacitor(C) to act asa bypassfor af crrrents. A receiverdesigpincorporatingtwo stagesd amplificationwith stabilized front endis sbo\mio FA. 16.6.This,in fact,is circuits,followinga conventional although aboutasfar asit is p'racticalto go with srch a basiccircuit because, any additionalcompletestagesprovidenoregainandgreaterfnaloutputpower, deficiencyin the circtit is alsoaggravated-notabln lack of sensitivityand eelectivityin tlre front end. pointto insertavolumecontrol Theamplifierstagesalsoprovideaconvenient into the receivercircuil This takesthe form d a potentiometerwhichcan thecurrent orbeplacedinserieswitbtheflowof replaceonedthebiasresistors, (Fig. 1G7).Tbisintroduces minimumdistorafterthefirst stageof amplification 6on over the volumecontrolnqge" Stagee Amplifier0utft Singletransistoroutputswork in ClaseAoperation,rvhichmeansthat the vzluesof biasandsignalvoltageappliedto the transistorensurethat collector 711

t! **

l* TN

FA. Iffi. Transkbramllifur sbgcuithtubiliud bias.bpicat oatwsfor lofltrudiumpowr transistors RI- 22h R2- 10h R3- 7h C- 10nicmfarads currentahnys flows.Figure1&8 showsa basicClass-Aoutputcircuitincorporating transformercouplingto a loudspeaker. A moreeconomic wayof producingsatisfactory outputpoweris to useasingle transistordriver workinga complementary pair of transistors(anNPN anda PNPselect€dwith netchedcbaracteristics) in pustr-pull configuratioaTheout-

l.

a Orttrrl -l t d

t:l

- J

FA. \ffi.

Twostageanllifier circuit. 115

I

'l O,rtgt ;' Losd

QT

R8

Fig.1&7. Twostageamplifierwith oolumccontml.

more put powerobtainedfrom a pairof transistorsin push-pullis considerably type. t11e same of ihan-doubtethe powerobtainedfrom a singletransistor thetransistorsarebiasedto nearlycutofr,sothatonly IYithClassBoperation, ".rtgio.t *rr"nt flowsunderquietconditions.Push-pulloutputsrnay,howwith ratherhighercurrentdrains' i.o U"desigredfor ClassABoperation, "uet, is lowestwith ClassA operation,whileClassB operation disto:rtion Basically, providesthe lowestcurrentdrainbut introducesthe possibilityo{ crossover by applyinga slightforwardbias distortionbeingpresent'Thiscanbeovercome betweenthe two. to eachtransistor.classAB offersa compromise

Fig. lffi. BasicClassA amplifiercircdt. 116

1\ro typesofbasicpush-pull outputciroritsareshowninFigs.1Sg and1610, oe with directcouplingandthe otherusingtrandormercoupling,bothinterandto theloudspeater. Thecoupling stage(betweeodriverandpush-pullinpu$ traadormercanprovidevoltagesteFup.An outputtrandormer,on tle otber hand,is invariablya stetrdownb?e to adjusttie loudspeakerimpedancetothe reguiredoutputimpedance. Both typesd circuitshavetleir advant4ges and disadvantages, althorghforsimplerreceivers,all-transistorcircuitsareusn[y preferred. Muchalsodepends of the receiver.To operatea soall on the requirements loudspeaker an audiopoweroutputof 5 nA or betteris reguired successfrdly, (higherstill for largerspeakers, of course).At the otherextreme,about10;rA represenb,for most people,the thresholdof audibilityin higb-impedance phones;and0.1 mA is a normalminimunfor comfortable listeningandreaf For easylisteoingwith higb-impedance identificationd sormdsin headphones. phones,anaudiooutputpowerof upto 0.5 nA is deirable.Hig[er signallevelr teodto gwampheadphones but can,dcourse, bereducedby a voluneffioL

T I I

9Vc

I

I I

tlrt-J

Fig.b9. Pt dr'ruillontliier. &nPrcn* C2R7- I C3M-I QlR3- 7 Q2Rl- 7h Q3R5- 7h IMsfiah,erC7- 5 nhmlarads

2ilmicmfards 50 nicmlatds SK I SKfiO7A SK3835 8ohns

n7

amflifur sith tra*{ottttt corflittg.QI itttb drirlr; Q2a;il 03' Sf,CII4 Tl}dul q* Fig.b10. Push-pall Ftuntuhus R7,M- bnatchQl (defeillitrgn09cd) C2- 60micmfards R3:Ih C3- 50nicmlards R4:100 T1- cmllitgtm*slorrul R5- L7h T2- uttltt tmwrorfr" R6- 70 ue li*ed andlotdspea&crs earpiecee, Tpical claracteristicsof headphones, here. Headphones o High-impedance type: dc resistance2,000-4,000 ohms,tlpical impedance 10,000ohms(at 1 kIIz) 80 ohns(at 1 r tnw-impedance tlTe dcresistance15ohms,typicalimpedance ,*lz) o t owimpedance 80ohms,typicalinpedane 120obms(at 1 type dcresistance kllz) Earpieces 15 ohs o High-impedance t1rye:dc resistance2,000ohms,tvpicalimpedance (at l kHz) . Highimpedance 14ohms,tjDicalim@nce 60 ohns(at 1 type dcresistance kHz) 250ohns(at o Highimpedance 60 ohms,typicalimpedance type:dcresistance 1-lz)

118

Loudspeakers Typicaldc resistance3 ohms,tlTical im@ance8-16 ohms. StepDown Transfomero phones,a It is obviousfrom a study of thesefiguresthat low-impedance low-impedance earpiece,or a loudspeaker will bea mismatchfor couplingto an outputrequiringa high-impedance load(asin the caseof mostof the simple all-transistoroutputcircuits). phones,earpiece,or a loudspeaker To employlow-impedance with anoutput requiringa high-impedance load,a matchingstepdowntransformer(output transformer)mustbe used.The primaryof the transformertlrenprovidesthe requiredoutputload,indireclycoupledto the secondary to whichis connected phones thelow-impedance or speaker, Fig. 1&11.Theturnsratiorequiredfrom tlte transformeris easrlycalculated as:

Y\

pho""o@

/

Sometypicaltransformerratiosandtheir suitabilityfor matchingare:

Fig. 1&11. Usingan outpttttransfotncr ta balancettc lud

119

Rado 4lzL 35:1 30:1 18:1 14:1 9:1 5:1

n0

Listening Device Speaker 4ohm earpiece Speaker 14ohmearpiece lSohm earpiece 60-ohmearpiece 80ohmheadphones

Equivalent Output Load ImpedanceOh'nr 29,000 20,000 10,000 20,000 20,000 20,000 20,000

L7 Oscillators An oscillatorcanbedescribed in simpletermsasanalternatingcurrentgeneraarerequired,asinmainselectricityforexample, tor. Wbereonlylowfrequencies rotatingmachines offera straighfforward solutionandcanalsooperateat high powerlevels.ln electroniccircuitswherevery muchhigheracfrequencies are required,theseareprovidedby oscillatorcircuits.Theyfall broadlyinto three types:resonant-frequency oscillators,crystal-controlled oscillators,andphaseshift oscillators. RESONANT.FREQUENCY OSCLLATORS Resonant-frequency oscillatorcircuitsarebasedontheparticularpropertyof (L) (C) to e:rhibitresonanc€ an inductance and an associated capacitance qscillatesfrompositivetonegativeina wherebythecurrentflowinginthecircuit mannerat a frequencydeterminedonly by the valuesof L andC. sinusoidal Figure18-1illustratessucha circuit. Intheinitialstate,withtheswitchinposition, thereisasteadyorrentflowing tlrough the coilbut with no appreciable voltage,andsotle capacitorremains ucharged.Movement oftlreswitchto position2 putsalargenegativebiasonthe gateof the FET so that the draincurrentis cut off. The resultantcollapseof magneticflux in the coil opposes tlte charge,sothat currentcontinuesto flow intothecapacitor, chargingit up.Voltageacrossthecapacitorcanonlyincrease of decreasing current,however,soa pointis reachedwherethe at the expense currentfallsto zeroandtle voltageis a maximum.

Fig. 17-1.Demonstrationcircuitfor LC osillator. At this point, the capacitorstarts to discharge,generatinga curent in the reverse direction, with falling voltage. This continues until the voltage has droppedto zero, correspondingto maximum reverse current. All the energy st*"{ in the LC circuit is now returned to L but with reverse polarity. Consequently, the voltage starts to increasein a negativedirection, with the current decreasing,endingup with the capacitorchargedin tlre reversedirection (maximum negitiue voltage) when the current againreacheszero. The circuit now starts to rise againin a positive direction with falling negativevoltage to complete a full cycle of oscillation,as shownin Fig. 18-2. The cycle of operationis repeatedover and over againat a frequencygiven by:

ZnxJfr Theoretically at least,with an idealinductanceandan idealcapacitor,neither havinganyelectricalresistance,the original store of energyis exchangedindefinitetf i.e., oscillationcontinuesindefinitely in a sinusoidalrumner with no further energyfed into the circuit (the original sourceofbattery energyis switched off oncethe switch is movedfrom position 1 to position 2).Ina practicalcircuit' however, there is someresistancepresent, which producesa gradualdecayor damping of.the amplitudeof the oscillators(i.e., reductionin rnaximumvoltage and current values).

'{l

Yoltage

Switch Operated

I Fig. 17-2 Cfle of oberationof LC osillabr. 122

FA.nA. Self+xillat'ngosillatorcircuit The degreeof dampingpresentis dependenton tlre ratio of react nce to resistancein the LC circuit,or Q. The lowerthe valueof Q, the greaterthe dampingpresent.If the Q is aslow as 1:2, the circuitis criticallydampedand doesnotoscillateat all.In otherwords,if conditions arerightfor theLCcircuitto start oscillating,this is dampedout completelyin the first cycle.Equally,tle highertheQ, thegreaterthelengthof timeoscillation However,in a continues. practicaloscillatorcircuit,someenergymustbesupplied to thecircuitto keepit oscillating,to replacethe energydissipatedin tlte resistancepresentin tle circuit. Also, tlris supplyof energyneedsto be providedautomatically in a practicalcircuit.Figure17-3showshowthe originaldemonstration circuitcan bemodifiedto provideiust this. Here, tlre smallcoil inductivelycoupledto L generatesa voltageoactly in theLC circuit.Theralue synchronized with the sinusoidal voltagegenerated of thisvoltagedepends onthenumberof turnsin thesmallcoilandhowcloselyit is coupledto L. Theinducedvoltagecanworkwith or againsttle voltagein the oneway.If the LC circuit.[n otherwords,it onlyworksif the coilis connected the right way,sucha circuitcanbe couplingis close,andthe coilis connected selfoscillatingstartedup by randomelectronmovements. PRACTICAL LC OSCILLATORS practicalresonant-frequencyorLC oscillatorare Thebasicrequirementsfora with somemeansof maintaining thusanoscillatorycircuit(L andC),associated oscillation(usuallysomeformof amplifrersuchasa transistor,FET,or opamp).

()

BasicltartleyOscitlator

()

BasicColpitts Oscillator

F4.174. Twobasbosillator circuits.

123

Fig.17-5.PracticalHartletosillatorcircuit. shown Twowaysof doingthisusinga transistorasthemaintainingamplifierare in Fig. 17-4.In thefirst (A),theLC circuitis tappedontheinductiveside.Thisis the basisof the Hartleyoscillator.In the second(B),the LC circuitis tappedon side.ThisisthebasisoftheColpittsoscillator.Practicalcircuitsfor capacitive t11e typesof oscillatorsareshownin Frgs.17-5and17{. important tlese two CRYSTAI-CONTROLLEDOSCILLATORS A quartzcrystalwith electrodesplantedon oppositefaces,is the electrical (Cf) in series, (L), resistance(R), andcapacitance equivalentof aninductance (c2) with a muchlargercapacitance acrosstlem, seeFig.l7-7.It is, in fact,a maynmgefroma completeLC circuitwith a specificresonantfrequencywhich onthecrystaltype,size,howit iscutandhow fewkHzto severalMHz,depending of ahighQwith goodstability'making It alsohasthecharacteristic it ismounted. qscillator circuitaround.It is alsoa moreor less it amostusefuldeviceto basean standardchoicefor a fixedfrequencyoscillator. is shownin A basicconfigUration A wholevarietyof suchcircuitsarepossible. Fig. 17-8wheretheactivedeviceis anFET. It shouldbenotedt}at, althougha theoscillatorfrequencyisdetermined included, tunedcircuit(LCcombination)is

Fig. 174. Prrctieal Colpittsoxillator circuiL

1124

I I EI

EquivalentElectricalCircuit

Fig. 17-7.Eleetricalcircuit equioalentof a crystal. essentiallyby the resonantfrequencyof the cr!'stal and not by the rest of tle circuit. The tuning of the LC circuit can in fact, be relatively broad aroundthis frequency,the crystal providing stabilizedoscillator at a specificfrequency. The frequencyof a crystal-controlledoscillator can be adiustedwithin very narrow limits (usuallylessthan 5 parts in 10,000)by meansof a rariable capaci-22Y

Fig. 17A. 1 MHz crystal osillator.

125

rI I

f,--;,---,,---'l ln

l

I

I

I I

I Ott I I I

I

I

I

Flg.17-9.Pha**hifiosiltabrfusdonfltet cir*iL devices inexpensive aresimple, sincecrystals it. However, acToss tor connected easiestmethodof of achangewstalisusrullythe asocket, wnicncanptuginto adjwtnenl frequency PEASF-SSIFTOSCILLIITORS byemploy' onthehmed-circuitprinciple anoscillato,r toproduce Itis possible suchcircuitsarefilternetworks Basically, [Aot inductances. dffJffiirot RC6lter A three'stage withlqamplifier. associated Groa.O nCconbinations) between d 180degrees to providea totalphasedifrerence iit*it is needed gain o!thearrp$er.Abasic the by nratcUeO toss srnall with a *lt"g., o-*t-d fortle activeelemenl dr*itd thirqrBJii.Uo*oinFig.1?-9,usingatransistor fc such"-cit*it to *otf is tlat tbeinputto theamplifier I GiLqdfii*t gainof the r*t t" .i lu".t Uf timestheoutputin strength,wheref-is the gAin witha cgrrent d tj3o.i.to" Thisgdera[tcals for theuseofa transistor tbeorderof 50or better.

Fig. 1?-10.Wienhddgeoxilla|rr'

n6

I I I I I I I

FA.n-U. Basicresnantcircuitosillatorwithtransisbramfrlifur. oscillatorsis t}at tley canreadilybe A particularadrantageof phase-shift ganged threevariablecapacitors, range, using wide variedin frequencyovera gsefirl is froma range working most Their togetherandvariedsimultaneously. range and MIIz the frequencies-in At higher few Hz to hundredsof.lrtlz. oscillators. generally resonant+ircuit inferior to above-tley are BRIDGE OSCILLATORS oscillator Oneottrertype of oscillatorworth describingis the Wien-bridge (anotherphase-shifttype) wherea balancedbridgeis usedas the feedback network(Fig. 17-10).To work, the loopgainmustequalunity andmusthave zerophase.Thisis generallygiveobysuitableselectionof resistorraluessotlat R2/Gf + R2)is lessthan%. The frequencyof operationwith this type is tle frequencyof ttre balanced I

valueson Notethatboththetwo resistorvaluesandcapacitor 7;rt5; theleft-handsideof thebridgeareidentical.Thefrequencyof tle oscillatorcan by usingdifferentvaluesfor R, or, alternatively,for a variable'frebe changed quencyoscillator,tlretwo capacitors arereplacedby gangedvariablecapacitors of identicalvalue,variedsimultaneously. Themajorityof oscillatorcircuits,however,arebasedonresonantcircuitor secondcoilto gqF form with a clcsecoupled of conventional LC combinations moteselfoscillation.A circuitof this typeis shownin Frg.L7'LL. bridge,or

u7

18 Circuit Diagrams and A circuitdiagramis a planof a particularcircuitshowingallthecomponents (see by symbols represented The are all the circuit connections. components simplyandclearly,avoidingcross" Chapter1),arrangedto showall connections po.ssible. sinceit doesnot showthe diagam It is a theoretical ing linesasfar as position in a built-upcircuil nor their actual actualsizeor shapeof components, plan practical circuit fromwhichthe diagramorworking It hasto beredrawnasa is actuallyconstructed. applyin drawinga circuit diagram,but tlese are not Certainconventions alwaysfollowedrigldly. The first is that the diagramshouldreadfrom left to right.Thatmeanswhateveris inputto thecircuitshouldstartat theextremeleft andbefedthroughthecircuitfromleft to right.In thesimpleradiocircuitshown currentfeedingthe bytheantenna in blockformin Fig. 18-1theinputis supplied tunedcircuit,tlen passingto thedetector,thento the amplifier,andfinallytle output.Thepowersupplyfor thecircuit(sayabattery)is shovmon loudspeaker ofthenrle,if you thefar right of thecircuit.At first thismayseemacontradiction put think of the powersupplybeing into the circuit.It is not a true input,but is merelya supplyto work t}re circuit. Otherwiseit hasnothingto do with the circuit,soit is depictedoutof thewayontheright.Thereis anothergoodreason in termsof left-tofor this.Althoughthe supplyfeedsall the stagesbachnrards power the first stage.Hence,it is right reading,it probablyis not requiredto logicalto showfeedfrom the rigbt, stoppingat the approprhtestage.

12Et

Fig. 1&1. fusicstages in a simlb radioreceiter. Nearlyall circuitsarebasedona commonlineconnection; i.e.,components in variousstagesareconnected to onesideofthesupply.Thiscommonline isdrawn at thebottomof thediagram,asshownin Fig. 18-2.It is generallyreferredto as tlte commongroundline, althoughit maynot haveany actul connectionto ground. Asimilarcommonlinecanalsobedrawnat tle topof thediagram,representing the othersideof the supply.Conventionally, this top line is the *, andthe bottomor commonlinethe-. Thisis notalwaysconvenient in designing transis. polaritymaybe reversedon somediagrans. tor circuits,sothis conventional TVorking onthisbasis,andreplacingthe boxeswithindividualcomponents, the circuitdiagnmlookssomething likeFig. 1&3. Eachcomponent is identifiedby a number,or mayhaveits actualvaluegivenalongside. Reading thecircuitisfairly - with a little practice.Startingfromtle left, theinputfromthe straighdoward antennais fed to L andCl formingthe tunedcircuit.Fromthere,it is passed throughthe diodedetectorto the amplifier(Q1).Ql thenfeedsthefinaloutErt stage(transistorQ2)drivingthe loudspeaker. A supplyvoltageis requiredonly byQl andQ2,sotheuppercommon linestopsstrortat Ql stage.Thereistors in tlte top halfregulatethe supply;andthosein the bottomhalf(connecting to the commonbottomline)establishthe workingpointof the transistors.Additiond componmts(C2andC3)are requiredfor couplingbetweenstages.

Fig. 1&2 Stages fuun wik commonground and fiouers'uppljconnections.

129

Stage3

-

--t

I

1

I

l

t: I I L

I I I I

rcquired. andall connections drawnwithall comfonents Fig. 18.g.Thesamestages linesmeetat right angles,andwheresuchconnection Notethatallconnecting clearby a '. If a line on the diagramhasto cross made occurs,this is further to thisline,it is simplydrawnasa crossing anotherlinewithoutanyconnection (righ\ lineswith a ' at thepointofmeeting . Crossing 1 8-5 line,asshownin Fig. at that point.To avoidpossible indicatethat allfourlinesare,in fact,connected '), linesfrom eachsideof connected (or the missing confgsion accidentally of Fig' 18-4' diagrun right in the shown as anotherline canbe drawn but diagrams, theoretical in reading at all no confusion Sofar, thereshouldbe complex more becomes circuit followwhenthe to moredifficult theycanbecome trick of the Onecornrnon or containsa largenumberof individualcomponents. in (which mistakes lead to could lines crossing tradeusedto avoidtoo many point, seprately or connecting a arrow is to followinga particularconnection) (Fig. 18-5).Arrowingis usuallyappliedto ".ot onlineconnection designale stage(or asaninputto a separate is connected outfuts,indicatingthat this line wittr is ground common connection separate circui$.Showinga evena separate clearly It indicates line. bottom and line .o*pon"ot .o*"cted betweentlretop to the groundline,andavoidshavingto is to beconnected that the component or crossingotherlines. proximity to othercomponents drawtlis lini in close is a circuitincorporatconnections TVhatis lesseasyto readin termsofactual (or more)separate perform two whichmay ing a singlephysicalcomponent

Ttatonthelight is ofun fureferredforclaitl' indieatingconnectiontoacommonpoint Fig.I84 Twometlpttsof

1il

t

+

(C) is connected Fig.1&5. Tlv capacitor btupentopandbottom lincs,krt mayb shownin a circ*it diagramin eitlvr ofthese twoway.Otttlp rightarethreewalsof drawingcrosinglkes uith noconnectiol. Method1 ie theuaul waX;mctM 2 is clenrerstill. functions.Asa verysimpleexample, a volumecontrolpotentiometerforaradio mayalsoincorporate onoff switching . Thefrrofunctional fatwe-softhissingle component mayappearin quiteseparatepartsof the circuit;e.g.,the volume controlpriorto anamplifierstageandtle switchfunctioninthe supplylineat the far right (Fig. 18-6). Thiscanbeevenmoreconfusingat firstwherea gangedtuningcapacitcca gangedswitchis involved,with its separate sectionsappearing in differentparts of the circuit, althoughit is actuallya singlephysicalcomponent. This is the logical-and by far tle simplest-way of showingthetheoreticalconnections of thecircuit,butwhenit comesto actualconstruction ofthecircuit,connections fromtwo ormoredifferentpartsofthecircuithavetobetakentooneparticular position component

F!s.1Y:-Inthisenmfle,asirybcon2oaent(ttcfuotent;ometerconncctions)apfoars inhmvparate partsofthe circtit diagrem.

Actual Size PhnDrawn WithAllComponents Working layoutdrawingorwo*inSPlorr.Thisb thesne cinuit Fig.1&7. Tyficalcomponent as Fig.18-?, followingthctunedcircuit(uhiehwouldbeaferriE radrcrial shouldfollowtlesame Asfar aspossible, actualphysicallayout of components path positions in flow as thetheoreticalcircuitdiagram,adjusted asnecessary to getcomponents intosuitablepositionsfor makingconnections. Exceptions must arise,particularlyasnotedabove.Justasa theoreticaldiagrarnis designed to presentthecircuitin assimplea manneraspossible with alltleoreticalconnectionsclear,the workingcircuitmustalsobeplannedto beasneatandsimpleas po*sibleandalsoas logicalas possibleas far as placementof components is It shouldbepreparedasa complete wiringdiagram,whenit becomes concerned. a workingplan.Almostinevitably,it will lookmoreconfused thanthetieoretical diagram,with probablya fair numberof crossingwires(unlessit is plannedasa printedcircuit-see ChapterZO)andleadsrunningin variousdirections.Commonconnectingpointsare still indicatedby a ., but crossingleadsare better asin Fig. l8-7. Thenthereis lessrisk of wondershownasdeftritecross-overs, ing whetheror not a . hasbeenmissedat that pointin preparingthe working diagram,particularlyasit is lesseasyto checkconnectionsonaworkingdiagnm thanon a theoreticalcircuitdiagram. Thetheoreticalcircuitdiagram,however,remainsthecheckreferencefor the workingplan-and for checkingthe circuitwhenbuilt.It mayalsobethe only guideavailable for establishing tlrecorrectwayto connecta diodeora polarized (electrolytic capacitor).Followingthe directionof curent flow (and capacitor re thusthe polarityat anyparticularpoint),shouldbe fairly straighfforward, Collector Current

Current In

Collector Cunent

CurrentOut

CurrentIn P-N-PTransistor

CurrentOut N-P-NTransistor

Fig. l88€.Directionof currentfow throughPNPand NPNtransisbr*

12

memberingthat with a * top line,the directionof currentffowis downwards onsuchpaths.If the (fromtoplineto bottomline),tlrough rariouscomponents -, easyto checkthe It is also reversed. is obviously flow direction is the line top the emitterin the alrow on by the transistors flow through of current direction thecollector of the transistorvia orf of currentflow Direction symbol. transistor into the baseof the flow current Direction of same direction. in the follows 18-8should Figrre direction. arrow the emitter of to tlat is opposite transistor makethis clear. Theserulesfor readingthecurrentflowthroughtransistorsshouldalsomake ittairly simpleto deterrrinethecurrentflowwithhorizontallyconnectedcomponentsonthe circuitdiagram,andtlus establishthecorrectpolariff for electroin theselines. appearing lytic capacitors

133

19 Circuit Construction Oneof the big problemsfacingmostbeginnersis howto constructa working circuit;i.e., turn a theoreticalcircuitdiagarninto a connected-up assembly of (with dl the connections components correct,of course!). Thefollowingdiagrams showsixvery simpleandstraighfforwardmethods of tacklingelementary circuitconstruction-dl capable of givinggoodresultswith the minimumof trouble,andespecially recommended for absolutebeginners at practicalelectronics. PINBOARD CONSTRUCTION Drawout tlte component layoutfor the circuiton a pieceof thin ply (or even hardbalsasheet),usinga ballpointpn (not a leadpencil).Drawin all connectionsandmarkpointswhereconrmonconnections occurwitl a blob(ust ason circuitdiagrams). Cut out the panelto a suitablesize.Drive coppertacksinto eachbloband simplysolderthecomponents in position.Complete additionalconnections wittr plainwire, seeFig. 1$1. SKELETON ASSEMBLY Startagainwith a component drawing,thistimeonpaper.Iay components in place,bendingtlte leadsof resistorsandcapacitorsto completeconnections. Otherconnections canbecomplete with t'ro lengtlsof l&guge copperwirefor 1A

Circuit Diagram Drawn on Ply or Balsawood

CopperTacks

Ferrite Rod Aerial

Transistor

Tuning Capacitor

For Common LinesJoinTack WithBarelVire

(c1)

Fig. 191.Pinbardconstruetion starts$rdrawinga uo*ing ptonoftlu cittuit ona panelofrtlyor folsa.Thendioe in copper tochsat eae;h connection fioint.sotder comfoncnt leadstotachs, andcomfleVcircuitasnece.ssary withary additionalpiring. "top" and"bottom" control lines.Solderall theconnecting points,addingtransistorslast (Fig. 19-2).Properlydone,sucha skeletonassembly canbe quite rigrd. BONDED MOUNTING Thisis very similarto skeletonassembly exceptthat individualresistorsand capacitorsare stuckdownto a rigid basepanelcut from plasticsheet.use five'minuteepoxyfor gluingthecomponents in place(Fig.19-3).Thisproduces a very strongbondin a fewminutes.Withthe maincomponents rigidlymounted, youcanthenbendleadsto producethe necessary connections andconnection pointsfor othercomponents (e.g.,transistors). BUS.BAR ASSEMBLY Thisisaneatandmoreprofessional wayto tacklecircuitconstruction. Thetop andbottomcommonlinesof the circuit are laid downfrst by mountingtwo lengtls of l6-gaugetinnedcopperwire in a sheetof plasticas shown.This Transistor SpaceFor Territe RodSerid

TinnedCoppcrWirc A

16 GaugeTinnedCopperWire

Fig.192. Thesamecircuit as Fig.lCI tuchledb justconnecting cotnfonents tqether oia tluir ofun lcads,and usingbarewirefor tofrand bttom lines.

Transistc Tloating:

Gluedto hse MainComponents AdditionalWiring

(erce,ttransisFie.1C3.hniled mountingissimilarto Fig.192btqtall components tors)areglwd dounb a baepanel.Turnuf endsofleadstnformcoanatingloints permitsmostof the resistorsandcapacitors to be mountedwitl oneleadsolplace. asfor skeletonassembly deredin Completethe rest of tlte connections (Fie. 19a). TAGBOARDASSEMBLY Moredr.rableandneaterthanthe previousmethods,tlis involvesmounting the ateachofthemainconnectingpointsof stripsof solderkgs(calledtagstrips) circuitlayout.Thesetagscanberivetedor boltedthroughthe panel.Individual are components are then mountedbetweentags.Any additionalconnections to formedbyshortlengthsofwirebetweentags(Fig.f 9-5).Moretimeisneeded constructa propertagboardthan with tle previousmetlods,but complete tagboardstopsarealsoavailablewith up to 36 individualtagsmountedin two parallelrows. PEGBOARDCONSTRUCTION Youcanbuy specialterminalpillarsto pressinto tle holesin ordinarypeg(Fig. 1S6). boardandso set up connectingpointsfor mountingcomponents Diode

16 GaugeTinnedCopPerWire-Bend EndsUp Under Panelto Hold in Place

Fig. 194. Lay down the to| and bottomlines in tinned copferwin, pernwrcntll mo*ntedon tlu panel. Slnrter lenglttsof bareuin can b ucd for othcrcommoa connating points.Cotnflctefu nldeing comlonentleadsin place. 136

Transistor

IndividualTags

..2?^-.

Tag Strip

Fig. 1!N, All thc connating foints are fonted b tag strils or indiaiduat tags mounted on tlu funeL Soldctconforcnts b approfriate tags,and completcuith additional wiing as naaessary.

points,soyoucanavoidsolderingcomTheseterminalshavescrewconnecting ponentsin place.Themaindisadrantage is thatafairlylargepanelsizeis needed, evenfor asimplecircuit,with components spreadout.Butit isaneasymethodof buildinge:rperimental circuits. Thereare variousproprietarysystemsbasedon variationsof the pegboard method.Veroboardsarepanels with rowsof copperstrips,eachstripdrilledwitl a numberof holeseither l mm (0.04in) or 1.3mm (0.052in.) diameter. Matchingpins(Veropins)canbeinsertedin appropriate holesto forrr terminal pillars,andthe copperstrips cut as necessary points. to separateconnecting Special toolsareusedfor insertingtheVeropinsandforcuttingthecopperstrips. Yerosti| is a similartne of boardorceptthat the boardis narower (LVzin wide)andthe copperstripsnrn acrossthe boardwith a breakdownthe center. Components canbemountedacrossor alongtle strips. Numeroussolderless breadboards havealsobeendeveloped wherecompe nent leadsare simplypushedinto the boardswherethey are heldby spring contacts.Contactpointsarearranged in parallelrows,with eitheraprearranged patternofinterconnection, or with basicbusbarconnections ontop andbottom rowsandottrersin commongroups.Interconnection betweengroupscanbe madeby wirespushedinto sparepointsin eachseparategroup. Theadvantage of sucha systemis that,apartfromavoidingsoldering, circuits

137

cencdy be uff phregiqgbtoa bt Fld r cqo.dd H difierentpoden For permanentcircuits,if you'rea beginner,youwill prohbly fnd pegboad

propsition youhave gained assembly thebest after some erpericnce incirqrit construction-and confdencein beingableto draw out componentlayoutr accurately. The ultimatefor all formsof compactcircuit constructionis, however,the printedcircuit.Herecomponents aremounteddirectlythroughholesin a plastic (or glassfiber)panelonwhichthecircuitwiringhasbeenreproduced byetching. This is a separatetechniqueon its own,but easyenoughto learn(seeCbapter 20). GENERAL RULES Connections shouldalwaysb solderedfor bestresults.Thisappliesevenon pegboards fitted with screw-typeterminalpillars.Theoneexceptionto thenrle

Spacefor FerriteRod Aerialto /-- O -

-a o:-\"-. Spacefor

-

ffi

x<,\

l': TerminalPillars

TuningCapacitor

Pegbarilaavnbb is sinilar to Fig.IlN enert t ut thefunt isrndliUadb Fig.196d. occeftterminalposts. Luatc thcv pastsatatitabb lointsbutasconrctconnatirE foints.Conncctcomfonentstofo$s,thencomfletcthcadditionaluiitrg osllr;cw;ty Note;all thes constwctionaldrawingsshowthcvttu citxuit-a si;Phtitrgbtnssistorradio receiwr withpreamflifur. Con@wats nquind an: Fenite rodurial with cottpliry ail CI- 0-5N0F tuningcapacitor C2- 0.0l nicrofarcd R7- 7 Megohn R2- 22h Dode - anygcnraniun cqstal dide Transistor- an1 rf transistot Tle citxuit w*s of a %ooltfutbry conwted b A andB (|obrity depnds ottohcfrrr thc transistorued is a PNPor NPNW). For lidelrhE,,coanat high in qlu hoad-plnusto pointsX and B. In cas theurial coil connutionsan rntclun o Tlp endsof thc main coilconnectb thc tw tagsott tltc tuning cafuitu (CO c Qaecadof thecotpling coil connutsb thccornm(m c(mrrg;tinrNnt of Rl a*l@ o Thcotlur cnd ol thccuglittg coilconnuts to thofusclcod (b)of thetlur*ti*n 18

are madeby springclips.In any whereconnections is solderless breadboards case,neverrely onjointswhichareformedsimplyby twistingwirestogether. Usea smallelectricsolderingiron for makingall solderedjoints,andresincoredsolder(electricalgade). Neveruseanacidtypefltrxonsoldered iointsin electricalcircuits. heat.Whensolderingin placeto a by excessive Transistorscanbedamaged (at circuit,leavethe leadsquitelong least1 inch).Gripeachleadwith flat ncse pliers behindthe joint whensoldering.The jaws of the pliers tlten act as a heatsink,preventingoverheatingof the transistor.Onceyou are proficientat in tle especially soldering,however,this precautionshouldnot be necessary, caseof silicontransistors. The mostconrmonreasonwhy a particularcircuitdoesnot work is because havebeenwronglymade.Thisis far morelikelyto be oneor moreconnections AlwaWcheckoverall connecthe causeof thetroublethana faultycomponent. asthebasic madethem,usingthetheoreticalcircuitdiagram tionsafteryouhave reference.Also,with transistorcircuits,bestreto connxt thebatterytheright way(asshownon the circuitdiagnm).

139

20 Printed Circuits laminate.For generaluse,t}ese phenolicresinlaminate(SRBP)or glass-fiber (cladononesideonly)andnominally1.5mmthick(about Loardsaresingle-sided %oin.). The procedurefor makinga PCBinvolves: 1. Cuttingthe boardto the requiredsizeandcleaningthe coppersurface. requiredfor tle circuitonttrecopperin a 2. Makingadrawingof theconductors resrsfink. copperareasin a chemicalbath. 3. Etchingawayuncovered areasor pads. 4. Removingthe resistink to exposethe copperconducting leads' component 5. Driling the copperpadsreadyto taketIe to ensuretlat ttrepadstake theboardsasnecessary andcleaning 6. Degreasing solderreadily. PLANNING THE CIRCUIT DRAWING ontle to beaccommodated Familiaritywith the physicalsizeof components positioned. There correctly be can etc, leads, for holes thit so is essential, board or horizontally mounted: be can capacitors and resistors in which ways aretwo reduces as tlis for resistors is usual (Fig. mounting Horizontal 20-1). vertically leadlengthto a minimum.Holesare thenspaceda sufrcientdistanceapartto on the leads.The sameconsideration allowfoi easyg0degreefinger_bends horizontally. mounted capacitors, appliedto tubular

tun

ktt upnoresprce, onaPCboanltahes ofcomponents Fig.2G1.Hoimntalmounting is uwal$ moreconoenbnt thanoerticalmounthg(erceitfor transisbrs). however,maybemuchlargerthanresistors, Thephysicalsizeof capacitors, whenverticalmountingmaybe preferredto savespace.Mountingholestlten rnatching the onlyneedto bealittle morethanhalfthediameterofthecapacitor, positionof the top leadtakendownttre sideof the capacitor.Somecapacitors on havebothleadsemergingfromthe sameend,especiallyforvedicalmounting a PCB.Spacingbetweenholes,however,shouldnot be lessthan twice the thicknessof tle board(i.e.,% in.). Transistorsneedreasonably wide spacingfor their leads.Exceptionsare transistort"pes with leadsintendedto plug directlyinto a PCB,andcertain powertransistorsneedingspecialmounts.In thesecases,holepositioningfolcircuitsnormallyplugdirectlyinto lowsthetransistorleadgeometry.Integrated matchingIC sockets,the latter beingmountedon the boardin holesdrilledto matchthe pin positions. positions, sketchingin the Layoutstartswith a tentativedesignof component required,(i.e.,the areasof copperwhichwill eventuallyform tle connections pads).No connections on a PCBcancross,anda certainamountof conducting altering trial-and+rrorsketchingis usuallyneededto achievethisrequirement, positionsasneoessary. If it seemsimpossible to achievea complete component thensuchpointscanbeterminatedontle circuitwithoutcrossingconnections, completed PC drawingon eachsideof the crossingpoint, andsubsequently duringassembly of the circuitby bridgingwith a shortlengthof insulatedwire, just asa component conductors(Fig. norrnallyacts asabridgebetweenadjacent 20-D.

W Fig.20-2.If it is (or *ems) imfossibleta aooida cmssingconnectionot a PCbodrd, withabidge ofin*lated sbf thecrossing linesshort(bft) andcomfrlete theconnection uire sldcred in place. 141

FINAL DRAWING I{avingarived at a suitablelayout,with connectingpointsfor component leadsindicatedbyblobs('), a tracingcanthenbemadeof thisPCBplan.Certain generalnrlesapplyin preparingthe fuialdrawing: Conductors shouldnot bele*sthan%oinchwide. Conductors shouldbe spacedat least%zinchapart. Thereshouldbeat least%zinchbetweenaconductorandtheedgeof apanel. Bendsofjunctionsin conductorsshouldbe radiusedor filleted,not sharf edged. pointsothat the copper 5. Allowsuffcientareaof copperarounda connecting width at this pointis at leasttwicethat of the holesizesubsequently to be drilled throughit, and preferablymore.(Typicalhole sizesfor miniatnre resistors,capacitorsandtransistorsare ys2inch).

1. 2. 3. 4.

Thesepointsareillustratedin Fig. 20-3. It is not necessary to drawdl conductorsneatlyanduniformin thickness. Relativelylargesolidareasc:rnbe left to accommodate a numberof common connectingpoints,simplifyingt}re amountof drawingnecessary(Fig. 21a). Largesolidareasshould,however,be avoidedin anypart of a circuitcarrying highcurrentasthiscouldcauseexcessive heatingof thecopper,possiblymaking it delaminateas it eraands.Thus,on a PCBfor a mainsoperated circuit,for example, thenurximum areaof anyparticularcopperpadshouldnotbemorethan about1 squareinch. Thefinaldrawingis transferredin reverseonto tlre copper(hencetheuseof tracing paper).This is becausethe circuit, as origindly planned,showsthe comfonentsrdeof theboard,whichis theplainside.Thusthetruepatternfor the Conductors at Leastil" lride

At least $" in fromEdgeof Panel

rnd Off Corners At Least*" Width of Copperfuound Holes

Fig.m& fusic ncommculationsfor blannhgcoadrcbruidthsands!rchgs ottN furdt le

I I I

t

l

r l

l t

I I I

1 l r l I

powered circuits. battery Fis.fu4. QuiElargeanasofcoltercanbelefionlowooltage (or shaies) ofconductors. consistcnt wiilths Thereis noncedfor fashion.Butbeforetransferringthedrawcoppersideis reversed,mirror-image ing to the board,the coppersurfacemustbe cleaned.This is doneby washing with detergentandthendrying. up undera tap andlet is to holdthe boardcopper-side A test for cleanliness waterrun on to it. If the waterflowsfreelyovertJrewholearea,it is freefrom gease. If dry patchesappearon tlte copper,theseareasare still greasyand requirefurther cleaning. this patternis After tracingthe (reversedimage)patternon to the copp
o Alwaysdrill with the coppersideuppermost;i.e., drill through the copperinto the board. o Always use a sharp drill bit (preferably a new one). o Alwaysusea backingof hard material under the boardto prevent the point of the drill from tearing a lump out of the backof the boardwhenthe point breaks through. o Spot the point to be drilled with a small center punch to prevent the drill running off its correct position when starting to drill. Use of an electric drill in a vertical drill stand is best for drilling PCBs. However, becauseof the small size of drill used,breakagerate of drills can be high if the work is pushedtoo hard. The original tracing comesin handy againfor marking the componentpositions on the plain side of the board, as a guide for componentassembly(Fig. 20-5). Componentsare always assembledon the plain side, with their leads pushedthrough their mountingholesuntil t}le componentis lying flush with the board (Fig. 20-1). The exceptionis transistors, which shouldbe mountedwith their leadsleft fairly long (andpreferablywith eachleadinsulatedwitha length of sleevingto prevent accidentalstrorting if the transistor is displaced).

Fig.20-5.Erampleof a pinted circuit dcsignwith coPferareaslnwn sladed and position indicated. of comfionents

lt%

MOUNTING AND SOLDERING Before mountingcomponentsin positionfor soldering,the coppersideshould be cleanedagain.It will probablyhavepickedup greasemarksthrough handling. An ordinarydomesticpowdercleaneris bestfor this, usedwet or dry, andrubbed on with a soft cloth. The running water test can againbe usedas a check for cleanliness,but if the board is wetted, dry with a cloth. Componentsare normally solderedin place,one at a time, with their full lead length protruding. Excesslength of wire is then cut offas closeaspossibleto the solder. Provided solderedjoints are completedrapidly, in not more than about 3 seconds,heatdamageto either the boardor a componentis unlikelp If the iron has to be held in contact with the lead for longer than this, then somethingis wrong with the solderingtechniqueandheat damagecouldresult, either to the componentor by the lifting of the copperpadon the PCB.The most likely causes of overheatingare usingan iron which is not hot enoughor too smallfor the job; attempting torework a solderedjointwhich hasnot takenproperly; andtryingto remove a lead which has been solderedinto the wrong hole. SIMPLIFIED PRINTED CIRCUIT CONSTRUCTION As a supplementto drawing-and for making neater straight lines-there rub-off transfer sheets of lines, bends, blobs for connecting points, etc. are available,which can be used to build up the required pattern on tlte copper, supplementedwith ink drawing where necessary.These transfer symbolsare resistant to etching fluid, so serve the samefunction as drawn or paintedlines. It is also possibleto buy self-adhesivecopperfoil precut in the form of lines, bends,etc, similar to transfer strips, but which canbe presseddownon to a plain panel to completea printed circuit directly, without the need for any etching treatment. Further shapescanbe cut from self-adhesivecopperfoil blanks.With PC boardsmadeup in this fashioncontinuous(conductor)sectionscan be made up from overlapping pieces, provided positive connection is made by solder appliedover the joint line.

1/t5

2L Radio at a specific Radiobroadcasts consistof a radiofrequency(rf) signdgenerated m audio frequencyallocatedto a particularstation,on whichis superimposed signal. @f) frequency Onlyrf workstor transmissioz.Theafpart,whichistle actr:alsoundcontent of the signalis, almostliterdly, carriedon the backof the rf signal,the two togetherformingwhatis calleda modulatedsigrnl. in hn'odifrerentways-amplitude,or signalcanbeproduced This combined (actuallya knownasAM; andfrequenrymodulation upanddownmodulation, knownas aboutits stationfrequencT), verysmallvariationinrf signalfrequency FM. AM is the simplertechniqueandis widelyusedfor longwave,mediumwave Broadcasting hasdwaysbeenreferredto in termsof andshortwavebroadcasts. recently.Therelauntilcomparatively insteadof signalfrequency, wavelengths frequency is: and wavelength tionshipbetween (meters): =300'000'000 wavelengtlrs frequency,IIz

IIz: frequency,

300,000,000 meters wavelength,

(Thefigure300,000isthespeedof lightin meterspersecond, whichisthespeed at whic,hradiofrequencywavestravel) 16

are used-and it is In the caseof FM, very hightransmittingfrequencies generallyreferredto asVHF(veryhighfrequency). Actualwaoelengthsare very short,andsoit is muchmoreconvenient to speakoffrequency,the usualrange for FM broadcasts being90- 100millionHertz(90- 100MIIz).A simplecalculationshowstlat thismeansa wavelengt!of about3.2to 2.9meters,or about3 meters. is AM or FM, anyradiofrequency signal Regardless of whetherthebroadcast receivingit. Thepresence hastle samebasicrequirementfor oftlissignalhasto stations.The be"found"andthensortedoutfromsignalsfromotherbroadcast "6nding" deviceis the antenna,and ttre "sorting out" deviceis the tuned circuit. Togetler, theyformthe front endof a radioreceiverasshownin Fig. 21-1(theextremeleft-handpart of a circuitdiagram-see alsoChapter18). A tunedcirarllconsists,basically, of a coilandrariablecapacitor,whichcan responseto a particularsignal be adjustedto showresonance or ma:ximum frequencyappliedto it. A full explanation of tlis behavioris givenin Chapter7. All the broadcast signalsreachingthe tunedcircuitarevery, very weak.Only thatsignalto whichit is tunedismagntfidbyresonance, sotlat it standsoutat a very muchhigherlevelof signalstrength. An actualwire antennaconnected to the tunedcircuit mayor maynot be necessary. In the caseof AM reception,the coilwindingalsoactsasanefrcient with the needfor an wire antenna,if woundon a ferrite rod. This dispenses is tlat thetunedcircuitis directional, externalantenna. Theonlydisadvantage mininumsignalstrengthbeingreceivedwhen theferriterodispointingtowards the transmittersendingthe signal,andma:rimumsignalstrengtl whentle on ferrite rod is at right anglesto this direction.This effectis mostnoticeable Toreceivecertain smallradioreceiverswhichhave onlymoderate amplification. Aerial

Y

CouplingCapacitorto MinimiseEffect of Aerial Capacitanceon Tuned Circuit

Coil Woundon Ferrite Rod

VariableCapacitor

Fig.2I-1. A oaiabh capacitorandcoilformtheusvaltunedcirttit. Strictlysfcahing this tuncstheantcnna,if an erternalaerial wire is uvd. MostAM receiarsue a fenitc rodanbnna whbhdoesnot requirean ertctzal anbnna. 147

stationsatgoodlisteninglevel, evenwitlmaximumadjustment ofvolume,itmay be necessary to adjustthe positionof the set. Laryerreceiversnormallyhave enoughamplificationto compensate for this, but the effectcanstill be quite noticeable. Also,it is alwaysbestto operatea receiverbelowmaximumampli6cationbecause this minimizesdistortionof the signal. The FM receiverdoesnedan externalantennabecause a woundcoil or a ferrite rod antennajust doesnot work at this rf. For satisfactoryresults,this externalantennaalsoneedsto bea specialt1pe,knownasadipole,whichitselfis tunedbymakingits lengthonehalfofthesignalwaoelength.The lattermayvary from11.5feetto 9.5feetin the90- 100MHzFM band,soa meanwavelength figureof about10 feet is adopted,glvinga realisticdipolelengthof 5 feet. ThethreepracticalFM antennaformsarea verticaltelescopic aerialextendingto 30inches;a horizontalwire (or rodantenna) with 30 inchlegs;or a folded dipole,asshownnFig.2L-2. DETECTION mayappear Thetunedcircuitis muchsimplerthantheforegoingdescriptions valuesright, andworking to imply.It is reallya matterof gettingthecomponent Designofthetunedcircuit withhighefficiency(see alsoChapter6 andQ-factor). is a little morecomplicated whena radiois intendedto receivemoretltan one waveband. EvenanAM receiverneedsseparateantennacoils(or at leastseparated wingson a singleferrite rod) to coverlong wave,mediumwave,and Sothetunedcircuitdesignfor anAM receivercouldinvolvetlree or shortwave. by a switch. moretunedcircuitsselectable In the caseof anFM receiver(or the tuningcircuitfor the FM sectionof a multi-bandreceiver),tJrcreis really no practicalform of woundantennacoil whichcanbe used(a theoreticalcoilof this typewouldprobablyrequireonlya part of a singleturn). Sothe startingpointis a dipoleantenna.This itself is a to be resonantwith the meanfrequencyto be hmedcircuit (i.e., desigrred

r

l

70-75 OhmsImpedance MatchingFeederCable

r

-

l

280-300OhmsImpedance MatchingFeederCable

Fig. 21-2 FM anbnnas tued connecting to tlv et aia a correct$rmaklvd fealcr cablc.

18

coveredin the FM band),but its amplification of signalis not nearlyasgoodas that of the coil-and-capacitor tunedcircuitof anAM receiver. To compensate for tlis, the FM receivernormallyusesan amplifierstage immediately followingt.heaerial,knownasapreamplifier or rf amplifier(because it is anamplifierof sigrralsat radiofrequency). Thisamplifiedsignalis fedto the next stageof tre receivervia a tunedoutput.A tjrpicalcircuit of this type is shownin Fig. 21-3. Thedetectorstage followingthetunedcircuitcanbeextremelysimple.In the caseof AM, it onlyneedsto bea diodeconnected to a potentiometer asits load. This potentiometer alsoactsasthe volumecontrol-Fig.2l-4. Thesignalpassed onfromthe tunedcircuitto the detectoris a strengtlened versionof the originalmodulated broadcastsignal.In otherwords,it contains bothafandrf. Therf parthasnowdoneitslobin gettingthesignalintothetuned circuit.Nowit needsto beremoved, whichcanbedonebyrectifyingttresignal.A diodedoesthisjobbychopping offonehalfof therf signalsothattheoutputfrom tltediodeconsistsof half-cyclesof rf. Thesehalf cycleshavetheaf contentofthe signalstill imposed, sothenextrequirement is to filter outtherf partto turn t.he outputinto anundulatingdc signal.Theseundulations followexactlythe same variationsasthe af signaloriginallyimposedontlre transmitterrf signalat the transmittingstationby a microphone, or recording. As orplainedin Chapter6, a resistorandcapacitorcanact asa filter for any specificfrequencyrequired.Thusthediodedetectoris associated with a matclringload(resistance) andassociated capacitorforming therequiredfilter circui$

Tunhg Capacitor

Output to Mixer

Itrputfro@DpoleAerial RFTransistor

Fig.21-3.fusic rf amplifier (orhreamblifur) circuit as usedin manl FM receioers. ltt9)

at outputfromthedetector e.g., seeFig.21.4- sothatonlyraryingdcispassed coupledto thene!ilstagebyacapacitor, whichhastbefurther stage.Itis usually thevaryingdcsignalaboutitszeroline(1e.,givingitpositive effectof balancing andnegativeralues,ratfier than"all positivevalues). In practice,theoutputload(R in Fig.214) is usuallyavariableresistor,which component tlen alsoactsasa volumecontrol.Thefacttbatthisisfollowedbya couplingcapacitoralsoavoidsanyflowof dcthroughtle movingcontact(wiper) noisebymovement d this of thiscontrolandreducesanytendencyto reproduce control isthatthe signalpassed Theaimin selectingthedetectorcircuitcomponents by the diodeis exactlythe sameasthe originalsignalgeneratedby ttre studio (withcertainlossesandpossible Fedto a microphone microphone distortions!). workinginreverse(i.e., headphones or aloudspeaker) theywouldbeheardasttre originalspeechor music.But the signalsat thisstagearestill tooweakto have or a loudspeaker, so the ne$ step is to enoughpowerto drive headphones by the detector. amplfy the af signalpa$sed FM DETECTOR It has In tle caseof anFM receiver,thedetectoris a little morecomplicated. to detecthowthefrequcncyofthesignatisvarying,notits amplitude, soit hasto aswellasapplyrectification. FM receiversinvariextracttle originalfrequency ablywork on ttre superhetprinciple,so the frequencyto be srtractedis the frequencyof if. A basicdetectorcircuitemploysa three-winding intermediate hansformerwithprimaryandsecondarytuned to theintermediate frequency Oy capacitorCl andC2in Fig. 21-5).Thethird windinginiectsa voltageinto the circuit,eachlegof whichcarriesa diode,Dl andD2,associated with a secondary capacitorC3 andC4. in terms Theworkingof thiscircuitis to detectvariationsin signalfrequency of an af output,so that the finaloutputis exacdythe same,in termsof signal content,asthatfromanAI\ddetector. Thusitcanbedealtwithbthesameway.

RF+AF Sisnal

Capacitor to Filter OutResidudif on Superbets

Conpling CapacitcHelps to Eliminate'noise' fromVolumeControl

VoluneControl

Fig.214 fuh AM detcctorcircuil

150

r-

t-

l*' L .

FA.zrc. fush FM detetorcitx*il Rl andC5 slpwn on tie dingranare to suppreso The additionalcomponents unwart€dsignalswhichmaybepresentafter detection AI}IPLIFIER STAGES A singletransistorcanprovideamplification of signalstrengthupto 100times (tr more(seeChapter9 for typicalanplifiercircuits).Themainrequirementof the amplifierfollowing thedetectoristhat thetransistorbespecificallysuitable (whenitcanactasafurtherblockto foramplifyingaf remainingin anyresidualrf the inputsignalto tle amplifier).Ideally,tlere shouldbenorf signalpresentat the inputto the amplifierstage(it shouldhavebeenfilteredoutin the detector stage),sinceanyrf voltagepresentedto the amplifierstagecouldcauseoverloading. Theoretically, at least,anyamountof amplification canbeproducedbyadding additionaltransistor-amplifier stages(Fig. 21-6).This does,however,greatly increasethe chances of distortingthe signal,sottrerearepracticallimitsto the numberofstageswhichareacceptableinsimplecircuits. Muchbetterresultscan be producedby more sophisticated circuits, particularlythe superhet(see below),wherefirst someintermediatesignalis amplifiedbforedefrcton;and amplifiedegaiqafier detection subsequently

is notnecesvi$ agod thingfor amplifuingan af Fig.214. Atkling anPlifnr stages sigwl ascuh stagecan anllib tlistortionlrodrced in thc fl.aniolrsstqe"

OUTPUT STAGE (orlastafamplifier stageoutputifmoretlanonestage Theafamplifieroutput power as in Fig. 21-6, drive a loudspeaker, to enough is used)can develop (amplifier) outputto the problem the in matching some may be althoughthere (oudspeaker) input,particularlyusinglow to mediumpowertransistorswhich havea loadimpedloadto match.Mostloudspeakers requirea highimpedance trandormerto to use an output here is solution A basic anceof only4- 16ohms. in Fig. 21-7. as matchthe differentloadcharacteristics, This relativelysimplesolutiondoes,however,haveoneparticularlimitation (for the moretechnicallyminded,it is calleda ClassA output).It is relatively listeninglevelfrom inefficientandsodrawsa highcurrentin providingasuitable represents tooheavya radios, but in car for use It is satisfactory theloudspeaker. receivers. currentdrainfor mostotler battery-powered a ClassB outputcircuitwherethelastamplifiertransistor Thesenormallyuse circuitoperating effectivelyworkinapush-pull drivesa pairof transistorswhich more tlnn double power is obtained considerably The output the loudspeaker. is not transformer and output froma singletransistor;also, the poweravailable push-pull output in a end up Most modernaf amplifiersfor radios neoess3ry. stageof this type,like the circuitshownin Fig. 21-8. limitaThelimitationsof simpleradioreceiversaremainlyconnectedwiththe input voltage of rf with an tionof a detector.A detectoris mosteffectiveworking

- 1

FW4

I I I

Eil^l

I

-

AFSignallnput

r-uuPrurs CouplingCapacitor I

Output Transistorr

Fig.21-7. fusic clas A otttfut citt*it. 152

- I tmoedance uitoing Traosfcm

I I I

I I

Coupling Capacitc

Irputo-l

Q1I

pecoupling

Fig. 218. fusb classB outputcircuit Ql actsasa |reamplifier. Q2is theilriter. Q3 and Q4area complemen"push-prll." Tle two resistorsshownby dudrcdlines may be tall fair of transistors,wothingalternate$in addcdto improw t te stabiliti of thc circuit. The* onfuneedto b of oerylow wlue (e.g.,7 ohm).

I volt or more.Signalsderiveddirectlyfromanantennacircuitareseldommore thana few millivoltsin strength,andthe weakerthe signalthe lesseffectively theywill bedetectedin anycase.In otherwords,therangeof stationsthatcanbe pickedupis limited,andnoamountof amplificationaftzrdetxttoncanrnakeup for this limitation. This limitation,or lack of vnsitioitl, canbe overcomeby amplifyingthe incomingsignalbeforedetection,so that the detectoris alwaysworkingwit} theantennasignalby goodsignalstrength.Thiscanbedonebyrf amplificationof introducinganamplifierstageright at the beginningof the circuitasin the FM receiver(Fig.21-3);or by the superhetworking.Thelatter alsoimprovesthe vlectility of a receiver,or its abilityto tunein sharplyto wantedsignalsand rejectnearbystationsignals. TIIE SUPERHET outputstage,it is equallytrue to saythat Havingarrivedat a standardized whichis considerably are nearlyall modernradioreceivers of thesuperhcttype,

153

more complicatedthan the circuit traced through above.The whole front end works on an entirely different principle. Starting point is the tuned circuit (ferrite rod antenna)in the caseof an AM receiver; or a dipoleantennafeedingan rf amplifrerin the caseof anFM receiver (the latter amplifying the modulatedradio signal in conjunctionwith a tuned resonantcircuit). In both casesthe boostedtuned signalis fed to an osillatormixer. This is a two-functioncircuit, althoughits duty is usuallyperformedby a single transistor associatedwith a tunedoscillatorcircuit. This tunedcircuit is mechanically coupledto the antennatuning in the form of a gangedcapacitor(i.e., two separatevariable capacitorscoupled,or gangedto move together when the tuning control is adjusted),sothat it tracks the aerialcircuit tuning while remaining separatedfrom it by a constant frequencl. This differenceis known as tJ1e intermediatefrequenq or i-f, andis usually455 kHz abovethe aerialfrequency (it canhaveother valuesin certain sets,andcanalsobe belowrather than above the antennafrequency). The oscillator part of the oscillator-mixeris concernedwith generatingthis fixed intermediatefrequency,tracking exactly above(or below) the signalfrequencyto which the aerial circuit is tuned. The two signalsare combinedin the mixer part of the oscillator-mixer,which alsolns a firedttned circuit (actually the primary side of a transformer associatedwith a capacitor)which responds only to the intermediatefrequency-Fig. 21-9. This i-f signalalsonow hasthe sameaf modulationas the original signal.In other words, it is a duplicateof the wanted af signal, but at this stage superimposedon a fxed intermediate frg.' quency. There are a number of technical advantagesto this seeminglyunneoessary complicationofincomingsignaltreatment.First, theprocessof superheterodyning gives much better selectiaitl or rejection of unwantedsignals.Then, t}1e signaloutput from the mixer is at a constantfrequency,makingit easyto amplify with the further possibility of eliminating any remainingunwantedfrequencies since an i-f amplifier has fixed-and virtually exact-tuning. In practice,i-f amplificationis usuallycarried out in two stages(AM receivers) or three stagos(FM receivers).The detector tlen follows after the i-f amplifier stages-Fig. :1-10. Each i-f amplifier stage consistsof a tuned transformer, adjustmentof tuning being done by an iron dust core in the transforrrer coil former. Oncecorrectly adjusted,ttre cores are sealedin this position. SeparateAmplifier Stages

Fig,21-9.Front endofa xrperlvt receiter,showinghowtlv inconing rf flus af signal is transformedinto an af signal nowimfuovdon a fired intermediatcfrequenclG-fl. Thismahesamplificationwitlnut distortionmoresimpleto achboe.

154

Fig. 21-10.Basicdesignofan AM/ FM receiaer slnwn in bloch forn. Theon\t common circuit is an af amplifier (usuallya classB output)followingthedetectors. The remainder of the radio circuit follows as before-rf amplifier stage(s) following the detector, terminating in (usudly) a push-pulloutput stage. But there is just onefurther refinementwhich canbe added.By feedinga proportion of the sigrralpassedby the detector backto the first if amplifierstage,automatic volumecontrol (normallycalledautomaticgaincontrolor agc)canbeachieved.If the signalstrength passedby the detector starts to rise to a point where it could become distorted, then feedbackvia the agc line automatically reduces tlrc amountof signalenteringat thispoint, somainteningthedetectorworkingunder optimum conditions. Agc appliesonly to the control of amplificationof signal in the i-f amplifier stages.The output or gain of the final i-f amplifier stage(s)is governed by a separatevolume control (potentiometer), typically located before the frst af amplifier stage. This potentiometer, incidentally, usually has a capacitorconnectedin parallelwith it toflter out anyresiduali-f whichmayhave got past tle detector.

155

22 Television Televisionmakesspecialuseof a cathoderay tube,whichin turn hascertain with avacuumtube(seeChapter12)'It hasaheater,a in common characteristics cathodewhichemitselectrons,ananodeto whichelectronsareattracted,anda .oottot gfid. Unlikea tube,however,the electronsaredirectedat the enlarged end - of tue tou" or &feenwhichis coatedwrthabhos,hormaterial. raytube.Thenarrowendof thetubeacts It is,in fact,aspecialtypeof cathode asanelectrongun,shootingelectronspasttheanodesection.Electronsimpingtheanode, screengeneratemoreelectronswhichareattractedbackto ingontJ1e dui*t"ot in eff; to eachelectronreachingthescreenbeingbouncedbackto andhencenocharge,actuallycollectonthescreen' theanode.Thusnoelectrons, Meantime,however,eachelectronreachingthe scteenmakestle phosphor badr' hasbeenbounced gfo*, -tt. *ni.n p"t irts for a shortperiodaftertheelectron phosphor of type on the brightnessof the glowproducedis dependent of (whichalsi governsthecolorol *re glow),andthe strengthof bombardment grid.Inother appliedtothe tle htter is controlledbythebiasvoltage etectrons. is the brightnesscontrolona TV tube(Fig' 22-1)' words,grid biasadjustment by ane:rtremelythin layerof aluminum tne adua brightnessis alsoenhanced mirror, but depositedou"i th" phosphorto act ratherlike an outward-facing transparentfrom thl othersideasfar aselectronsareconcerned. two other controlsare To producea picturefrom electronbombardment, o*""ory. The first is a meansof deflectingthe electronbeamsotlat a single spotcantraceout a particularpathcoveringallthevariationsin pictgredensity lffi

Fig.22-1.Ttc TVtub andiEbasiccontrols, slpwnin simplifwdfon.

is ameansoffocusingtheelectronbeam overtlte wholescreenarea.Thesecond into a tiny spotsothat the tracedpictureis sharp,notfiizzy. Defuctionisachteved bydirectingthestreamof electronsthroughtwosetsof parallelcoils setat rightanglesto eachotlerlike theXandYdeflectionplatesina simplecathoderay tube (Chapter12). Signalvoltagesappliedto the X-coils deflectthebeamsideways; signalvoltages appliedtotheY-coilsdeflectthebeam vertically.Combined X andY signalsthusdirectthebeamtowardanyspotonthe screm,depending on tlte resultanteffectof tlte two signals. Fuusing, meantime,is achievedby usingsupplementary cylindricalanodes arrangedto work asan electroniclens,with the focusingeffectadjustable by varying&evoltageappliedto one(ormore)oftheseanodes. Theseanodescome beforethedeflectionplates;ie., in theparallelor gunsectionof thetuberather thanin the divergentsection. Blectroniccircuitscanrespondveryrapidly-which is howtelevisioncanbe madeto workat all!To paintapictureontlrescreenaspotoflight(produced bya focused electronbeam) hasto traversetbewholepictureara,ag-zagfashion,at least30 timespersecondif thepictureis to appearreasonably freeof flicker.It doesthis in a numberof parallellines,usuallyrunningfrom left to rigbt, with rapidflybackbetweenlines,Fig. 22-2. The greaterthe numberof linesthe clearerthe picturewill be, i.e., tlre better the definition.The standardcommonlyadoptedin the UnitedStatesis 525 lines(per picture).The actualfre' quencyat whichlinesappear,calledthe line frequency,is 30 X 525: 15750 persecond. Thislinepatternis knownasa raster.Thebnes makinguptheraster canactuallybeseenif youexaminethetelevisionscreenclo,se up,or turn uptle brightnesscontrolwhennopictureis beingtransmitted.Onlythe parallellines canbeseenin the raster.Duringflyback,the cathoderay tubeis cut off andno linesappearon the screen.

=-r-__L-:, --t--

->--+

Fig. 2ZZ Illustrating tlw formation of a raster. First tlp dd nunbred linesare xanned from lefr to nSM (solitllines) withflybachfuhrceneachlinc (dadredlircs). Downwardmoaement is controlledb thetimobase.After xanning half thepicture lines,tlv timebavfies backtot tcto|. AA t teewn numberlinesaretlpn sanned, with flfiach futweeneachline (right handdiagram} At thcendof thelastline thctimefuy flics bachto tlp left to start thcvquenceall oter again. Thex tuo diqrams aQerimpo*d repre*nt a complctcraster. In practice the picture is scanned60 times per second,not 30. This is fast enoughto eliminate any trace of flicker, but using an optical trick, the actual picture frequezcyis still only 30 per second.Scanningtakes placein two stages - first the oddlinesonly, tlen the evenlines.Eachscantherefore buildsup only half the picture, the two halvesfollowing eachother to present the complete picture. Movement of the lines downwardsis accomplishedby the time fos circuit starting with the first (odd)line andrestarting a line at the left firmpositionsdown eachtime. This continuesuntil the scanninghasreached525 + 2 - 262rhlines. Thespotthenfliesbacktothetop again,startinghalfwayalongtlrefirstevenline andrepeatstlle processto scanthe 262r/z evenhneswhich makeup the second half of the picture. This processis knownas interlacing. Actrnlly a few lines get left out in this changeoverprocess,but this doesnot show up on tlte picture. Picture transmissionand picture reception operate in reverse mode. The television camerascansthe sceneto be transmitted in 525 lines at a picture frequency of 30 per second,and turns the light spot responseinto electrical signals.The numberof lines hasbeenquotedasgoverningpicture definition,but this is not the wholestory. A scanof 525 lines givesgoodpicture definitionfrom toptobottom, e.g.,the pictureisbuilt uptopto-bottomfrom 525 strips.Thereis also the questionof how manyindividualpicture elementsare coveredby each strip. The answeris about 600 as an absoluteminimumfor goodpicture definiThe orthe equivalentof600 phosphordotsmakingupeachline. tionside-to-side, totalnumberof individualdotsor pictureelementsineachwholepicturetraceis thus 315,000. Sincethe picture frequencyis 30 times per second,this callsfor a transmitted signalfrequency of.2.5 tvftlz. 18

\

Theseintelligencesignalsarebroadcast like anyotler radiotransnissionsuperimposed ot a carrier waoeto producea modulatedsignalwhichcanbe pickedup by a receiveranddecodedon a similarprincipleto ordinaryradio reception,exceptthat the decodernowhasto handleradiofrequencies of 2.5 MHzandnotafsignals,sotheTV receiverdecoderis considerably morecomplicatedthana radioreceiverdetector. Thereis alsoanotler importantdifference. Carrierwavefrequencies haveto bemuchhigherthanmodulation frequencies for satisfactory results.Hencettre frequencyof televisionpicturesignalsis in theVHFrange.It is invariablyanAM broadcast with sidebands, operatingwithinachannel widthof 8 MIIz. EitherAM or FM canbe usedfor the accompanying soundsignal(FM is standard). The fairly widechannelwidth or frequencyspreadoccupiedby a television transmission doesnotmakeit susceptible to receivingspurioussignalsupsetting the picture(but not the sound,whichis operatingin a narrowbandlike any ordinaryFM receiver).It alsolimitsthenumberof televisionstationsthatcanbe accommodated in the VHF bandwithoutinterferingwith eachother. Thisparticularconsideration alsomakesthedesignofacolortelevisionsystem evenmore complicated than it needbe usingfirst-principleelectronics.For example,it wouldmeane:rpanding the bandwidthto threetimesits black-andwhite figureto transmitthree separatepicturessimultaneously in the three primarycolors.Sincethisis not anacceptable solution,colorinformationbasto be containedwithin the 8 MHz channelallowedfor black-and-white transrnissions,whichbecomes anextremelycomplicated subjectandvirtuallyimpossible to describein simpleterms.Strangelyenough,however,it doessimplifythe requirement otler problem-the essential that a colortelevisionshouldalsobe ableto receiveblack-and-white transmissions in blackandwhite.Thebroadcast stationsstill havethe oppositeproblemof ensuringthat colortransmissions can bereceivedon black-and-white setsin black-and-white! Theconventional colorTVtubeis madewithtlreeguns,oneforeachcolorgreen, red, andblue;with eachdotonthescreenformedbyseparate red,gr@o, phosphors picture andblue arrangedin a triangle.The is thus scannedby a triangleof beamsconvergingon eachtriangleof dotsat the samerate asin a picture.The resultingsharpness black-and-white of the colorpicturedepends primarilyontheaccuracy of convergence, andonsometubesmayvarynoticeably from centerto edgeand/ortop to bottom.This maybe a limitationof that particulardesignof tubeandassociated circuitry,or merelya matterof conver(whichis usuallyfactory-setandcanbequitecomplex). genceadjustment There in thisrespectin thecaseof somemoderntubes;e.9.,simpliareimprovements problemsby usingin-linegunsandcolorspots. fyingcovergence

159

23 Microprocessors l l .

techof analoganddigitalcomputing Theyears1939- 45 sawthedevelopment electronic of appearance first tle era, "iq"J., ."4, towardsthe end of that *iiput"tr u.sedontubesandrelays.Theyears1948 50sawdigitalcomputers of the development significantly-tle *t"iti.n"A usingtubes,and-more transistor. thefirst Tenvearstater(1960),printedcircuitboardsfirst appeard,andalso Transi' circuits' integrated hybrid and circuits ..rU-*"f" monolithicini'e'grated circuits integrated Medium-scale tiofi.orp"ter circuitstdk overfromtubes. (LSI) in 1970' circuits integrated iMSO .ppu.tua in 1965,andlarge scale digital of aspects many ICs, of tt oooout this periodof rapiddeveloprnent oi the 6rst .otpufrtg or"r" irrth", extended,cuttinating in the appearance date,there tlnt From in 1971. computer,or microprocessor, micro-miniaturirea in micro'miniature circuits of digitalintegrated hasbeenintensiveaevJtopment period' iot tr,tougttDTL to TTi to MosFET andcMos' etc'Throughoutthis most wittr, improved, be to havecontinued tf,"..p.Uifltius of microprocessors of all, a fall in prices. sigrrifrcant - -Essentially, the microirocessoris a digitalcomputerin micro-rniniaturized digitalcomof a conventional with thefull capabilities form,but not necessarily programmable controller Logic il;;. -tpi|l o"" of its mainaiplications,io r.t ,:.. upwards so, or Tyithinthelastfiveyears r*u.ti*. industrialapplications. aswellas designedforindustrialuse, appeared, of o0differentPLCsystemstrave with its ownperipheral otherswith moreopencapabilitiel.Bactris associated (i.e., prograrnming devices)' software devicesandassociated 180

usingdiscretecompo asone-bitprocessoln, Originally,PLCsweredesigned or, in somecases, nents.Today,theyareinrariablybasedonmicroprocessors, anda one.bitprocessor.Someof the of both a microprocessor combinations morerecentlyintroducedtypesincorporatedul-languageprocessonbasedoa bit-slicetechnology. is its size-not its Oneof the mostimportantfeaturesof microprocessor physicalsize,but ttrelengthof the datawordit operateson andthe numberof is measured by tle nr.tmlanguage wordsits memorycanstore.Mcroprocessor word microprocessor ber of bit per word,a Ditbeinga binary digit. C,ommon lengtlrsare4, 8, L2,and16bits.An 8-bitdatawordis by far themostcomnon, andis givena specidmme-a byte. for onebit, it followstlat Because tlere areonlytwo pc*siblecombinations possible in a binarywordin a givenlengthis 2 thetotalnumberof combinations raisedto the Nth power,whereN is the numberof bits in a word.Thus,for an is 28-256. &bit word,the total numberof combinations sizeisthelengthofitsword,thisreflectson Asonemeasure ofmicroprocessor the sizeof ttrememory.The memorymustbe ableto storea givennumberof with a mema byteorientedmicroprocessor wordsof thislength.For example, orytlat canstoreupto 10 wordshas80bitsofstoragecapacityinthememory.It is accompanied by a memorry that a 4-bit microprocessor is generallyassumed with 16-bitwordsof memory. storing4-bitwords,anda 16-bitmicroprocessor by thenumberof wordsit It follows,therefore,that memorysizeis measured haslittle or ao maystore.Often,ttreIC whichis referredto asa mictoprocessor in otherICs. memory;the memoryfor tle unit is contained ontheaddressing memorycapacityof a PLCsystemdepends Thernaximum practice, to tlte maxiis related the maximum orpandability But, in capability. physical of control the complexity inputs and outputs, and also of number of mum is: 10-15 requirements for calculating memory A nrle of thumb algorithms. (or per for se' tle average application one output on instnrctions commands) quentialcontrol,with fewarithmetic,timing,or countingfunctions.Forapplicaperoutput controlalgorithm,15- 20instructions tionswitl amoresophisticated programs if memory space several mayberequired.Youhaveto considermore mode by a manual operation whichareselectable areto bestoredsimultaneously, production The machine. at one switch,for example,for frequentlychanging (one ranges from 256 words PLCs market onttre surximummemorycapacityof expressed as word corresponds to oneinstnrction)to 1600words(commonly from a minimum and expandable 1610andmore.Memorysystemsaremodular "starterset" of ylKin increments of.L/+K,lK,and4K. tbreebasicparts:the central comprises In simpleform,the microprocessor (CPU), processing always memory,andthe I/O devices.A microprocessor unit The I/O well as an device. memoryas containsa CPU,andin someinstances, (address) the I/O memoryof informationto eitlerthe CPUhastbeabilityto send (see Fig. 24-1). device beforedatamaybetransferredto or Justasthememorymusthaveanaddress from it, so muchthe VO devices.Normally,ttrereis morethan oneinput or outputdeviceona system.Therefore,theCPUmustdecidewhichoneit wishes to traasferdatawith, andtbis is doneby addressing.

maysetasideacertainarea Toexecuteaparticularprog&m,theprogrammer arebesetasidefor of memory program areas Other storage. for of thememory program.The for a different change may assignments datastorage,andthese positionof illustrates the 23-1, in Fig. form in schematic Texas510PC,shown storage, package memory includes which the total within t}1emicroprocessor (PDS). (IR) pushdown stack and register locationarea,image are madewith a plug-inprognunmer.This can All entriesto the sequencer readoutandmodifyalllocationswithinthememorystoragelocationarea.TVhm anentry is made,it is enteredinto andstoredwithinthis storagearea.Timers andcountersaswell asseriesandparallellogicare containedtherein. and controlsthe Model510PCmemoryduringprogramming The processor programmer are program. from the instructions Logic executionof tl1euser program (user scan, each At start of the memory). enteredintothestoragearea tlte statusof theinstructionintheregisterandtran$ersthe tlteprocessorplaces statusof the selectedoutputto the output(Y) lines. I/O modulesare commonlyavailableas plug-incardsor plugon boxesin and of 2,4, 8, 16, and32linespermodule.However,8-stagemodules increments with I/O cards of variety lGstage modulesseemto be the bestbuy.A broad Some levelsare availablefrom manymanufacturers. differentsignal-voltage (TTL);24Ydc; Logic typesof I/O modulesare: 5 Vdc Transistor-Transistor prefer48 Vdc; l2}l24Ydc;24 Vac;andL20124Vac.In practicalapplication, modules canbe I/O signalvoltage,sofaulty enceshouldbegivento onestandard replacedby a spareVO moduleof the sametype. Wherepossible, technique. I/O cardsis afast,sure,troubleshooting Swapping of thesamesigfraltypeshouldbeused,which controlelementsandI/O modules The outputcurrentloadper stockof sparecomponents. meansanine:rpensive line of variousoutputcardsrangesfrom 0.5A to 2A, but somePLCson the marketmayhaveaproblem-namely,notalloutputsonacardcanbeenergized Prograrnmablecontroller

Fig.2,91. Bluh diagran of Te*as510Programmdlclqic controller. 1@

the carddoesnot havethe curent-carryingabilityto because simultaneously persystem ofinputs/outputs energizealloutputsat once.Themaximumnurrber rangesfrom 32 for srnall,128for medium,andup to 4096for larges]'stems. However,at tlis point,it mustbenoticedthat a largentmrberof inputs/outputs, or largeprograms,mayheavilyreducetlte response, aswell ascorresponding cycle,time of an application.The distributedsystemsconceptmaybe more for a largeandcomplexapplication. appropriate or optionswhichallowcontinuous SeverallargerPLCshavesomecapabilities PID(Proportional-Integalprocess controlwith algorithmsbuilt-infor standard oflogic Differential)controllerswith analoginputsandoutputs.Thecombination controlandanalogloopcontrolmeansthat thesePLCscanbeusedforbatchor processing however,they are not ableto completely applications; continuous controllers.Usually,loopcontrollersare built as replacet5rpicalprocess-loop plug-inor plug-onmodulesfor extendedPLCsystems.OptiondavailableA'/D (Analogue-to-Digital Converters)accept, respectively,supply signals of 4-20mAor 0-5/0-f0 Vdc.Thesignalresolutionis ratler low.Precisemeasure. mentsare not possibleat Wpicallythree{igit BCD(BinaryCodedDecimal) value. toolsof a PLCsystem.NomiCounterandtimer functionsareindispensable nally,up to 64 internalcountersor timersarebuilt into mediumto large'scale PLCs.The time baseis generatedthrougha qnrtz oscillatorclockthat can commonlydelivertlree differenttime standards:0.1 s, 1.0s, and 1.0min. upanddowncountersand timers provideconvenient Three{igit cascading suchas For fast countingapplications, toolsfor a widerangeof applications. fastcountercardsarenecpo.sitioning separate transducers, or angle-indicating for usinginternalcountersis the countingfre e$sary.A criticalspecification the actualcount-rate quencyunderthe worst-case timingconditions,because very muchonhowthe programis structured;therefore,specialattendepends problems. tion mustbe paidto timing-related

163

24 Batteries, PowerSupplies, and Chargers some acquirespowerfrom equipment Virtuallyall portableelectricaVelectronic primarybatteries, Batteriescanbedividedintotwomaingrougs: tneof battery. actionis irreversible(oncethe battery hasbeen wherethe electrochemical batteries,wherethe elecandsecondary reenergized); it cannotbe discharged (theycanbe anddischargedrepeatedly). c.harged actionisreversible trochemical batteries Primarybatteriesarepopularlycalleddry batteries;andsecondary buttley correct, not strictly are arecalledstoragebatteries.Thesedescriptions pritnary form have some batt€ries In fact,dl andwidelyacccpted. areconvenient "wet" jelly); (usually paste secondary and many or in theformd a electrolyte of "d4y''cells(implyrnS theuseof a non-liquidelectrobatteriesmayhavesocalled cannotalwaysbe strictly tyte). And just to showhow generalclassifications (non-rechargeable) batterysystemsare,in praccorrect,sometypesof primary tice, rechargeable. DRY BATTERIES ERIMARY BATTERIES) A batteryis madeup of oneor moreindividualcells.Eachcell developsa on the batterysystem.To builda battery specificnominalvoltage,depending given by a singlecell, two or morecellsare that with a highervoltagethan in series. connected oncostandutilizationof ttrebattery. Choiceof thebestsystemsis dependent popularbecause theyarerelatively most byfar the dry batteriesare Carbon-zinc sizes.(The size voltages and range of cheapand readily availablein a wide Iil

ggvens the capacityof tbe battery,or tle amormtof electricalenergyit can store.) SECONDARYBATTERIES batterywaswidelyusedin the earlydaysof radioasa Thefamiliarlead-acid batterywhictrhasa signifrlow-voltagebattery.The onlytlpe of rechargeable batcantapplicationin presentdayelectronicsis the modernnickel-cadmium batterysptem in which"gassingf'canbe tery. It is the onetypeof secondary soitcanbeconstructedinfullysealed+ellform(althottgb eliminatedoncharging, manytypesare,in fact, constructedwith resealingventsasa precaution). duradvantages-nodeterioration Nickel*admiumbatteriesofiernumerous state(exceptthat a chargedcell ing storagein either chargedof discharged cycle suffersa lossof about20 percentof its capacityamonth);charge/rectnrge (depending possibly on charges of thousands hundred and least severd life of a (and high rates with vented rates charge for high discharge suitabliity actualuse); wideoperatingtemperaturerange(-40' to +60' C.);and cells);robustness; suitabilityfor operatingin dmostanyenvironrtent. in are highinitial cost(dthoughthis is generallyrecoverable Disadvantages per 1.2 volt* is voltage cell only nnominal longcyclinglife).andtle factthat the flaL are substantially However,the dischargecharacteristics USING BATTERIES To increase thevoltageof abattery,increasethenumberof cellsconnectedin seriesto rnakeup the battery.For a batteryof givenvoltage: Numberof cellsrequired-

VOLTAGE BATTERY REQIJIRED VOLTSPERCETL

is not a wholenumber,usethe nextwhole If the numberof cellssocalculated numberup.For ourple: Batteryvoltagerequiredis 9 volts C,cllsto be usedarenickel-cadmium Voltsper cellis 1.2

o

-frNumber ofcellsrequired

7.5cells

in series.(The actual firerefore, makeup the batteryfrom 8 cetlsconnected theadditionalvoltage batteryvoltageis then8X 1.2: 9.6volts.If necessary, canalwaysbedroppedin a circuit by nsinga droppingresistor.) To increasethe capacityof a battery,connecttwo (or more)batteriesof the in parallelhalvetbecurrent in parallel.Twobatteriesconnected requiredvoltage in fact,tle capacdrainfromeachbattery,thusdoublingthecapacity.Basically, ity of the originalbatteryis nultiplied by the numberof similarbatteriescoonect€din parallet ,6

OTHER DC POWER SOI'RCES with a bridgerectifier of a stepdowntrandormerassociated The application capacitorto providea lowvoltagedc supplyfromaaacmains anda smoothing supplybasalreadybeendessibedin Fig. &5 Chapter8. Rathermoresophisti' catedcircuitsmaybeusedwhereitisdesirabletoensureastabledcvoltage;a9, for operatingaaFM transistorradiofrom the acmainsinsteadd a battery. raluesspecified A circuitof this typeis shownin Fig. 2&1, the component gving a stabilizeddc outputof 6 volts from 120V AC. The bridgerectifier followingthetransformerprovidesfullwaverectification,smoothedbycapacitc manner. Cl in the conventional The input to outputvoltageis droppedacrcs hansistorQl. The emitter voltageof transistorQ2is setbytheZenerdiodeat 2.7volts.Theoutputvoltage is dividedbyR , R5andwhentle voltageacrossR5is about3.2rolts, Q2begins

F;g.2+L Stabili*do*tfut dccluryeropratiry frcm U0VACfuuel. funluants ac R7-ffiohns R2- Sil ohttts R3- Tohm R1- 39 ohmsfor 6 wlts dcutt lNohmsfor 7.5wlts tlc utt R5- ffiohms Ct- 1000micmfard 3il olnnfor 9 wlts dc utt C2- 70 micmlarad C3- 0,0l micmfarad T - Lil| 15 oolttransfonur BR: bridgcratifiu Ql- 5K3190 Q2- SKSI!44 Q3-SK I ZD- hpriliode - SI{2V7

166

to conduct.Thisdivertssomeof tlte currentflowingthroughRl intotle baseof Ql sothat Ql skrts to turn off. Thus,sincethe basecurrent,andtle voltage junctionof Ql, is controlledby Q2,theortput dropacrosstle collector-emitter voltageis stoppedfrom goinganyhigherthanthe designvoltage. Conversely, if a heavyloadis appliedto the outputit tendsto causea dropin outputvolkgeandso alsothevoltage onthebaseofQ2tendstofall.Theeffectof thisisthatQ2startsto turn off,allowingmorecurrentto flowintothebaseofQ1 whichturnson to maintainthe outputvoltage. ismaintained Voltagestabilization untiltheoutputcurrentrisesto theorderof greaterthanthe 400-500mA. At this point,the voltageacrossR3 becomes fromthebaseof voltage starts This taps curent turnon of Q3,which to conduct. Ql, causingQl to start to turn off, reducingthe outputvoltage,so tlnt the currentdoesnot rise anyfurttrer.In otherwordsthe circuit is automatically protectedaginst overload,evendownto shortcircuitconditions. In the latter maintrined case,tle voltagewil fall to almostzero,with the curent still at 400-500milliamps. Thecapacitors C2andC3arenotstrictlynecessary,brutare additionalsmoothingcomponents. Thecircuitcanbeadapted to provideanumberof differentdcoutputvoltages, selectable byswitching.To dothis,resistorR4is replaced byachainof resistors

DCOutrut

Fig.2tL2 Switchingcircuit for alteing dc outputooltage. R4A: 39 ohms R4B: 150ohms R4C- 750ohms 7d7

R4A,R4B,R4C,asin Fig. 24-2.Thevaluesgiven,togetler with the previous circuitcomponent values,provideselectable outputsof 6 volts,or 9 volts,with voltagestabilizedin eachcaseup to a maximumcurrentdrain of 400-500 milliamps. BATTERY CHARGERS Eitier a bridgerectifiercircuit,or thevoltage'stabilized circuitjustdescribed, canalsobeusedfor batterycharging.ln thiscase,smootldng is notsoimportant, asthe presence of a certainamountof ripplein the dcis heldto bebeneficial for charging.Normally,however,at leastonesmootlingcapacitorwouldbedesirablein the chargercircuit. Sinceit is notalwaysevident whetherachargerisworkingornot,anindicator lampor ammeteris normallydesirablein a chargercircuit. A lampmerely indicatesthatthechargerisonandtheoutputcircuitisworking.It canbetapped directlyacros{rthe circuit at anypoint.The preferredform of lampis anLED sincethis drawsminimalcurrent,althougha smallfilamentbulbwill doaswell. An LED needsto be associated with a ballastresistorto dropthe necessary voltageat thispoint;afilamentbulbdoesnot,butaresistoris needed to workasa voltagedropperifa &volt bulbis usedin thiscircuit(seeFig.24-3).Notetlratan indicatorlampon the mainssideof the trandorrrer, or on the secondary side

Fig,24-?.Alternatioearrangements for clurgitg indicabr lanpa A b& ofafupnyiab dtagc ottld b *sedis otu of tb uttptt bads uithott a ilrofping nsitur. 1A

Yotts ttrput

Fig.2+1, Stobiliud6d, dcclurgero|cmtingfmna 12ooltfuttery.&nfulrlnts an: R7R2R3RlR5ClC2eI-

N dws Mohtw Iohn #)ohtrc dildms 70 nicmfarads 0.0l micrclards SK3t90

e2: SK3&l Q3- SKi],!14

ZD- kwrdido- SK2V7 CRl: LNffit(orc$tiulen[ betweenthetrandormerandthebridgerectifier,wouldnotnecessarily codrm tbat the outputwasworkingwith anoutputloadconnected. In thecaseof a meterindicator,tlis wouldsimplybeanammeter(or milliar metetr,asappropriate) connected in eeriesin onec anotler dthe outputlines.

1N

DC INPUT CHARGERS Thereis alsoa callfor chargerswhichcanchargelowvoltagebatteriesdirect fromanotierbattery,suchasa l2-volt carbattery.In thiscase,sincettreinputis dc,a transformercannotbe usedto set the requiredvoltage,nor is a rectifier neoessary. 6-voltdc to providea stabilized Figure24-4showsa chargercircuitdesigrred it is thesame voltage)froma l2-volt inputsupply.Essentially, output(charging asthat of Fig. 24-1withouttlretransformerandrectifier,buta diodeis included to protectthetransistorsin thecircuitagainstreversevoltages.T[orkingofthe circuit is the sameas that describedpreviously,with automaticshort*ircuit protection.Like the previousmainscircuit,too, it canbeadaptedto providea rangeof outputvoltages,usingexactlythesamevaluesfor thechainofresistors asin Fig.24-2.

170

25 High-VoltagePower Supplies Thebasicwaytoobtainahigh-voltage supplyisto stepupanacmains supplyviaa transformer.At thesametime,thetransformercanbetapped,or providedwith separatewindings,to produceanyotherloweror intermediate voltageswhich mayberequired.Voltagestepupor stepdown,usinga trandormer,is po*sible ooly with an alternating-current input. The resultingoutputis alsoac, from whichit followsthat further components are requiredin a powersupplyto providestepped-up or down direct-currentvoltages,suchasrequiredfor the anodeof a tube.Basically, thisinvolvesrectificationof tlretransforrredvoltage, with theadditionof smootling,if necessary, to removeanyremainingripple,in the dc output. voltageregulationmayalsobenecessary, evenif it isonlyaimedat limitingthe valueof transientvoltageswhichmaybeintroduced in thepower-supply circuit. In thatcase,weareconcerned withthepeakinversevoltages(piv)whichmaybe introducedin thepower-supply circuit,affectingtheloadingof t.hecomponents. Voltageregulationitself canbe orpressedasa percentage:

regulationtml:ff El is thenoloadvoltage(nocurrentflowingin theloadcircuit),andE2is thefull loadvoltage(ratedcurrentflowingin loadcircui$. Threebasicrectifiercircuitsareshownin Frg.2*1. Asingtediodeprovides

Bridge

Half-Wave

Full Wave

Fig.%-1. Threefusic rectifnr circaits

half-waverectjfication. Two diodescan provide full-waverectification, with the circuit completedthrough the transformer center tap. Alternatively, the bridge tpe circuit can be usedfor full-wave rectification. Either tubes (diodes)or metal rectifiers can be usedin such circuits. Metal rectifiers require no heater supply, but have to be fitted with cooling fuis to dissipatethe heat generatedby their relatively high forward resistance.Tubes alsoget hot, andboth needplenty of spacewithin the cabinet,andgoodventilation. Power suppliesof these types, therefore, tend to be heavy and bullry. Siliconpower diodesare generallypreferred to tubesormetal rectifiers. They can be producedin virtually miniature size,require no heater current, and have relatively low heating (and thus much higher efficiency),becauseof their very low forward resistance.TVhilethis ltter feature is highly desirable,it doesalso emphasizethe potential weaknessof the silicon diode.High voltage surgescan developwhich may destroy the diode.This is becauseof the relatively low piv valuessuchdiodescanwithstand. Unfortunately, too, silicondiodesalsoteod to 172

failin ashortedcondition,ratlertlan open,sothattailureofonediodeinaseries couldreadilycausethe remainderto fail aswell. Seriesconnectionof silicondiodesis generallyneces{nryto realizethe piv by the rating required.This is determinedby the piv likely to be developed rectifiercircuit.In the caseof a singlediodecircuit,the piv acrcssthe diodeis 1.4timestheacvoltageacrosstle transformercoil.Thecenterapproximately tap circuityieldsa piv of about2.8 timesthe acvoltageacrosseachhalfof the transformercoil. The bridgecircuit againyieldsabout1.4 timestlre voltage acrossthe coil. Therequiredratingcanbebuilt upby connecting asmanydiodesin seriesas necessary to factortleir individual ratingby2,3, 4 times,etc.,allowingasuitable in marginof safety.This,however,is onlyvalidif thediodesareexactlymatched (particularlytheirreverseresistance). Thisisunlikelyin practice, characteristics andso equdizingresistorsare normallyconnectedacrosseachdiode-Fig. 25-2.Alternatively,equalizingcapacitorsmaybe usedin somecircuits.Both incidentally,dso act as transientsuppressnrs to protect the configurations, aremoreeffectivein this diodesagainstsurgesof highcurrent.Sincecapacitors respect,resistorsandcapacitorsmaybe usedin seriesacrosseachdiodeas the equalizing/damping devices.Furtherprotectionmayalsobeincorporatedin rectifiercircuitbyincludingafuseto open+ircuitachainof diodesin theeventof overload,or failureof oneof tlte diodes. is to balance whichmaybenecessarywith silicondiodes Oneotherprecaution their rating againsttemperature.Althoughthey do not generatemuchheat is temperaturedependent, andthe ma:rimum tlemselves,their performance ratingapplieswith a temperaturelimit. If they are to be workedat a higher is necessary. Temperatnres ambienttemperature,deratingtheir performance ratingmayrangefromas lowas25'C to ashighas 70"C, fora maximumcurrent Derating,tlryically,is of the orderof 10 depending on tlpe andmanufacture. percentper 10"Ctemperatureriseabovethe ratedtemperature. FILTERS Theoutputfroma rectjfiercircuitis pulsatingdc.To renderthisin theformof thismaynotbestricdynecessary for smoothdc,filteringmustbeapplied.TVhile it isabsolutely necessary tubeoperation, to eliminate(oratleastreduce)thehum stagesof atransmitterorreceiver contentoftlepowersupplyappliedtovarious circuit. Effectivesmoothing of the supplyis readilyachieved bymeansof a capacitorinputflter, whichmaybegithsl singleFs€ctien or two-section-Fig. 25-3.The

Diode

SiliconDiodes

Fig.2tt-2 Didcs with equalizittgnsistors

tn

4.,

Single.Section

-

-{.-

R is BleederResistor

TweSection

Fig.zffi. Singlcand ttwwtion ntnthingfiltera for radiotransmitters,but tlte twofilter is generallyadequate single-section sectioncircuitispreferablefor radioreceivers.Theadditionofa bleederresistor to discharge thecapacitorswhen thepowersupplyis is generallyrecommended, itdrawsl0 shouldbechosensothat notinuse.Thevalueof thebleederresistance percentor lessof the ratedoutputctrent of the supply.(It canbe calculted directlyasf 000E[ ohns, whereE is theoutputvoltage,andI is theloadcurrent in milliamps.) and The ripplevoltageremainingis governedby the valuesof the capacitors (although Cl can be inductance.Typicalraluesare 8pF for the capacitors reducedto 4 pF in the two-sectioncircuit),with an inductanceof 20 to 30 andinductanceare made henries.Ripplevoltagegets srnalleras capacitance ralues,andsatisfaclarger.Fewproblemsareimposedin matchingcomponent 6lter circuit,however,extory smoothingis readilyobtained"Capacitor-input propertieswhenusedwith wrying loads. hibit poorvoltageregulation Ttre choke-inputfilter providesbetter voltageregulation,but lesseffective Again,it canbesinglesectionor twosection-Fig. 25-4.Thetwosmoothing. Noteagaintheuseofa sectioncircuitisgenerallysuperiorasregardssmoothing. bleederresistorto dischargethe capcito(s) wbenthe power$Dply is not in use. can,with advantage, beof thesringingchoketype-that fire fust indictance overarangeof about5 to 20henriesoverthe is,havingswingingcharacteristics thereis nooutput full outputcurrentrange.Thehighestraluethenapplieswhen loadon the powersupplyother than the bleederresistor.Thesecondchoke shouldthenbavea constantinductance of 10to 20 henrieswith Erying dcload currents. With this t)'pe of circuit, it is possibleto usecapacitorswith lower rated for a capacitor-input filter (whichhaveto havea voltagethanthosenecessary ratinghigherthanthepeaktransforrrervoltage).However,a similarhigh-volt-

Fig.2e1^Clrotu-infut filte* 174

ageratingisusuallyadvised, asintheeventd&ihre dthe bleederresiEtorthe voltageswouldrise to thesepeakfigues. OUTPTTTVOLTAGE Basically,the dc ortput voltageis about0.9 timestbe acvoltageacrosstbe tran$orner secondary in the caseda singlediodeor bridgecircuit;andabout 0.45 timestle acvoltageacrosstle trandormersecondary in the caseof the bridgecircuit.Withcapacitor-inputfilters, thesecondaryrmsvoltagerequiredis thts 1/0.9or 1.1timestherequireddcoutputvoltage,to allowfor voltagedrops in the rectifierandfilter circuits,andin the transformeritsef. Io the caseof a center-tapped circuit, this voltagemust be developed acrosseachsided the secondary centertap. With a chokeinputfilter circuit followingthe rectifia, the requiredtrans. formersecondary voltagecanbecalculated directlyfrom:

E-1.1(t"*S*t*t) wtere: E - full loadrms secondaryvoltage Eo- requireddc outputvoltage(The opencircuitvoltagewill usuallybe anythingfrom5 to 10 percent higher.) Er: voltagedropin tbe rectifier Rl, R2 : resistancein fitter chokes

VOLTAGE STABILIZATION is bytheuseof a voltage'regA basicmethodof obtainingvoltagestabilization ulatingtubein serieswith a limitingresistor,asshownin Fig. 2S5. The initial (unregulated) voltageneedsto be higherthanthe startingvoltageof the tube, whichis nsuallyabout30percenthigherthantheoperatingvoltage.Theralueof thelimitingresistoris chosentoiust passthema:dmum tubecurrentwhenthere is noloadcurrent.Withloadadded,thetubecanthenworkdownto its minimum cnrrentcondition.Withinthisrangethevoltagedropofthetubeisconstant, thus providinga pointfor tappingoffa stabilizedvoltage.Voltageregulationbetter than10percentcanreadilybeachieved; andwiththetubesin series,stabilization is frrther improveddownto aboutI percent.Theuseof nro tubesin seriesalso enablestwo differentvaluesd regulatedvoltageto be tapped,onefrom each tube.

staffilization with VRtub. Fig.25-5.Voltage Figure25-6showshowZenerdiodescanalsobeusedto stabilizea high-voltdiodes(Zl afi agesupplyobtainedfroma transformer.Thelow-voltageTnrrcr configurationacrossa low-voltage 22) are simplyconnectedin back-to-back windingon the transformer. BIAS VOLTAGES to arebasicallyafixedvoltageof therequiredr"alue Bias-supplyrequirements setthe operatingpointof a tube.Theoutputshouldbewell-filtered,andcapacitor-inputfilters are commonlypreferred.A bleederresisteris effectiveas a

e-r

I

ac Inzut

o

I -

1

Fig. 255. Stabilintion b Zencrdiodcs. 176

Fig.25-7.Stabilimtionb Uasooltage. voltageregulatorsinceit providesa dcpathfromthe grid to thecathodeof the tubebeingbiased.However,to bereallyeffective,tlis needsa low resistance valuesothatthecurrentflowingthroughthebleederresistoris severaltimes the maximumgrid currentto be expected,whichis wastefirlof power. In particularcases,therefore,it maybe expedientto adoptmoreefficient methodsof bias-voltage Two suchstabilizingcircuitsareshorrnin stabilization. Fig.25-7. Oneusesa triodeasaregulator,andtle otier aVRtube.Thelatteris onlyapplicable wherethe voltageandcurrentratingsof thetubespermittheir application VOLTAGE DIVIDERS Theconventional t1ryeof voltagedivideris basedonthecircuitshownin Fig. 25-8.Basically,it comprises a seriesof resistors(or a resistorwith a seriesof tappingpoints),fromwhichvoltageslowertbantheinputvoltagecanbedrawn

Fig. %A" Conuentional ooltagediaider.

ln

by connectingto an appropriatetap. The end resistoris only as a bleeder, carryinga bleedercurrentwhichis normally10percentor lessof the total load current.The raluesof resistorsrequiredtlen followfrom:

*t-fl nz- E=2191 15t11

na:fr1ff F-E'9

the Voltageregulationis very poorwith voltagedividersof t}is typebecause onthecurrentdrawnfromthetap(andwitt voltagetakenfromanytapdepends thusvary with varyingload).Thus,while they are suitablefor constant-load applications, additionalvoltageregulationwouldhaveto beappliedfor stabilization with varyingloads. YOLTAGE MULTIPLIERS in integerfactors-a feaRectifierscanalsobe usedasvoltage'multipliers, for example, It is possible, to accept ture whichcanoftenbeusedto advantage. anacinputdirectintoarectifiercircuit,witlrouthavingtoemployatran$ormer, andobtainbothrectificationandvoltagedoubling.Sucha circuitis shownin Fig. to the samedcvoltagefromtlte two 25-9.Eachcapacitoris chargedseparately intothesameloadcircuit(t}usdoublingthe diodesandthen dischargedinseries dc outputvoltageobtained). Figure25-10showsanextensionof this principle,utilizingfour diodes.The outputfromthiscircuitprovidesbothvoltagedoublingandvoltageqrudrupling. Aswith voltagedividers,voltagemultiplierstendto offerpoorvoltageregulation, althoughthis is lessmarkedwith silicondiodesas comparedwith diode tubesandmetalrectifiers.

iv

I Fig.2e9. Voltqc+nultiflier circuit 178

Fig.2en Vottagc-;attillb?ciruit *sittgdio&l

VANHBI,E-VOLTAGE STTPPLIES A simpletypeofvariablevoltage power supplyfc usewith aconstant-voltage supplyis shownin FA. 2F11. Thiscircuitelirninates seriesresistorsasa sottroe maiqtrins4 substantiallyconsant of voltagedropand,asaconsequence, sortrce inpedance.Voltageregulationis alsoprovided,aswell asvoltagernariation vh the variableresistor,althoughthe degreed stabilization deteriorateswith increasi4gvdtageor@uLIt is, however,anotler emmpled howsimplecircuits canoftenbeusedto providesolutionstoparticularrequirenentsintransmitter and/orreceivercircuit&

t

I I I I

srntt I I I

I I I

Fb. 2&1L Yariable ooltqesuttlt 1?9

o__ ac Input

{

-{ -{

I #

Fig.%-12. Stabilizedheatersuppll.

STABILIZED TUBE HEATER SUPPLIES The heatersupplyfor tubestendsto beregardedasnon-critical,andconve. nientlysupplieddirect from a separatelow-voltagecoil on t}re powertransit is generallydesirable former,withoutrectification.Asa minimumprecaution, to useseparateheatersupplies(e.9.,separatetransformercoils)for oscillator maywell be considered asa methodof firther tubes,andvoltagestabilization improvingthe overallstabilityof the stage(s)involved. possibilitiesinthisrespectisshown A simplecircuitwhichoffersconsiderable bridgecircuitconfigutwoZenerdiodesin back-to-back in Fig.25-12,employing ration.Thepotentiometer actsasa trimmerto setupthecircuit,its valuebeing about20 percentof the total resistancevalueof the lowerarm of the bridge. of betterthanI percentis claimedfor this circuit,witl a Voltagestabilization transformervoltagechangeof up to 13 percent.

+

6.3V

t I

6.3

I

Fig.%-13. Transistorpoucrsuypll. lU

Fig. 25-14.Sinfuleregulationcircuit.

TRANSISTORPOWER SUPPLIES simplify Transistorcircuitsrequireonlylow voltagesandthusconsiderably particularlyasonlya singlevoltageis usuallyrepowersupplyrequirements, quird. They may,however,be fd from an ac supply,in whichcase,similar requirements applyasregardsrectificationandsmootlingfollowingtle trans. (theZenet a Zenerdiodeis norrnallyemployed former.For voltagestabilization, diodeis virtuallytle counterpartof the VR tubein higbervoltagecircuits). A typicalmoderntransistorpowersupplycircuitis shownin Fig.2S13,which is alsonotablefor incorporatingelectronicsmoothing.There are numerous variationson a similartleme but, in general"shuntregulationis takinggr€ferenceoverseriesregulation,astlris will permitt}e outputto beshort
I Stabilized Output

I I Fig.%-15. Sinple lowlossstabilizingcircuit. 181

is lessreadilyprovidedactossa directbatteryfeedto ahansistor Stabilization usingZenerdiodesandresis. methodsof stabilization circuitsinceconventional torsalmostinevitablymeana largeincreasein currentdrain,figther loadingthe to combatthis,suchas solutionshavebeenproposed batteries.Variousingenious the use of current-limitingcircuits(whichalsosafeguardtransistorsagainst circuit,basedaround overload).Figure25-15showsa simplelow-lossstabilizing theuseof atransistorasaconstantcurrentdevice,whichcanreadilybeextended to two stagesif necessary.

1&

Appendix: Symbolsand Equations fire followingequations areusedoftenin the fieldof electronics. In additionto equations discussed in the previoustent, this sectionalsocontainsmanyother equations for futurereference.

Sfmbol A B

c

D d

dB E F t G H

SIMBOLS USED Meaning Iength of thesideadjacent to 0 in theright triangle,in sameunitsas the othersides (measured in seimensor mhos);thereciprocalof reacSusceptance tance (measured in farads);Celsiustemperature Capacitance Dissipation factor;reciprocalof storagefactorQ in centiThickness of thedielectricmaterialin acapacitor(measured meters) Decibel;the ratio betweentwo amountsof power in volts) Electromotive force(measured Temperature(measured in degreesFahrenheit) Frequency(measured in hertz) (measured Conductance in siemensor mhos) Lengthof the blpotenusein a right triangle,in sameunitsas the othersides 183

Current,in amperes in ioules) Energy,work, or quantityof heat(measured Couplingcoefficient,dielectricconstant,G t€mperaturein kelvins (measured in henries) Inductance (measured in henries) Mutualinductance Generalsymbl for numbers LengthofthesideoppositetoOinariglttriangle,insameunitsasthe other sides Power(measuredinwatts) Powerfactor Qualityofaninductor,orquantityofelectricitystored(measuredin coulombs) Resistance(measuredinohns) in squarecentimeters) Areaof oneplated a capacitor(measured (meastredin ohms);measuredoppositionof a circuitor Reactance to an alternatingsurrenl component in ohms) reactance(measured Capacitive in ohms) Inductivereactance(measured thereciprocalofimpedin siemensormhos), Adnittance(measured crrrent in a flow d dt€rnatilg to the ance;tle lack of oppositio reactivecircuit (measured in ohms),the reciprocald admittance;the Impedance oppositionto the fow d alt€rnatiqgctrrent in a reactivecircuit 9G0degrees in degrees)in a right triangle,simplyan Phaseaqgle(measured angle (measured in meters) Wavelength 3.1416... Cutofrfrequency

I J K L M N O P pJ. a R S X X X Y Z d e X n etc

ADMITTANCE 1 (t)Y:16f7 1

(2)Y:2 L

(3)Y:GqEu AVERAGEVALUE (1) Average rralue:0.637(peakwlue) (2) Average vralue) value:0.900(R.M.S.

1A

CAPACITANCE (1) Capacitors in parallel: Crmrl:Cr*C2+C8 . . . etc. (2) Capacitors in series CmrL:

rsrq +.*.+..

(3) Twocapacitcsin serieE c*.. - CtC' -tu'&

q+Ct

(4) Capacitance da capaciton

C:o.o885IqP (5) Quandtyof electricitystore& Q-CE CONDUCTANCE

(1)G:it (2)G-;

I

(3) GmrAr: Gr+ Gs+ Gr . . . etc.(Resbtfisinpardlel) (4) IprAL:EG1m11 COSECANT ( 1 ) c - s0c = E

o

(2) csc 0 = sec(904)

( 3 ) c sr =c f r1 COSINE A

(f) coe0:E (2)m0-sin(90-O

(3)cos0:&

1

,ffi

COTANGENT A

(1)cot0-6

(2)ot 0-tat (90- 0) 1

(3)cote-ffi DECIBEL

P.

(1)dB- t0 log#

Ez

- 20 log*t (2) 8dts areequal andloa impedance - D , 2 O"t if source I.

(3) dB- 20log* Onlyif source areequal andloadimpedance rt

(0 dB- zoblyf9Source arermequl andloadimpedances Ez{Q' areunequal andloadimpedances A - 2glor]&source O(5) lz'lZs FIGT'NEOF MEruT (1)Q-tan0 Y

(2 )Q: p

FILTERS,ACTTVE utdband*top. bandpass, lowpass, Therearefourbasicfiltertypes;highpass, "constant-K' practical fc and are flter typesarereferredto as Thefollowing mootpurpooea Low-Ba$andf,igb-pass _ l _ R t-a c-fr rvlere: CLRco.-

t6

in farads capacitance in Henries inductance - frd nominalterminatingresistance OttOfffreqrency- 2 rf" - 6.28f"

Amplitude

FA.A-Z Lo*|assfltercfnbn fnq*mey

Frequency--;

Banrtpars @ z - @ t

,"t:EF A

C' , :

.l r r '

1 R(c+ - cor)

R

--

@z-(lt

R(ar-ar)

-

t":T

rvhere: C1Ca: Lr : La R:

Seriescapacitance in farads Sbuntcapacitance in farads Seriesinductance in henries Shuntinductance in henries - [ffi= Nominalt€rninatiqgresistance

frequency
- #r-

fW

2xf"-6.28q

: Iowerortofrfrequency X 6.28 - Uppercutofffrequency X 6.28

Bandctop A

I

!,-mTG'

.

4-T

Fig.A& H$-Wflter

R(crlr-c,lt)

circui| 187

Amplitrdc

I

Wc

Frequency-*

FA.A1 Hlgh-/rss filter+tut oilfn$.erw

-

Cr-ffi @c

QJt

-

Lt:-

R @z -

Qtr

where:

: (@
- U!E;q- - [email protected]

ue the sameaEband-pas* All otherespressions FILTDRS ACTTVE anpfificr, Activefilters incorpcateanactivedevice(transistor,operadonal etc.)to replacetbedificult to fnd inductors.Actirc filtersaresmallermdeat&r to modifyfor specificneeds.Theycanatsofovide high€rQ valuesrhantheir passivecounterpartsThefollowingequations toffi andtablesarreapflicable anplifiers(indtdingFEtr tJ'pes). IC operational Lm-PassFiltsr filt€r. Th€valuedCl FigureA-9i[ustratesa ommonformd activelow-pass is determined bydividingthedesiredcut
Fb. A.A fund-fu$f,lbr ciro*it 18

t

I

- - TI- -

Anplitudc

.l

I I Wr

Wo

W.

--fD Freqnency

Fb. AA fund-fus fltcr+tat ot fnryeaey.

Componcnt

For Gainof2

FcGainof 1O

Crtn/F) Rr0nKO)

0.150X vahnofCl 1.612X scalefrctfi 9,223Xscalefacttr 2.068X scaleftctor

X Yalue 0.0:13 dCr 1.021X scalefrctos l0.2ll X scalefrctc 2.96tX scahhcttr

&(rnI(o) Rs0nKo)

Erample: Desired fc : 500Hz;@n- 2

q:#:

o'oz1ts

- .003/F Cr: (0.02)0.150

scalefacror-#-#: r8.9 : 25.63k0 f,, : (1.612)15.9 - 51.24k{} Rc- (3.223)15.9 - 32.88kO Rr: (2.068)15.9

r89

Amplitude

wt

w"

Wr

--> FrequencY

FA. 48. fund.tto/filtcrefat onfnqtercy.

f,igh-Pass Filters flter. fite ralueof Figrre A-10illustratesa commonformd activehigh-pass q$ofrfrequency ({,) into 10.Theresult the desired by dividing is determined Q C1 Nsrt, multiplytheralueof inmicrofarads. this calculationistheralueof of Q factor. product This result is the scale into 159. this fc and divide with thedesired of theremainingcomponents The scalefactoris neededto calculatethernalues from tle followiqgtable:

Component

For Gain of 2

Cz0n/F) Cs(inpF) Rr (in k0) Rr (in kO)

0.500X valueof C1 f .000X valueof Ct 0.566X scalefactor 3.536X scalefactor

190

For Gain of l0 0.100X ralueof C1 1.000X ralueof C1 0.673X scalefactor 14.849X scalefactor

Fig.A-10.High-|a$filtcr.

Exarple: fc - 50llz; Gain:2 Desired to

Q:fr -o'02 tE

Cc: (0.02)0.500:0.01p1t - 0.02tE Cc: (0.02)1.000

scalefrctor-H-f;- ts.e : 8.99k0 &: (0.566)15.9 f,r: (3.536)15.9:56.22ka BandpassFilter FigureA-11illustratesa cornmon formof activebandpass filter. Thevalueof (fo)into 10.Theresult Cris determined bydividingthedesiredcenterfrequency dthiscalculationisthevalueof Cl inmicrofarads.Next, multiplytderalueof C1 with thedesiredfo anddividethisproductinto 159.Theresultis tle scalefactor, usedto calculatethe raluesof the renainiqgcompooents from tle followiag tabte:

Cmpooent

For Gain of 2

FcGalnof 10

Cr(tnpF) &(ro kO) RrGnkO) Rr(in kO)

1.000XwhedCg 1.000X scalefactc 0.333X scalefrctm 4.000X scalefactc

2.000XvaluedQ 0.200X scalehctc 1.000X scalefrcttr 3.000X scalefrctor p1

Fig.A-11.tundlassflt /. Example: Desiredfo: 500IIz; Gain: 10 10

Ct:Bii':0.02#F : 0.0apF Cc: (2.000)0.02 factor:g - 119: rb.9 scale lu tU, Rr: (0.200)15.9:3.18kO Rr: (1.000)15.9:15.9kO : 47.7kA f,r: (3.000)15.9

a higherQ valueis required.Theraluesof R1,R2, For a narrowerbandpass, andRscanbeadjustedby multiplyingthemby thefactorsgivenin thefollowing table: For Gain of 1O

For Gain of 2

DeeiredValue ofQ

Rl and R3

1 6 8 10

2 3 4 5

Rr 0.400 0.258 0.189 0.153

Rt aodRg

2 3 4 5

R, 0.105 0.06r 0.0/t4 0.034

Dxample: maintainingthe calculationswhile Desirea Qof 8 fromthepreviousbandpass gainat 10. 1%

Rr - 3.18kQ X 4 : L2.72l<{2 Rz: 15.9kQ X 0.044: 0.699ko Rs:47.7 kO X 4: 190.8k0 BandstopFilter (Band-Rejector Notch Filtet) filter. The valueof C1is FigureG12 illustratesa conmonformof bandstop determinedby dividingthe desiredcenterfrequency([) into 10.Theresultof Next,multiplythe valueof C1 is the valueof Q in microfarads. this calculation product 159. Theresultis thescalefactor. into this and divide the desired with i gain of Ct . For uniB that is twice capacitance C2's Gainof 1)' thefollowingtable on the tlree resistorsdepending values of obtain the used to can be of multipliers product The multipliers. factor by the the scale multiply Simply the desiredQ. wiUbein kilohms.

Deeir€dVdue ofQ I 2 3 4 5 6 7 I 9 10

Rr 0.500 0.250 0.167 0.125 0.100 0.083 0.071 0.063 0.056 0.050

R,

2.000 4.000 6.000 8.000 10.000 12.000 14.000 r6.000 18.000 20.000

Rr

0.400 0.235 0.162 0.L23 0.099 0.083 0.07r 0.062 0.055 0.050

Fig.A-12.tundstop6otch)filter. 193

Bample: Desiredfo - 500IIa Q- 5 1n

Cr- 3tr: 0.02 tF

Cz:2(0.024F)- 0.04#F

factor scale

#

:

T;

: 15.e

- 1.59kO Rr- (0.100)15.9 159ko Rz(10.000)15.9: - 1.57kO Rr - (0.099)15.9 FREQUENCY

(Df-=Lq 1

(2)f::,

IMPEDANCE

(l) Z:trl* (2)Z-G4BPP

($ Z:4 cosu

(4)z-#Y (5)z:i

F

p (6) z-fu

Et cos0 l:t)L:T INDUCTANCE (1) Inductorsin series: Lrm11-Lr*Lr+Ls. (2) Inductorsin parallel: Llorrr

1 , 1 , 1

L-t-f, 1g

. . etc.

...€tc

(3) Two inductorsin parallel: L,L, _ -Tfi;

Lrm.rr

(4) Coupledinductances in serieswith fieldsaiding: Lrorer,: Lr * L2+ 2M (5) Coupledinductances in serieswith fieldsopposing: L1661.: Lr * h- 2M (6) Coupledinductances in parallelwith fieldsaiding: Lrmrr,:

1 + 1 ' L 2 +M Ll+M (7) Coupledinductances in parallelwith fieldsopposing: I

Lrmrr,:

1 + 1 ' L"-M Lr-M (8) Mutualinductionof two rf coilswith fieldsinteracting: M - L'-L' 4 TVhere of bothcoilswith fieldsaiding L1is the total inductance of bothcoilswith fieldsoppming L, is the total inductance (9) Couplingcoefficientof two rf coilsinductivelycoupledso asto givetrans. formeraction: M 'lLtL, f , - -

METER FORMTTI,/TS (1) Ohms/volt:

1

f

Where I: Full scalecurrentin amperes (2) Meter resistance: It

R nrn:ry TFUIJ,SCAIS (3) Currentslnmt: R--^--5srr

- RMerEn N _

I

Where (both bytheoriginalfull-scalereading reading divided N isthenewfull-scale in sameunits) OEM'S LAW FOR DC CIRCUITS (1) I:.Rg

(z)I-

n \l.R:

6) I:;

(4)R:? (5)R:i F3

(6)R-f

(7)E-IR

(8)E-? (9)E:ffi (10)P-IzR (11)P-EI

(u)P-tr.2 OEIII'S IITW FOR AC CIRCTIITS

$b2 (2)r:

G)I-E#

(4) Z:+ (5)z-# 1%

E2cos0 (o, Z:--F(7)B:lZ p

(8)E:d;0 t v 7

(9)E:{.*e

(10)P:lzZcs9 (1 1 )P :IE co s0

s2)p-ry PEAT VALUE (1) VraAr: 1.414(rursvalue) (2) Vtgltr: 1.570(average value) PEAT TO PEAK VALI'E (1) Vp-p- 2.828(rns ralue) (2) Vp-p- 3.140(aver4ge value) PEASEANGLE x

0-arctanfr

POW-ERFACTOR (1)pJ.-cos0 (2) D-crlt0 NEACTANCE (1) & -2nfl 1 (2)xc-fu,

r97

NESISTANCE (1) Resistcsin series: Rrprp-Rt+Rs+Rs . . . etc. (2) Resistors in parallel: Rrvrrr.-

1

1

'

'

1

Rr Rr Rr

"'-'

(3) 1\ro resistorsin parallel:

Rm^,-ffi RESONIINCE 1

(l) fnrsorwcr:ffi,

QrL4fu @>c-Th ruGgT TRIANGI,E (1)fta-* (2)cc A-*

(3)tano-l (4)sec0-*

(5)cot0-8 (6) csc0-

H

O ROOTIIIEAN SQUARE (SINE WAVE SHAPEONLY) (1) rms- 0.707Geakvalue) (2) rns: 1.111(avsagevahre) SECANT

(1)sec A-x tg

otf,^

(2) sec0- cosecant (90- d) (3) sec0- I = CGI U

SINE

(1)sinA:i

o

(2)sin0- cos(90- d) ( 3 ) s i-n Q - ' ' . > cosecant A 1

SUSCEPTANCE x.

(1)B,-pft (2 )B- .:i r \

t c

(3)Bmmr:Br+Bs+Bs . . . etc, TANGENT o

(1)tan0_i

(2 )tm0 :o t(9 0 -d ) 1

(3)tan?-;i.re TEMPERATTIRE (1 )C :0 .5 5 6 F -1 7 .8 (2)F: 1.8C + 32 (3 )K : C +2 7 3 TRANSISTORS, BIPOIITR (1)I"-Iu f (2) I"= 1" (3) B" - Eu- 0.7(silicontransistors) (4) E. - Eu- 0.3(gernaniuntransistors) F

(5)I":ff

I\G

19

(6) 4-R.F A i+ - F Wh€res of a comnmco[ectcemi$radm &is tle porvergain TRANSISTORS, FISII) EFFECT "- --AIoAVcr Wbere: ralue Gbis thetransconductance rroltage in gateto source AVcsi8a change in &ab curreot change AIDiEa suhequent INANSFONUERRATIO

N ' E t - t { rT- ,

-----

N

WAVEI,ENGTg

m

E. t

lz,