Em-unit-2

UNIT-2 TRANSISTORS AND AMPLIFIERS

Topics v Bipolar junction transistor- CB, CE, CC configuration and characteristics-Biasing circuits. v Class A, B and C amplifiers. v Field effect transistor-Configuration and characteristic of FET amplifier. v SCR, Diac, Triac. v UJT-Characteristics and simple applicationsSwitching transistors. v Concept of feedback-Negative feedbackApplication in temperature and motor speed control.

Construction of PNP and NPN transistors.

Ø The middle region of each transistor type is called the base of the transistor. This region is very thin and lightly doped. The remaining two regions are called emitter and collector. Ø The emitter and collector are heavily doped. But the doping level in emitter is slightly greater than that of collector and the collector region-area is slightly more than that of emitter. Ø A transistor has two p-n junctions. One junction is between the emitter and the base, and is called the emitter base junction, or simply the emitter junction JE. Ø The other junction is between the base and the collector, and is called collector-base junction

Ø The transistor can be constructed using one of the five basic techniques and accordingly they are classified as v Grown type v Alloy type v Electro chemically etched type v Diffusion type v Epitaxial type current components in BJT v The holes crossing the emitter base junction J and reaching the collector base junction Jc constitutes collector current I Not all the holes crossing the emitter base junction J reach collector base junction J because some of them combine with the electrons in the n-type base. v Since base width is very small, most of holes cross the collector base junction Jc and very few recombine, constituting the base current . v The emitter current E consists of hole current l (holes crossing from emitter base) and electron current (electrons crossing from base into the emitter) v If doping is more, conductivity is more and if doping is less, conductivity is less. v In a commercial transistor the doping of the emitter is made much larger than the doping of the base. v When emitter is open circuited, E = 0 and hence I =0 In such condition, the base and collector act as a reverse biased diode, and the collector current I equals the reverse saturation current L.

Common Base Characteristics

v It is current) constant is taken axis.

the curve between input current E (emitter and input voltage V (emitter-base voltage) at collector-base voltage VC8. The emitter current along Y-axis and emitter base voltage along X-

Common- Emitter Configuration v input is applied between base and emitter, and output is taken from collector and emitter. Here, emitter of the transistor is common to both, input and output circuits, and hence the name common emitter configuration. v Common emitter configurations for both npn and pnp transistors .

Breakdown characteristics v At high collector junction voltage, there is the possibility of voltage breakdown in the transistor. v Two types of breakdown are possible: Avalanche breakdown and punch through or reach through. Construction and Characteristics of JFET Construction of JFET v A small bar of extrinsic semiconductor material, n type is taken and at its two ends, two ohmic contacts are made which are the drain and source terminals of FET. v Heavily doped electrodes of p type material form p-n junctions on each side of the bar. v The thin region between the two p gates is called the channel. Since this channel is in the n type bar, the FET is known as n-channel JFET.

v The electrons enter the channel through the terminal called source and leave through the terminal called drain. v The terminals taken out from heavily doped electrodes of p type material are called gates.

v These electrodes are connected together and only one terminal is taken out, which is called gate. Construction of p-channel JFET v The device could be made of p type bar with two n type gates then this will be p-channel JFET. v The principle of working of n-channel JFET and pchannel JFET is similar; the only difference being that in n-channel JFET the current is carried by electrons while in p-channel JFET, it is carried by holes.

V-I Characteristics of JFET Drain VI Characteristics for n-channel JFET v The curves represent relationship between the drain current D and drain to source voltage VDS for different values of VGS.

From this characteristics we observe following points 1. VGS and Vds both = 0 2. Self pinch off at no bias (VGS 0) 3. VGS with negative bias 4. Breakdown Region 5. Ohmic and Saturation regions 6. Cutoff Drain V-I characteristics for p-channel JFET v In a p-channel JFET the source is positive with respect to the drain. v Here the source is the source of holes which flow through the channel to the drain. v The pinch-off is achieved by making the source to gate voltage, V negative (i.e. VGS positive) there by reverse biasing the pn junction diode formed by the channel and the gate.

Transfer Characteristics for n-channel JFET v The curves represents relationship between the drain current Id and gate to source voltage VGS.

From this characteristics we observe following points

a. The relationship between the drain current Id and gate to source voltage VGS is non-linear. This relationship is defined by Shockley s equation

b. 2. c. A point A at the bottom end of the curve on the VGSaxis represents VGS and point B at the top end of the curve on the Id axis represents T (maximum drain current at VGS = 0). Thus, this curve shows the operating limits of a JFET.

SILICON CONTROLLED RECTIFIER: v It is a four layered PNPN device and is a prominent member of thyristor family. v It consists of three diodes connected back to back with gate connection or two complementary transistors connected back to back. v It is widely used as switching device in power control applications. It can switch ON for variable length of time and delivers selected amount of power to load. v It can control loads, by switching the current OFF and ON up to many thousand times a second. v Hence it possesses advantage of RHEOSTAT and a switch with none of their disadvantages.

Construction: As shown in fig. it is a four layered three terminal device, layers being alternately P-type and N-type silicon. Junctions are marked J1,J2,J3 Whereas terminals are anode(A), cathode(C) and gate(G). The gate terminal is connected to inner P-type layer and it controls the firing or switching of SCR. Biasing: v The biasing of SCR is shown in fig.1 The junction J1 and J3 become forward biased while J2 is reverse biased. It is seen that now junction J1 and J3 become reverse biased and only J2 is forward biased.

SYMBOL FOR SCR

BIASING FOR SCR

Operation of SCR . v In SCR a load is connected in series with anode and is kept potential with respect to cathode when the gate is open i.e., is applied at the gate. v Under this condition, junctions J1 and J3 are forward biased and junction J2 is reverse biased. Due to this, flows through RL and hence the SCR is cutoff. v SCR starts conducting and is said to be in ON state. The SCR offers very small forward resistance so that it allows infinitely high current. The current flowing through the SCR is limited only by the anode voltage and external resistance. v If the battery connections of the applied voltage are reversed the junction J1 and J3 are reverse biased. J2 is forward biased. If the applied reverse voltage is small the SCR is OFF and hence no current flows through the device. VI Characteristics of SCR v The volt-ampere characteristics of a SCR for IG = 0.

1. As the applied anode to cathode voltage is increased above zero, very small current flows through the device, under this condition the SCR is off. It will be continued until, the applied voltage reaches the forward break over voltage (point A) 2. If the anode-cathode (applied) voltage exceeds the break over voltage it conducts heavily the SCR turns ON and anode to cathode voltage decreases quickly to a point B because, under this condition the SCR offers very low resistance hence it drops very low voltage across it. 3. At this stage the SCR allows more current to flow through it. The amplitude of the current is depending upon the supply voltage and load resistance connected in the circuit. 4. The current corresponding to the point is called the holding current (IH). It can be defined as the minimum value of anode current required to keep the SCR in ON State. If the SCR falls below his holding current the SCR turns OFF. 5.If the value of the gate current I is increased above zero(IG > 0)the SCR turns ON even at lower break over voltage. 6. The region lying between the points OA is called forward blocking region. In this region SCR is OFF The region lying between the points BC is called forward conduction region. 7. Once the SCR is switched ON then the gate looses all the control. So SCR cannot be turned OFF by varying the gate voltage. It is possible only by reducing the applied voltage. To obtain the “reverse characteristics” the following points are followed. 1. In this case the SCR is reverse biased, if the applied reverse, voltage is increased above zero, hence a very small current flows through the SCR. Under this condition the SCR is OFF, it continues till the applied reverse voltage reaches breakdown voltage.

2. As the applied reverse voltage is increased above the breakdown voltage, the avalanche breakdown occurs hence SCR starts conducting in the reverse direction. It is shown in curve DE. Suppose the applied voltage is increased to a very high value, the device may get damaged. Firing and Turning Off Mechanism Firing Usually SCR is operated with an anode voltage slightly less than VBO (forward break over voltage) and is triggered into conduction by applying low power gate pulse. Once the SCR is switched on, gate has no control on device current. Turning Off SCR remains ON, even when triggering pulse is removed. This ability of SCR to remain in the ON state even when the gate current is removed is called latching . SCR can be turned off either by any one of the method. a) anode current interruption b) reversing polarity of anode - cathode voltage c) reducing current through SCR below holding current H Applications Main application is that it is a power control device, other common applications are a) relay controls, b) phase control, c) static switches, d) regulated power supplies, e) heater controls, 1) inverters, and g) motor controls.

TRIAC

v It is a 5-layer 3 terminal bidirectional device which can be triggered ON by applying either positive or negative voltages irrespective of the polarity of the voltage across the terminal anodes . v It behaves like two SCR s, connected in parallel and in opposite direction to each other. v Because of the inverse parallel connection the two terminals cannot be identified as anode or cathode. v The anode and gate voltage applied in either direction will fire (ON)a triac because they would fire at least one of the two SCR S which are in opposite directions. Construction v It has three terminals A A and G. The gate G is closer to anode A v It has six doped regions. It is nothing but two inverse parallel connected SCR S with common gate terminal.

Operation

V-I Characteristics of TRIAC Applications (TRIAC) v TRIAC is a bidirectional device hence it is used in many industrial applications such as (i) Phase control ,heater control

(ii) (iii) (iv) (v) (vi)

Light Dimmer control, speed control of motors. It is also used to control ac power to a load by switching ON and OFF light control motor speed control as static switch to turn on and off ac power. By adjusting R, the point at which conduction commences can be varied.

DIAC v A diac is a two terminal, three layer bidirectional device which can be switched from its OFF state to ON state for either polarity of applied voltage. v The two leads are connected to P-region of silicon chip separated by an N-region. v MT1and MT2are two main terminals by which the structure of the diac is interchangeable. v It is like a transistor with the following basic differences. (i) There is no terminal attached to the base layer

(ii) The doping concentrations are identical (unlike a bipolar transistor) to give the device symmetrical properties.

Operation v when Positive or negative voltage is applied across the main terminals of a diac, only a small leakage current IBO will flow through the device. v If the applied voltage is increased, the leakage current will continue to flow until the voltage reaches the break over voltage VBO. v At this point, avalanche breakdown occurs at the reverse-biased junction it may be J1or J2depending upon the supply connected between MT1& MT2 the device exhibits negative resistance v ie current through the device increases with the decreasing values of applied voltage. v The voltage across the device then drops to break back voltage V Applications of DIAC Some of the circuit applications of diac are • Light dimmer circuits. • Heat control circuits. • Universal motor speed control.

THE UNIJIJNCTION TRANSISTOR v Another device whose construction is similar to that of the FET is indicated in Fig. 14-31. v A bar of high-resistivity n-type silicon of typical dimensions 8 X 10 X 35 mils, called the base B, has attached to it at opposite ends two ohmic contacts, Bi and B2. v A 3-mil aluminum wire, called the emitter B,

v

v

v

v

v v v

v

v

v

is alloyed to the base to form a p-n rectifying junction. This device was originally described in the literature as the double-base diode, but is now commercially available under the designation uni junction transistor (UJT). The standard symbol for this device is shown in Fig. 143 lb. Note that the emitter arrow is inclined and points toward B1 whereas the ohmic contacts B1 and B2 are brought out at right angles to the line which represents the base. The principal constructional difference between the FET and the T is that the gate surface of the former is much larger than the emitter junction of the latter. The main operational difference between the two devices is that the FET is normally operated with the gate junction reverse-biased, whereas the useful behavior of the UJT occurs when the emitter is forwardbiased. As usually employed, a fixed interbase potential V is applied between El and B2. The most important characteristic of the UJT is that of the input diode between E and El. If B2 is open-circuited so that 1 = 0, then the input volt-ampere relationship is that of the usual p-n junction diode as given by Eq. (6-31). In Fig. 14 32 the input current-voltage characteristics are plotted for l = 0 and also for fixed values of interbase voltage VBB. Each of the latter curves is seen to have a negativeresistance characteristic. A qualitative explanation of the physical origin of the negative resistance is given in Ref. 14. The principal application of the L is as a switch which allows the rapid discharge of a capacitor (Ref. 13).

THE FEEDBACK CONCEPT v In the preceding section we summarize the properties of four basic amplifier type. v In each one of these circuits we may sample the output voltage or

Fig. 17-4 (a) A transresistance amplifier is represented by a Norton s equivalent in its input circuit and a equivalent in its output circuit. (b) Equivalent circuit of a commonemitter transistor transresistance amplifier.

v For this circuit Current by means of a suitable sampling network and apply this signal to the input through a feedback two-port network, as shown in Fig. 17-5. v At the input the feedback signal is combined with the external (source) signal through a mixer network and is fed into the amplifier proper. FEED BACK NETWORK. v This block in Fig. 17-5 is usually a passive two-port network which may contain resistors, capacitors, and inductors. Very often it is simply a resistive configuration. SAMPLING NETWORK v Several sampling blocks are shown in Fig. 17-6. In Fig. 17-6a the output voltage is sampled by connecting the feedback network in shunt across the output. v In this case it is desirable that the input impedance of the feedback network be much greater than RL as not to load the output of the amplifier.