0195136047.pdf

400 TRANSISTOR AMPLIFIERS

C

B

E

CE

CC

+

−

VCC

+

−

VL

Ground

iS

+

−

vS

RS

R 2

R 1

CB

RC

RE

RL

iL

v 1

AC source

Amplified

output

ac voltage

npn BJT

Load

resistor

Bypass

capacitor

(a)

Rin Ri

RB=R 1 ||R 2 RC RL

(b)

RS

+

−

vS

+

−

vπ rπ +

−

vL = Av 1 v 1

gmvπ

E

C

ro

iS iL

B = AiiS

Rin=RB||Ri Ri = rπ Ground

v 1 = vπ

Figure 8.3.1Common-emitter (CE) BJT amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.

another part. CapacitorCE, known as the bypass capacitor, bypasses the ac current aroundREso

that no significant ac voltage is generated acrossRE, and helps to increase the gain.

The small-signal ac equivalent circuit is shown in Figure 8.3.1(b), in which the small-

signal model of Figure 7.3.7 for the transistor is used. While omitting the details of analysis

and summarizing the results, we have

RB=R 1

∥

∥R 2 = R^1 R^2

R 1 +R 2

(8.3.1)

Ri=





resistance between

transistor base and ground

(as seen looking into base)



=rπ (8.3.2)

v 1 =vπ=

vSRin

RS+Rin

(8.3.3)