0195136047.pdf

404 TRANSISTOR AMPLIFIERS

C

B

E

CE

CC

+

−

VCC

+

−

vL

Ground

R 2

R 1

CB

RC

RE

RL

iL

RS

iS

npn BJT

+

−

vS

Rin

(a)

AC

source

Output

voltage

v 1

RC RL

+

−

vL

+

−

vS

gmvπ

E

C

rπ ro

Ground

v 1 = −vπ

Ri

RE

RS

B

+

−

vπ

Rm=REIIRi

IL

iS

(b)

Figure 8.3.3Common-base (CB) BJT amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.

Ri∼=

rπ

1 +gmrπ

(8.3.21)

Rin∼=

(rπ‖RE)

1 +gm(rπ‖RE)

(8.3.22)

Av 1 ∼=gm(RC‖RL) (8.3.23)

Ai∼=

gm(RC‖RL)(rπ‖RE)

RL[1+gm(rπ‖RE)]

(8.3.24)

Because of the low input resistance, the CB amplifier is often used as a current amplifier which

accepts an input current into its low impedance and provides an output current into a high

impedance. That is why a CB amplifier is sometimes referred to as a current follower.