chapter3_bjt(for it class)

7/26/2019 Chapter3_BJT(for IT Class)

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SJTU Zhou Lingling 1

Chapter 3

Bipolar Junction

Transistor (BJT)


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Outline

Introduction

Operation in the Active Mode

Analysis of Transistor Circuits at DC The transistor as an Amplifier

Graphical Analysis

Biasing the BJT for Discrete-Circuit Design

Configuration for Basic Single Stage BJT Amplifier

High frequency Model


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Introduction

Physical Structure

Circuit Symbols for BJTs

Modes of Operation

Basic Characteristic


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Physical Structure

A simplified structure of the npntransistor.


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Physical Structure

A dual of the npnis calledpnptype. This is the

simplified structure of thepnptransistor.


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Circuit Symbols for BJTs

The emitter is distinguished by the arrowhead.


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Modes of Operation

Modes EBJ CBJ Application

Cutoff Reverse Reverse

Switching applicationin digital circuits

Saturation Forward Forward

Active Forward Reverse Amplifier

Reverse

activeReverse Forward

Performance

degradation


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Basic Characteristics

Far more useful than two terminal devices(such as diodes)

The voltage between two terminals cancontrol the current flowing in the thirdterminal. We can say that the collectorcurrent can be controlled by the voltage

across EB junction. Much popular application is to be an

amplifier


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Operation in the Active Mode

Current flow

Current equation

Graphical representation of transistorscharacteristics


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Current Flow

Current flow in an npntransistor biased to operate in the

active mode.


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Collector Current

Collector current is the drift current.

Carriers are successful excess minority

carriers. The magnitude of collector current is almost

independent of voltage across CB junction.

This current can be calculated by thegradient of the profile of electronconcentration in base region.


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Base Current

Base current consists of two components.

Diffusion current

Recombination current

Recombination current is dominant.

The value of base current is very small.


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Emitter Current

Emitter current consists of two components.

Both of them are diffusion currents.

Heavily doped in emitter region.

Diffusion current produced by the majority

in emitter region is dominant.


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Explanation for Common-Base

Current Gain

Expression for commonbase current gain:

Its value is less than but very close to unity.

Small changes in correspond to very large

changes in .

1


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Recapitulation

Collector current has the exponentialrelationship with forward-biased voltage

as long as the CB junction remains reverse-biased.

To behave as an ideal constant currentsource.

Emitter current is approximately equal tocollector current.

BEv


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Graphical Representation of

Transistors Characteristics

Characteristic curve relates to a certain

configuration.

Input curve is much similar to that of the diode,only output curves are shown here.

Three regions are shown in output curves.

Early Effect is shown in output curve of CE

configuration.


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Output Curves for CB

Configuration


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Output Curves for CB

Configuration

Active region

EBJ is forward-biased, CBJ is reverse-biased;

Equal distance between neighbouring output curves;

Almost horizontal, but slightly positive slope.

Saturation region

EBJ and CBJ are not only forward-biased but also turned on;

Collector current is diffusion current not drift current.

Turn on voltage for CBJ is smaller than that of EBJ.

Breakdown region

EBJ forward-biased, CBJ reverse-biased;

Great voltage value give rise to CBJ breakdown;

Collector current increases dramatically.


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The Early Effect(contd)

Assuming current scale remains constant,

collector current is modified by this term:

Narrow base width, small value of Early voltage,

strong effect of base width modulation, strong

linear dependence of on .Ci CEv

)1(A

CEVv

sCV

v

eIi T

BE


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DC Analysis Steps

a. Using simple constant-voltage drop model, assuming ,irrespective of the exact value of currents.

b. Assuming the device operates at the active region, we can apply the

relationship betweenIB, IC, andIE,to determine the voltage VCEor

VCB.

c. Check the value of VCEor VCB, if

i. VC>VB(or VCE>0.2V), the assumption is correct.

ii. V C


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Examples

Example 5.4 shows the order of the analysis steps

indicated by the circled numbers.

Example 5.5 shows the analysis of BJT operating

saturation mode. Example 5.6 shows the transistor operating in

cutoff mode.


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Examples(contd)

Example 5.7 shows the analysis forpnp type

circuit. It indicates the the current is affected by ill-

specified parameter .As a rule, one should strive

to design the circuit such that its performance is asinsensitive to the value of as possible.

Example 5.8 is the bad design due to the currents

critically depending on the value of .

Example 5.9 is similar to the example 5.5 except

the transistor ispnptype.


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The Transistor as an Amplifier

Conceptual Circuits

Small-signal equivalent circuit models

Application of the small-signal equivalent circuit

models

Augmenting the hybrid model.


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Conceptual Circuit

(a) Conceptual circuit to illustrate the operation of the transistor as an amplifier.

(b)The circuit of (a) with the signal source vbeeliminated for dc (bias) analysis.


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Conceptual Circuit(contd)

With the dc sources (VBEand VCC) eliminated (short circuited), thus onlythe signal components are present.

Note that this is a representation of the signal operation of the BJT and

not an actual amplifier circuit.


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Small-Signal Circuit Models

Transconductance

Input resistance at base

Input resistance at emitter

Hybrid and T model


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Transconductance

Expression

Physical meaninggmis the slope of the

iCvBEcurve at the bias point Q.

At room temperature,

T

CQm V

Ig

msgm 40


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The Hybrid-Model

The equivalent circuit in (a)represents the BJT as a voltage-controlledcurrent source (a transconductance amplifier),

The equivalent circuit in (b)represents the BJT as a current-controlled

current source (a current amplifier).


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The T Model

These models explicitly show the emitter resistance rerather than the base

resistance rfeatured in the hybrid-model.


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Augmenting the Hybrid-Model

Expression for the output resistance.

Output resistance represents the Early Effect(or base width modulation)

'

1

. C

A

constvCE

Co

I

V

v

ir

BE


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Graphical Analysis

a. Graphical construction for the determination of the dc base current in

the circuit.

b. Load line intersects with the input characteristic curve.


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Small Signal Analysis

Graphical determination of the signal components vbe, ib, ic, and vcewhen a

signal component viis superimposed on the dc voltage VBB


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Biasing in BJT Amplifier Circuit

Biasing with voltage

Classical discrete circuit bias arrangement

Single power supply

Two-power-supply

With feedback resistor

Biasing with current source


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Classical Discrete Circuit Bias

Arrangement

Both result in wide variations inICand hence in VCEand therefore are considered to be bad.

Neither scheme is recommended.


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Classical Biasing for BJTs Using a

Single Power Supply

Circuit with the voltage divider supplying the base replaced with its Thvenin

equivalent.

Stabilizing the DC emitter current is obtained by considering the negative

feedback action provided byRE


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Two-Power-Supply Version

ResistorRB can be eliminated in

common base configuration.

ResistorRBis needed only if thesignal is to be capacitively

coupled to the base.

Two constraints should apply.


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Biasing with Feedback Resistor

ResistorRBprovides negative feedback.

IEis insensitive to provided that

The value ofRB determines the allowable signal swing at the collector.

1(BC

RR


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Biasing Using Current Source

(a) Q1and Q2are required to be identical and have high .

(b) Short circuit between Q1s base and collector terminals.

(c) Current source isnt ideal due to finite output resistor of Q2

A li i f h S ll Si l


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Application of the Small-Signal

Models

a. Determine the DC operating point of BJT and in

particular the DC collector currentIC(ICQ).

b. Calculate the values of the small-signal model parameters,

such as gm=IC/VT, r=/gm=VT/IB, re=/gm=VT/IE.

c. Draw ac circuit path.

d. Replace the BJT with one of its small-signal models. The

model selected may be more convenient than the others

in circuits analysis.

e. Determine the required quantities.


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Basic Single-Stage BJT Amplifier

Characteristic parameters

Basic structure

ConfigurationCommon-Emitter amplifier

Emitter directly connects to ground

Emitter connects to ground by resistorRE

Common-base amplifier

Common-collector amplifier(emitter follower)


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Definitions

Input resistance with no load

Input resistance

Open-circuit voltage gain

Voltage gain

LRi

ii

i

vR

i

iin

ivR

LRi

o

vov

v

A

i

ov

v

vA


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Definitions(contd)

Short-circuit current gain

Current gain

Short-circuit transconductance

0

LRi

ois

i

iA

i

oi

i

iA

0

LRi

om

v

iG


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Definitions(contd)

Output resistance of amplifier proper

0

ivx

xoi

vR

Output resistance

0

sigvx

xout

i

v

R


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Definitions(contd)

Voltage amplifier

Transconductance amplifier

Voltage amplifier


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Relationships

Voltage divided coefficient

sigin

in

sig

i

RR

R

v

v

oL

Lvov

RR

RAA

omvo RGA

oL

Lvo

sigin

in

vRR

RA

RR

RG

vo

sigi

ivo A

RR

RG

outL

L

vov RR

R

GG


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Basic Structure

Basic structure of the circuit used to realize single-stage,

discrete-circuit BJT amplifier configurations.


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Common-Emitter Amplifier


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Common-Emitter Amplifier

Equivalent circuit obtained by replacing the transistor with its hybrid-model.


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Characteristics of CE Amplifier

Input resistance

Voltage gain

Overall voltage gain

Output resistance


rRin

)////( LComv RRrgA

sig

oLCv

Rr

rRRG

)////(

Cout RR

is

A


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Summary of CE amplifier

Large voltage gain

Inverting amplifier

Large current gain Input resistance is relatively low.

Output resistance is relatively high.

Frequency response is rather poor.

The Common Emitter Amplifier


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The Common-Emitter Amplifier

with a Resistance in the Emitter

The Common Emitter Amplifier


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The Common-Emitter Amplifier


Characteristics of the CE Amplifier


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Characteristics of the CE Amplifier


Input resistance

Voltage gain


Output resistance


))(1//( eeBin RrRR

ee

LCv

Rr

RRA

//

))(1(

)//(

eesig

LCv

RrR

RRG

Cout RR

isA


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Summary of CE Amplifier withRE

The input resistanceRinis increased by the factor

(1+gmRe)

The voltage gain from base to collector is reduced

by the factor (1+gmRe). For the same nonlinear distortion, the input signal

vican be increased by the factor (1+gmRe).

The overall voltage gain is less dependent on thevalue of.


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Summary of CE Amplifier withRE

The reduction in gain is the price for obtaining the

other performance improvements.

ResistorREintroduces the negative feedback into

the amplifier.

The high frequency response is significant

improved.


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Common-Base Amplifier


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Common-Base Amplifier


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Characteristics of CB Amplifier

Input resistance

Voltage gain


Output resistance


ein rR

)//( LCmv RRgA

esig

LCv

rR

RRG

)//(

Cout RR

isA


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The Common-Collector


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Amplifier or Emitter-Follower



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The Common Collector




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The Common Collector



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Characteristics of CC Amplifier

Input resistance

Voltage gain


Output resistance


)//)(1( Loeib RrrR

)//)(1(

)//)(1(

Loe

Lov

Rrr

RrA

)//)(1(

)//)(1(

//

//

Loe

Lo

sigibB

ibBv

Rrr

Rr

RRR

RRG

1

//sigB

eout

RRrR

)1( isA

Summary for CC Amplifier or


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Summary for CC Amplifier or

Emitter-Follower

High input resistance

Low output resistance

Voltage gain is smaller than but very close tounity

Large current gain

The last or output stage of cascade amplifier

Frequency response is excellent well


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Summary and Comparisons

The CE configuration is the best suited for realizing the

amplifier gain.

IncludingREprovides performance improvements at the

expense of gain reduction. The CB configuration only has the typical application in

amplifier. Much superior high-frequency response.

The emitter follower can be used as a voltage buffer and

exists in output stage of a multistage amplifier.

Internal Capacitances of the BJT


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Internal Capacitances of the BJT

and High Frequency Model

Internal capacitance

The base-charging or diffusion capacitance

Junction capacitances

The base-emitter junction capacitance

The collector-base junction capacitance

High frequency small signal model

Cutoff frequency and unity-gain frequency

The Base-Charging or Diffusion


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The Base Charging or Diffusion

Capacitance

Diffusion capacitance almost entirely

exists in forward-biasedpnjunction

Expression of the small-signal diffusion

capacitance

Proportional to the biased current

T

CFmFde

V

IgC

J i C i


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Junction Capacitances

The Base-Emitter Junction Capacitance

The collector-base junction capacitance

0

02

)1(je

m

oe

BE

je

je C

V

V

CC

m

oc

CB

VV

CC

)1(

0

The High-Frequency Hybrid-


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The High Frequency Hybrid

Model

jede CCC Two capacitances C andC, where

One resistance rx. Accurate value is obtained form high frequency

measurement.

The Cutoff and Unity-Gain


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The Cutoff and Unity Gain

Frequency

0

)(

CEvB

Cfe

I

Ish Circuit for deriving an expression for

According to the definition, output port is short circuit



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C U y G

Frequency(contd)

Expression of the short-circuit current

transfer function

Characteristic is similar to the one of first-

order low-pass filter

rCCssh

fe )(1)( 0



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y

Frequency (contd)

rCC )(

1

CC

gmT

0

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