Bipolar Junction Transistor

Prepared By: Engr KSK 1

BIPOLAR JUNCTION TRANSISTORS (BJTs)

Friday, April 7, 2023

CHAPTER NO. 04


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


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


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


SCHEMATIC SYMBOL FOR BJTs


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


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.


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.


ILLUSTRATION OF BJT ACTION


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


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.


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.


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.


EXAMPLE 4-1

SOLUTION


TRANSISTOR CURRENT AND VOLTAGE ANALYSIS

Three transistor dc currents and three dc voltages can be identified as


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


TRANSISTOR CURRENT AND VOLTAGE ANALYSIS Since the emitter is at ground ,by kirchhoff’s voltage law, the

voltage across RB is


TRANSISTOR CURRENT AND VOLTAGE ANALYSIS


EXAMPLE 4-2


SOLUTION

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


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


COLLECTOR CHARACTERISTIC CURVES

The collector characteristic curve divided into three regions

o Saturation region

o Active region

o Cutoff region


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


CIRCUIT DIAGRAM FOR SATURATION REGION

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

The collector current also increases (Ic = βIB) and VCE as a result of more drop across the collector resistor (VCE = Vcc - IcRc)

When VCE reaches its saturation value VCE(Sat), the base collector (BC) junction becomes forward biased and IC can increase no

further even with a continued increase in IB


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


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


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


EXAMPLE 4-3


SOLUTION


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)


EXAMPLE 4-4


SOLUTION


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;


EXAMPLE 4-5

SOLUTION


MAXIMUM POWER DISSIPAATION CURVE A maximum power dissipation curve can be plotted on the

collector characteristic curve

Assume PD(max) is 500mW, VCE(max) is 20V and Ic(max) is 50mA

Transistor cannot operated in the shahded portion


MAXIMUM POWER DISSIPAATION CURVE Ic(max) is the limiting rating between point A & B

PD(max) is the limiting rating between point B & C

VCE(max) is the limiting rating between point C & D


EXAMPLE 4-6


SOLUTION


DERATING PD(MAX)

A transistor rating that tells how much the maximum allowable value of PD(max) decreased for each 1°C rise

PD(max) is usually specified at 25°C

A derating factor of 2mW/°C indicates that the maximum power dissipation is reduced 2mW for each degree

centigrade increase in temperature


EXAMPLE 4-7

SOLUTION


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.


AC QUANTITIES AC quantities always carry an lowercase roman subscript. For

example, Ib ,Ic and Ie are the ac transistor currents.

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.

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′


AMPLIFICATION

Definition:

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


TRANSISTOR AMPLIFICATION 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


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.


TRANSISTOR AMPLIFICATION The forward biased base emitter (BE) juction presents a very low

resistance re′ to the ac signal.

The ac emitter current is,


TRANSISTOR AMPLIFICATION


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


EXAMPLE 4-8


SOLUTION


TRANSISTOR AS A SWITCH

Transistor used as an electronic switch into two regions

o Cutoff region

o Saturation region


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


CONDITION IN CUTOFF REGION By neglecting the leakage current , all of the currents are

zero and VCE is equal to VCC


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


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


EXAMPLE 4-9


SOLUTION


SOLUTION

Bipolar Junction Transistor

Documents

single subscripted

bc depletion

bipolar junction

maximum power

dc collector

base collector

base emitter

ac voltages