Lecture 12

Bipolar Junction Transistor (BJT)

BJT 1-1 Wednesday 15/11/2017

Outline

Continue BJT analysis with small AC signal

Common-Emitter configuration

• Analysis using various biasing circuits

BJT 1-2

Common-Emitter Configuration

BJT 1-3

bbe III 1

The diode re model can be replaced by the resistor re.

e

eI

mV 26r

Can you deduce the input impedance ?

Common-Emitter Configuration

BJT 1-4

Input impedance:

Output impedance:

Voltage gain:

Current gain:

e

B

ii rZIV

o o

o L

Z r

Z R

e

LV

r

RA VV

i

o

oi rA o C

i B

I II I

RL

Vo

Vi

At load:

Common-Emitter Fixed-Bias Configuration

BJT 1-5

• The input is applied to the base

• The output is from the

collector

• High input impedance

• Low output impedance

• High voltage and current gain

• Phase shift between input and

output is 180

Common-Emitter Fixed-Bias Configuration

BJT 1-6

AC equivalent circuit

re model-based BJT circuit

Common-Emitter Fixed-Bias Calculations

BJT 1-7

Co 10Rre

Cv

e

oC

i

ov

r

RA

r

)r||(R

V

VA

eBCo r10R ,10Rri

eBCo

oB

i

oi

A

)r)(RR(r

rR

I

IA

C

iv

i

iR

ZA

ZA

VRV

II

i

Co

i

o

Current gain from voltage gain:

Input impedance:

Output impedance:

Voltage gain: Current gain:

B e

i B e

i e R 10 r

Z R || r

Z r

Co

O

R10rCo

Co

RZ

r||RZ

Common-Emitter Voltage-Divider Bias Configuration

BJT 1-8

AC equivalent circuit using the re model

Common-Emitter Voltage-Divider Bias Calculations

BJT 1-9

Current gain from voltage gain:

Input impedance:

Output impedance:

Voltage gain:

Current gain:

ei

21

r||RZ

R||RR

Co 10RrCo

oCo

RZ

r||RZ

Co 10Rre

C

i

ov

e

oC

i

ov

r

R

V

VA

r

r||R

V

VA

eCo

Co

r10R ,10Rri

oi

10Rrei

oi

eCo

o

i

oi

I

IA

rR

R

I

IA

)rR)(R(r

rR

I

IA

C

ivi

R

ZAA

Common-Emitter Emitter-Bias Configuration

BJT 1-10

AC equivalent circuit using the re model

Common-Emitter Emitter-Bias Calculations

BJT 1-11

Eb

Eeb

Eeb

bBi

RZ

)R(rZ

1)R(rZ

Z||RZ

Input impedance:

Output impedance:

Co RZ

Voltage gain:

Eb

Eeb

RZE

C

i

ov

)R(rZEe

C

i

ov

b

C

i

ov

R

R

V

VA

Rr

R

V

VA

Z

R

V

VA

Current gain:

bB

B

i

oi

ZR

R

I

IA

Current gain from voltage gain:

C

ivi

R

ZAA

Example

BJT 1-12

For the CE Emitter-Bias Configuration circuit: (a) Determine re and the internal small-signal transistor output resistance (b) Find Zi and Zo. (c) Calculate Av and Ai.

Solution

BJT 1-13

Emitter-Follower Configuration

BJT 1-14

• This is also known as the common-collector configuration.

• The input is applied to the base and the output is taken from the

emitter.

• There is no phase shift between input and output.

Emitter-Follower Configuration Calculations

BJT 1-15

Input impedance:

Output impedance:

Current gain from voltage gain:

Voltage gain:

Current gain:

i B b

b e E

b e E

b E

Z R ||Z

Z r ( 1)R

Z (r R )

Z R

eE rReo

eEo

rZ

r||RZ

EeEeE RrR ,rRi

ov

eE

E

i

ov

1V

VA

rR

R

V

VA

bB

Bi

ZR

RA

ii v

e

ZA A

r

BJT 1-16

Lecture Summary

Covered material Continue BJT analysis with AC signal

Common-Emitter and common-collector configurations • Analysis using various biasing circuits

Material to be covered next lecture

Effect of source/load impedance on current and voltage gain

Introduction to cascaded systems Introduction to FET transistors