Common Base and Common Collector Amplifiers

Reminder: BJT Small Signal Model

ib

r

E

B Cic

ib

ie

Definition of input and output voltage and current directions

Amplifier

iin iout

vin vout

Common Base Amplifiers

• Uses the same stabilised bias scheme as CE configuration.

Common Base Amplifiers

• Uses the same stabilised bias scheme as CE configuration.

• The base is grounded for a.c. signals using a large capacitor.

• Signal input at the emitter (between the emitter and the common base)

• Signal output at the collector (between the collector and the common base)

Common Base Amplifiers

+VCC

GND

RE

R1

R2 CB vin

RC

voutRL

Common Base Amplifiers

• We will ignore the a.c. coupled load RL

R1

R2 CB

RC

vin

+VCC

GND

RE

vout

CB Amplifier, a.c. equivalent.

E

B

C

ib

ic= iout

RC

r

vin RE

ie

iin

vout

ib

Common Base Amplifier

1i-

i

i

i gain Current

e

c

in

out

Common Base Amplifier

π

c

in

out

r

R β

v

v gain Voltage

Ccout R i- v

β

cπbπin

ir- i r - v

Common Base Amplifier

• Input impedance to transistor

)(1

r

i-

ri-

i-

v

e

b

e

in

Common Base Amplifier

E

B

C

ib

ic= iout

RC

r

vin RE

ie

iin

vout

ib

vin

Common Base Amplifier

• Input impedance to stage (whole amplifier)

)(1

r //R

E

Common Base Amplifier

E

B

C

ib

ic= iout

RC

r

vin RE

ie

iin

vout

ib

Common Collector Amplifier

• Signal input between base and common collector.

• Signal output between emitter and common collector.

Common Collector Amplifier

VCC

R1

R2RE

vin vout

CC Amplifier, a.c. equivalent circuit

vs

RS

ib

r

B

C

ib io

REvoutRB = R1//R2vin

ieE

Common Collector Amplifier

bcbe iiii )1(

]β)R(1[riRiriv Eπ bEeπ bin

Common Collector Amplifier

• Transistor input impedance =

]β)R(1[r iv R Eπ

b

in ib

Common Collector Amplifier

E bE eout β)R(1iRiv

]R )(1 [r i

R i β)(1

v

v gain Voltage

E πb

Eb

in

out

]β)R(1[riv Eπ bin

Common Collector Amplifier

1]R β)(1 [r

R β)(1gain Voltage

E π

E

)1(i

i

i

igain Current

b

e

in

out

Common Collector Amplifier

• To find the output impedance we note that:

• Hence

πb B Sbout r i-R// R i- v

lowβ)(1

)r R// (R

i

v R

πB S

e

outout

Common Collector Amplifier

• The voltage gain is slightly less than unity.

• The CC circuit is sometimes known as an emitter follower since the output voltage at the emitter follows the input voltage at the base.

Common Collector Amplifier

• The prime function of this circuit is to connect a high resistance source to a low resistance load (“a buffer amplifier”).

• Since voltage gain is approximately one this is achievable as the current gain is high.