Department of Electronics and Communication School of Engineering, Manipal University Jaipur Bipolar Junction Transistor BASIC ELECTRONICS

Department of Electronics and Communication School of Engineering, Manipal University Jaipur

Bipolar Junction Transistor

BASIC ELECTRONICS


Syllabus

• Introduction to Bipolar Junction Transistor• BJT Operation• BJT Configurations• Tutorials• BJT Biasing• Tutorials• BJT Amplifier• Tutorials


Reference Books

1. “Electronic Devices and Circuit Theory” by Boylestad & Nashelsky,

2. “Integrated Electronics” by Millman & Halkias,


Introduction

• Solid state transistor was invented by a team of scientists at Bell laboratories during 1947-48

• It brought an end to vacuum tube era

• Advantages of solid state transistor over vacuum devices:

– Smaller size, light weight

– No heating elements required

– Lower power consumption and operating voltages

– Low price


Introduction

Figure showing relative sizes of transistor, IC and LED

Figure showing different transistor packages


Introduction

• Bipolar Junction Transistor (BJT) is a sandwich consisting of three layers of two different types of semiconductor

• Two kinds of BJT sandwiches are: NPN and PNP


Introduction

• The three layers of BJT are called Emitter, Base and Collector

• Base is very thin compared to the other two layers

• Base is lightly doped. Emitter is heavily doped. Collector is moderately doped

• NPN – Emitter and Collector are made of N-type semiconductors; Base is P-type

• PNP – Emitter and Collector are P-type, Base is N-type

• Both types (NPN and PNP) are extensively used, either separately or in the same circuit


Introduction

• Transistor symbols:

Note: Arrow direction from P to N (like diode)


Introduction

• BJT has two junctions – Emitter-Base (EB) Junction and Collector-Base (CB) Junction

• Analogous to two diodes connected back-to-back:

– EB diode and CB diode

• The device is called “bipolar junction transistor” because current is due to motion of two types of charge carriers – free electrons & holes


Transistor Operation

• Operation of NPN transistor is discussed here; operation of PNP is similar with roles of free electrons and holes interchanged

• For normal operation (amplifier application)

– EB junction should be forward biased

– CB junction should be reverse biased

• Depletion width at EB junction is narrow (forward biased)

• Depletion width at CB junction is wide (reverse biased)





Un-biased transistor showing barriers at the junctions



C-B junction is reverse biased – increased barrier height



E-B junction is forward biased – aids charge flow



Electron-hole combination – leads to small base current



• When EB junction is forward biased, free electrons from emitter region drift towards base region

• Some free electrons combine with holes in the base to form small base current

• Inside the base region (p-type), free electrons are minority carriers. So most of the free electrons are swept away into the collector region due to reverse biased CB junction



• Three currents can be identified in BJT

1. Emitter current • This is due to flow of free electrons from emitter to base

• Results in current from base to emitter

2. Base current• This is due to combination of free electrons and holes in the base

region

• Small in magnitude (usually in micro amperes)

3. Collector current• Has two current components:

• One is due to injected free electrons flowing from base to collector

• Another is due to thermally generated minority carriers



• Note the current directions in NPN and PNP transistors

• For both varieties: ---(1)

C

E

B

IC

IE

IB

NPN

C

E

B

IC

IE

IB

PNP

BCE III



• As noted earlier, collector current has two components:

– One due to injected charge carriers from emitter

– Another due to thermally generated minority carriers

• Both results in current in the same direction. Hence

--- (2)

αdc is the fraction of charge carriers emitted from emitter, that enter into the collector region

ICBO is the reverse saturation current in CB diode

--- (3)

CBOEdcC III

E

CBOCdc I

II



• As approximation, we can neglect ICBO compared to IE and IC

• Hence approximate equations are:

• Like the reverse saturation current of ordinary diode, ICBO also doubles for every 10o C rise in temperature.

• So ICBO cannot be neglected at higher temperatures

• The parameter αdc is called common-base dc current gain

• Value of αdc is around 0.99

EdcC II

E

Cdc I

I



• We have

• Substituting for IE, we get

CBOEdcC III

CBOBCdcC IIII

CBOBdcCdc III )1(

)1()1( dc

CBOB

dc

dcC

III

CEOBdcC III

• Where and

)1( dc

dcdc

CBOdcdc

CBOCEO I

II 1

)1(

--- (4)



• Equations (2) and (4) are two alternate forms of BJT current equation

• Since value of αdc is around 0.99, ICEO >> ICBO

• However, ICEO is still very small compared to IC

• Hence approximation of (4) gives: or

• Parameter βdc is called common emitter dc current gain

• Values of αdc and βdc vary from transistor to transistor. Both αdc and βdc are sensitive to temperature changes

BdcC II B

Cdc I

I


Problems1. A BJT has alpha (dc) 0.998 and collector-to-base reverse sat

current 1μA. If emitter current is 5mA, calculate the collector and base currents.

(Ans: 4.99 mA, 10 μA)

2. An npn transistor has collector current 4mA and base current 10 μA. Calculate the alpha and beta values of the transistor, neglecting the reverse sat current ICBO

(Ans: 0.9975, 400)

3. In a transistor, 99% of the carriers injected into the base cross over to the collector region. If collector current is 4mA and collector leakage current is 6 μA, Calculate emitter and base currents

(Ans: 4.034 mA, 34 μA)


Transistor Configurations

• BJT has three terminals

• For two-port applications, one of the BJT terminals needs to be made common between input and output

• Accordingly three configurations exist:

– Common Base (CB) configuration

– Common Emitter (CE) configuration

– Common Collector (CC) configuration• (The last one is not discussed in this course)

Input Output2-port device


Transistor Configurations• Common Base configuration

(Resistors are not shown here for simplicity)

• Base is common between input and output

– Input voltage: VEB Input current: IE

– Output voltage: VCB Output current: IC



• CB Input characteristics

– A plot of IE versus VEB for various values of VCB

– It is similar to forward biased diode characteristics

– As VCB is increased, IE increases only slightly

– Note that second letter in the suffix is B (for base)



constVwithI

Vr CB

E

EBi

constIwithV

VA E

EB

CBV

• Input resistance ri

• Voltage amplification factor AV

• Both can be determined from the CB input characteristics



21

21

EE

EBEBi II

VVr

21

21

EBEB

CBCBV VV

VVA



• CB Output characteristics– A plot of IC versus VCB for various values of IE

– Three regions are identified: Active, Cutoff, Saturation– Active region:

• E-B junction forward biased• C-B junction reverse biased• IC is positive, VCB is positive• IC increases with IE

• For given IE, IC is almost constant; increases only slightly with increase in VCB. This is due to base-width modulation (Early effect)



CB Output characteristics



• Base Width modulation

– As the reverse bias voltage VCB is increased, the depletion region width at the C-B junction increases. Part of this depletion region lies in the base layer. So, effective base width decreases. Hence number of electron-hole combination at the base decreases. So base current reduces and collector current increases.

– Note that IE = IC + IB



• When IE = 0, IC = ICBO

– ICBO is collector to base current with emitter open

– Below this line we have cut-off region

– Here both junctions are reverse biased

• Region to the left of y-axis (VCB negative) is saturation region

– Here both junctions are forward biased

– IC decreases exponentially, and eventually changes direction



• Both can be measured from output characteristics

constIwithI

Vr E

C

CBO

• Output resistance ro

• Current amplification factor AI or αac

constVwithI

ICB

E

Cac


Transistor Configurations• Common Emitter configuration

(Resistors are omitted for simplicity)

• Emitter is common between input and output– Input voltage: VBE Input current: IB

– Output voltage: VCE Output current: IC



CE input characteristics

• Plot of IB versus VBE for various values of VCE

• Similar to diode characteristics• As VCE is increased, IB

decreases only slightly• This is due to base-width modulation• Note that second suffix is E (for emitter)



• CE output characteristics

– A plot of IC versus VCE for various values of IB

– Three regions identified: Active, Cut-off, Saturation

– Active region:

• Linear region in the output characteristics

• E-B junction forward biased

• C-B junction reverse biased

• IC increases with IB

• For given IB, IC increases slightly with increase in VCE; this is due to base-width modulation (Early effect)



CE output characteristics



• Note that VCE = VCB + VBE

• So if VCE is increased, effectively VCB also increases

• For saturation to take place, C-B junction should be forward biased.

• This happens when VCE is approximately 0.3 V (or less) for Si

• Note that when VCE= 0.3V, and VBE= 0.6 V, VCB= –0.3V (a forward bias of 0.3 V)

• So region to the left of the vertical line VCE=VCE(sat)=0.3V (for silicon) is considered as saturation region

• Region below IB=0 line (or IC=ICEO) is cut-off region



• ICEO is much larger than ICBO because of the relation:

dc

CBOCEO

II

1 • Note that value of αdc is around 0.99

• The values of αdc & αac, and βdc & βac are almost the same. Hence the subscripts can be omitted for simplicity



• Input resistance ri

• Voltage gain AV

• Output resistance ro

• Current gain AI or βac

constVwithI

Vr CE

B

BEi

constIwithV

VA B

BE

CEV

constIwithI

Vr B

C

CEo

constVwithI

ICE

B

Cac

• All these parameters can be determined from CE characteristics



• Experimental setup for determining CE characteristics:


Tutorials1. A Ge transistor with β = 100 has collector-to-base leakage

current of 5 µA. If the transistor is connected in common-emitter operation, find the collector current for base current (a) 0 (b) 40 µA.

Sol: Given that ICBO = 5µA, and β = 100

We know that

When IB = 0, IC = ICEO = (β+1)ICBO = 505 µA

When IB = 40 µA, IC = βIB + ICEO

= (100 × 40 × 10–6) + (505 × 10–6)

= 4.505 mA


Tutorials

2. A Ge Transistor has collector current of 51 mA when the base current is 0.4 mA. If β = 125, then what is its collector cutoff current ICEO?

(Ans: 1 mA)

3. In a transistor circuit, when the base current is increased from 0.32 mA to 0.48 mA, the emitter current increases from 15 mA to 20 mA. Find αac and βac values.

(Ans: 0.968, 30.25)

4. A transistor with α = 0.98 and ICBO = 5 µA has IB = 100 µA. Find IC and IE.

(Ans: 5.15 mA, 5.25 mA)

Transistor Biasing

• One of the most common applications of transistor is in amplifiers.

• E-B junction should be forward biased; C-B junction should be reverse biased (active region)

• For faithful amplification we require that transistor be operated in active region throughout the duration of input signal.

• To ensure this, proper dc voltages should be applied. This is called Biasing.


Operating point

When no input signal is applied to transistor circuit, and only dc voltages are supplied, currents IC, IB and voltage VCE will have certain values.

If these values are plotted over the transistor output characteristics, the point we get is called ‘Operating point’. It is also called ‘Quiescent point’ or just Q-point.



• In above figure, currents IBQ, ICQ and voltage VCEQ are plotted as point Q. In practice, we have to choose Q-point according to our requirement. If we want to operate in the middle of active region, we may choose Q as Q-point.

• If we want to operate near saturation, we may choose Q’ as Q-point.

• If we want to operate near cutoff, we may choose Q’’ as Q-point.

• Note that if no biasing is used, Q-point will be in the origin of graph.

• So, biasing is used to fix the Q-point according to our need.


Type of Biasing in Transistor

Fixed bias

Voltage divider bias


Fixed Bias

• Base resistor RB is connected to Vcc (Instead of VBB). Negative terminal of Vcc is not shown. It is assumed to be at ground.

• Applying KVL to the input,


B

BECCB R

VVI

Vcc is constant, VBE is almost constant (0.7V for silicon). So by selecting proper RB, we can fix IB as required.

Applying KVL to output side we get:

•IC is related to IB by β

•So, VCE can be fixed by selecting proper RC.


Load Line

We have,

This is an equation of straight line with VCE and IC as two variables. This straight line is called load line. Now, output characteristic is also a function of same two variables.

If RB and RC are held constant and VCC is varied, then load line shifts, maintaining same slope.



•If RB and RC are held constant and VCC is varied, then load line shifts, maintaining same slope. From these graphs we infer that:

•with everything else held constant, if RB is increased, transistor goes towards cutoff.

•if RB is decreased, transistor goes towards saturation.


• With everything else held constant, if RC is increased, transistor goes towards saturation.

• if RC is decreased, transistor goes towards active region.

• With everything else held constant, if VCC is increased, transistor goes towards active region.

• if VCC is decreased, transistor goes towards saturation.


Advantages of fixed bias•Simple to analyze and design•Uses very few circuit components

Disadvantages of fixed bias•Q-point is not stable. i.e., if temperature varies, β will vary, hence IC will vary.

•If transistor is replaced by another transistor having different β then Q-point will shift.


Voltage divider bias or Self bias

• Uses two resistors R1 and R2 instead of RB. RE is connected between emitter and ground.


• Input side of the above circuit is redrawn below,


• VTH is the open circuit voltage between points A & B in fig (a) given by,


• RTH is the resistance between A & B with VCC replaced by short circuit.

• Applying KVL to the input loop,

Substituting and rearranging, we get


• Applying KVL to the output loop, we get

Rearranging, we get

Also,

Where, VC is voltage from collector to ground.



• Also,Where, VE is voltage from emitter to ground.

Since β>> 1, we have (β+1) ≈ β. If βRE >> RTH, then equation of IB reduces to,

Now,

• Since equation for IC does not contain β, we say that IC is independent of temperature variation and transistor replacement.

Advantages of voltage divider bias

. Q-point is stable against variation in temperature and replacement of transistor.

Disadvantages of voltage divider bias• Analysis and design are complex• More circuit components required


Problem

1. Draw the DC load line and mark the Q- point on fixed bias cct. Assume Beta DC=100 and neglect Base –Emitter voltage. (Vcc=30V,RB=1.5 Mohm, RC=5 Kohm).

Ans: VCE,max=30v,VCEQ=20v,ICQ=2mA

2. In a fixed bias cct. Find the base current required to establish VCE=6v, also find RB & IE, (VBE=0.7v, Beta DC=120, VCC=12v,RC=2.2.Kohm).

Ans: IB=22.75 uA, RB=497 Kohm

3. Determine the region in which the transistor operates. (VBE=0.2v,RB=120kohm,RC=1kohm,VCC=15v,Beta DC=120).



End of Module 3

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Department of Electronics and Communication School of Engineering, Manipal University Jaipur Bipolar Junction Transistor BASIC ELECTRONICS