0195136047.pdf

408 TRANSISTOR AMPLIFIERS

R 1 R 2

(b)

RS

+

−

vS

+

−

vGS

+

−

vL

gmVGS

Output

Input

S

GD

ro

Rin = R 1 ||R 2 Ri − ∞ Ground

RSS RL

Figure 8.4.2Continued

the lowest frequency of interest in the input signalvS. Figure 8.4.3(b) shows its small-signal ac

equivalent circuit, whose analysis yields the following results:

Rin=

RSS(ro+RF)

ro+RF+RSS( 1 +gmro)

(8.4.10)

where

RF=RD‖RL=

RDRL

RD+RL

(8.4.11)

Av 1 =

vL

v 1

=

RF( 1 +gmro)

ro+RF

(8.4.12)

Ai=

iL

is

=

RF( 1 +gmro)RSS

RL[ro+RF+RSS( 1 +gmro)]

(8.4.13)

With practical values, quite oftenrois rather large andgmRSS>>1 so thatRin∼= 1 /gmand

Ai∼=RF/RL=RD/(RD+RL), which turns out to be less than unity.

CG

CG

RSS

RD

R 2 Ri

R 1

v 1

G

S

D

iL

RL

+

−

vL Output

voltage

+

−

vS

CS RS

Rin

AC Input source

+

−

VDD

iS

Ground

n- Channel

JFET

Figure 8.4.3Common-gate (CG) JFET amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.