Chapter 04 Full

CHAPTER # 04 BIPOLAR JUNCTION TRANSISTORS (BJTs)

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TRANSISTOR STRUCTURE The basic structure of the bipolar junction transistor (BJT) determines its operating characteristics. The BJT (bipolar junction transistor) is constructed with three doped semiconductor regions emitter, base, and collector separated by two pn junctions as shown in Figure

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PHYSICAL REPRESENTATION OF BJTs Physically BJTs are of two types. One type consists of two n regions separated by a p region npn, and the other type consists of two p regions separated by an n region pnp

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PHYSICAL REPRESENTATION OF BJTs The pn junction joining the base region and the emitter region is called the base emitter junction. The pn junction joining the base region and the collector region is called the base collector junction

The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions 4

SCHEMATIC SYMBOL FOR BJTs

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BASIC TRANSITOR OPERATION In order to operate transistor as an amplifier properly , the two pn junctions must be correctly biased with external dc voltages. Figure shows the proper biasing arrangement for both npn and pnp transitors for active operation as an amplifier. In both cases the base emitter (BE) junction is forward biased and the base collector (BC) junction is reversed biased

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ILLUSTRATION OF BJT ACTION To illustrate transistor action, let's look inside the npn transistor. •

The forward bias from base to emitter narrows the BE depletion region, and the reverse bias from base to collector widens the BC depletion region. •

The heavily doped n type emitter region is teeming with free electrons that easily diffuse through the forward biased BE junction into the p-type base region where they become minority.

• The base region is lightly doped and very thin so that it has a limited number of holes. Thus, only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base. These relatively few recombined electrons forms the small base electron current. 7

ILLUSTRATION OF BJT ACTION • Most of the electrons flowing from the emitter into base region do not recombine but diffuse into the BC depletion region because they are pulled through the reverse biased BC junction by the electric field set up by the force of attraction between positive and negative ions. • The electrons now move through the collector region. This forms the collector electron current. The collector current is much larger than the base current. This is the reason transistors exhibit current gain.

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ILLUSTRATION OF BJT ACTION

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TRANSISTOR CURRENTS The directions of the currents in an npn and pnp transistor and its schematic symbol are shown as;

The above figures shows that the emitter current (IE) is the sum of collector current (IC) and the base current (IB) ,expressed as follow

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TRANSISTOR DC BIASED CIRCUITS When a transistor is connected to dc bias voltages, as shown in figure. VBB forward biases the base emitter junction, and Vcc reverse biases the base collector junction. Generally, VBB is very small as compared to Vcc.

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DC BETA (βDC ) Definition: “The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta (βDC).” It is also called the gain of a transistor.

Typical values of βDC range from 20 to 200 or higher. βDC is usually designated as an equivalent hybrid (h) parameter, hFE , on transistor data sheets.

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DC ALPHA (αDC ) Definition: “The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha (αDC).” The alpha is a less used parameter than beta in transistor circuits.

Typical values of αDC range 0.95 to 0.99 or greater but αDC is always less than 1.

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EXAMPLE 4-1

SOLUTION

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TRANSISTOR CURRENT AND VOLTAGE ANALYSIS Three transistor dc currents and three dc voltages can be identified as

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TRANSISTOR CURRENT AND VOLTAGE ANALYSIS VBB forward-biases the base emitter junction, and Vcc reverse-biases the base collector junction. When the base emitter junction is forward biased, it is like a forward biased diode and has a nominal forward voltage drop of

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TRANSISTOR CURRENT AND VOLTAGE ANALYSIS •

Since the emitter is at ground ,by kirchhoff’s voltage law, the voltage across RB is

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TRANSISTOR CURRENT AND VOLTAGE ANALYSIS

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EXAMPLE 4-2

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SOLUTION As we know that .We can calculate the base, collector and emitter current as

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COLLECTOR CHARACTERISTIC CURVES • Collector characteristic curves plotted between collector current IC versus VCE for specified values of base current IB •

Both VBB and VCC are variable source of voltages

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COLLECTOR CHARACTERISTIC CURVES The collector characteristic curve divided into three regions o

Saturation region

o

Active region

o

Cutoff region

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SATURATION REGION •

Both BE (Base Emitter) and BC (Base Collector) are forward biased •

•

VBB produce certain values of IB and Vcc is zero

Base is approx. at 0.7V while the emitter and collector are at 0V

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CIRCUIT DIAGRAM FOR SATURATION REGION •

When Base Emitter (BE) junction becomes forward biased the base current is increased

•

As VCC is increased, the collector current also increases (Ic = βIB) and VCE is gradually increased (VCE = Vcc - IcRc) but remains below 0.7V due to forward bias Base-Collector junction.

•

When VCE exceeds 0.7V, the Base-Collector junction becomes reverse biased and transistor goes into Active Region.

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ACTIVE REGION •

BE (Base Emitter) is forward biased and BC (Base Collector) is reversed biased

• IC remains essentially constant for a given value of IB while VCE continues to increase

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CUTOFF REGION •

Both BE (Base Emitter) and BC (Base Collector) are reverse biased

• IB = 0 ,although there is a very small collector leakage current ICEO

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CIRCUIT DIAGRAM FOR CUTOFF REGION • The base terminal open, resulting in base current of zero • ICEO is very small so it could be neglected i.e; VCE = VCC

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EXAMPLE 4-3

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SOLUTION

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DC LOAD LINE •

A DC load line drawn on a family of curves connecting the cutoff point and saturation point •

•

Bottom of load line is at ideal cutoff where IC = 0 & VCE = VCC

Top of load line is at saturation where IC=IC (sat) & VCE = VCE(sat)

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EXAMPLE 4-4

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SOLUTION

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MAXIMUM TRANSISTOR RATING •

Typically , maximum rating are given for collector to base voltage , collector to emitter voltage , emitter to base voltage,collector current & power dissipation •

The product of VCE and IC must not exceed the maximum power dissipation, (means both cannot be maximum at the same time) • If VCE is maximum , IC can be calculated as;

•

If IC is maximum, VCE can be calculated as;

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EXAMPLE 4-5

SOLUTION

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DC QUANTITIES •

DC quantities always carry an uppercase roman subscript. For example, IB ,IC and IE are the dc transistor currents. •

VBE ,VCB and VCE are the dc voltages from one transistor terminal to another.

•

Single subscripted voltages such as VB,VC and VE are dc voltages from the transistor terminals to ground.

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AC QUANTITIES • AC quantities always carry an lowercase roman subscript. For example, Ib ,Ic and Ie are the ac transistor currents. •

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Vbe ,Vcb and Vce are the ac voltages from one transistor terminal to another. Single subscripted voltages such as Vb,Vc and Ve are ac voltages from the transistor terminals to ground.

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The rule is different for internal transistor resistance. Transistor have internal ac resistances that are designated by lowercase r ′ with an appropriate subscript. For example, the internal ac emitter resistance is designated as re′ 36

AMPLIFICATION Definition: “Amplification is the process of linearly increasing the amplitude of an electrical signal”

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TRANSISTOR AMPLIFICATION •

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A transistor amplifies current because the collector current is equal to the base current multiplied by the current gain β. i.e. (Ic = βIB) The transistor base current is small as compare to emitter and collector current so

• By keeping the above expression ,Let us consider a circuit in which an ac voltage Vin is superimposed on the dc bias voltage VBB by connecting them in series with the base resistor RB •

The dc bias voltage Vcc is connected to the collector through the collector resistor Rc 38

TRANSISTOR AMPLIFICATION •

The ac input voltage produces an ac base current which results in a much larger ac collector current •

The collector current produces an ac voltage across Rc, which produces an amplified but inverted signal at the output.

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TRANSISTOR AMPLIFICATION •

The forward biased base emitter (BE) juction presents a very low resistance re′ to the ac signal. • The ac emitter current is,

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TRANSISTOR AMPLIFICATION

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CONCLUSION •

•

We can say that the transistor produces amplification in the form of gain, which is dependent on the values of Rc and re′

Since Rc is always considerably larger in value than re′ , result the output voltage is always greater than the input voltage

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EXAMPLE 4-8

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SOLUTION

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TRANSISTOR AS A SWITCH Transistor used as an electronic switch into two regions o

Cutoff region

o

Saturation region

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CUTOFF REGION In cutoff region, the transistor behaves as an open switch because base emitter (BE) juction is reversed biased which cause an open action between collector and emitter

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CONDITION IN CUTOFF REGION By neglecting the leakage current , all of the currents are zero and VCE is equal to VCC

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SATURATION REGION In saturation region, the transistor behaves as a close switch between collector and emitter because both junctions base emitter (BE) and base collector (BC) are forward biased which cause the collector current to reach its saturation value

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CONDITION IN SATURATION REGION When transistor is saturated, the formula for collector saturation current is

Since VCE(sat) is very small compared to Vcc, it can usually be neglected. The minimum value of base current needed to produce saturation is

IB should be significantly greater than IB(min) to keep the transistor well in saturation 49

EXAMPLE 4-9

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SOLUTION

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SOLUTION

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