0195136047.pdf

402 TRANSISTOR AMPLIFIERS

Common-Collector (CC) Configuration

A common-collector (CC) amplifier is also known as an emitter follower (or a voltage follower)

due to the fact that the output voltage “follows” the input by being approximately equal to the

input voltage. The amplifier is shown in Figure 8.3.2(a), in which the collector forms a common

terminal between the input and output circuits, and resistorsR 1 ,R 2 , andREare determined by

biasing. CapacitorsCBandCEare chosen large enough to appear as short circuits at the lowest

frequency of interest in the input signalvS. The output voltagevLis taken across the load resistor

RL. The small-signal ac equivalent circuit of the amplifier is shown in Figure 8.3.2(b), whose

analysis yields the following results:

RB=R 1 ‖R 2 =

R 1 R 2

R 1 +R 2

(8.3.7)

C

B

E C

E

+

−

VCC

+

−

RL

Ground

iS

+

−

vS

RS

R 2

R 1

CB

RE vL

IL

v 1

AC source

Output

voltage

npn BJT

Rin Ri

Load

resistor

(a)

R 1 ||R 2 =RB

(b)

RS

+

−

vS

gmvπ

E

C

iS rπ ro

i

iL

B

RB||Ri=Rin Ri Ground

v 1

+

−

RE vL RL

+

−

vπ

Figure 8.3.2Common-collector (CC) BJT amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.