Chapter 04 Full

8/14/2019 Chapter 04 Full

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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 withthree doped semiconductor regions emitter, base, andcollectorseparated 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 nregions 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 iscalled thebase emitter junction. Thepn junction joining the baseregion and the collector region is called the base collector

junction

The base region is lightly doped and very thin compared to theheavily doped emitter and the moderately doped collectorregions


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SCHEMATIC SYMBOL FOR BJTs


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BASIC TRANSITOR OPERATION

In order to operate transistor as an amplifier properly , the twopn junctions must be correctly biased with external dcvoltages. Figure shows the proper biasing arrangement forboth npn and pnp transitors for active operation as anamplifier. In both cases the base emitter (BE) junction isforward 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 npntransistor.

The forward bias from base to emitter narrows the BE depletion region, and the reverse bias from base to collectorwidens the BCdepletion region.

The heavily doped n type emitter region is teeming with freeelectrons that easily diffuse through the forward biased BE

junction into the p-type base region where they becomeminority.

The base region is lightly doped and very thin so that it has alimited number of holes. Thus, only a small percentage of all

the electrons flowing through the BE junction can combinewith the available holes in the base. These relatively few

recombined electrons forms the small base electron current.


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Most of the electrons flowing from the emitter into baseregion 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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TRANSISTOR CURRENTS

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

The above figures shows that the emitter current (IE) is the sumof 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 shownin figure. VBB forward biases the base emitter junction, and

Vcc reverse biases the base collector junction. Generally,

VBBis very small as compared to Vcc.


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DC BETA (DC )

Definition:

The ratio of the dc collector current (IC) to the dc base current

(IB) is the dc beta (DC). It is also called the gain of a

transistor.

Typical values of DC range from 20 to 200 or higher. DC is

usually designated as an equivalent hybrid (h) parameter, hFE ,on transistor data sheets.


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DC ALPHA (DC)

Definition:

The ratio of the dc collector current (IC) to the dc emitter

current (IE) is the dc alpha (DC). The alpha is a less used

parameter than beta in transistor circuits.

Typical values of DC range 0.95 to 0.99 or greater but DC is

always less than 1.


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EXAMPLE 4-1

SOLUTION


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TRANSISTOR CURRENT AND VOLTAGE

ANALYSIS

Three transistordc currents and three dc voltages can beidentified as


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ANALYSIS

VBB forward-biases the base emitter junction, and Vccreverse-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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ANALYSIS

Since the emitter is at ground ,by kirchhoffs voltage law, thevoltage across RB is


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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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SATURATION REGION

Both BE (Base Emitter) and BC (Base Collector) are forwardbiased

VBB produce certain values ofIBand Vcc is zero

Base is approx. at 0.7V while the emitterand collectorare at 0V


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CIRCUIT DIAGRAM FOR SATURATION

REGION

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

As VCC is increased, the collector current also increases(Ic = IB) 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 becomesreverse biased and transistor goes into Active Region.


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ACTIVE REGION

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

ICremains essentially constant for a given value ofIBwhile VCEcontinues to increase


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CUTOFF REGION

Both BE (Base Emitter) and BC (Base Collector) are reversebiased

IB= 0,although there is a very small collector leakage currentICEO


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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 forcollector to base voltage

, collector to emitter voltage , emitter to base voltage,collectorcurrent & power dissipation

The product ofVCEandICmust not exceed the maximum powerdissipation, (means both cannot be maximum at the same time)

IfVCE is maximum , IC can be calculated as;

IfIC 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 dctransistor currents.

VBE ,VCBandVCEare the dc voltages from one transistor

terminal to another.

Single subscripted voltages such asVB,VCandVEare 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 actransistor currents.

Vbe ,VcbandVceare the ac voltages from one transistor terminal

to another.

Single subscripted voltages such asVb,VcandVeare ac voltages

from the transistor terminals to ground.

The rule is different forinternal 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


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AMPLIFICATION

Definition:

Amplification is the process of linearly

increasing the amplitude of an electrical

signal


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TRANSISTOR AMPLIFICATION

A transistor amplifies current because the collector current isequal to the base current multiplied by the current gain . i.e.

(Ic = IB)

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 resistorRB

The dc bias voltage Vcc is connected to the collector through the

collector resistorRc


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The ac input voltage produces an ac base current which results ina much largerac 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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The forward biased base emitter(BE) juction presents a very lowresistance reto the ac signal.

The ac emitter current is,


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CONCLUSION

We can say that the transistor produces amplification in theform of gain, which is dependent on the values ofRc 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 currentsare 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 beneglected. The minimum value of base current needed to

produce saturation is

IB should be significantly greater than IB(min) to keep thetransistor well in saturation


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EXAMPLE 4-9


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SOLUTION


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SOLUTION

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