0195136047.pdf

8.5 FREQUENCY RESPONSE OF AMPLIFIERS 411

R 1 R 2

Cs

RD RL

(a)

RS

+

−

vS

+

−

vGS

+

−

vL

gm vGS

Output

voltage

Input ac

source

S

CG

GD

CD

RSS

ro

R 1 R 2 Cgs ro Cds RD RL

(b)

RS

+

−

vS

+

−

vGS

+

−

vL

gmvGS Output

voltage

Cgd

Input ac

source

Figure 8.5.2Small-signal equivalent circuits for CS JFET amplifier.(a)For low frequencies nearωL(including
coupling capacitors and bypass capacitor).(b)For high frequencies nearωH(including parasitic capacitances
of transistor).

where

Av 0 =

−gmRGRDRL

(RS+RG)(RD+RL)

(8.5.3)

RG=R 1 ‖R 2 =

R 1 R 2

R 1 +R 2

(8.5.4)

ωZ=

1

RSSCS

(8.5.5)

ωL 1 =

1

RL 1 CS

(8.5.6)

ωL 2 =

1

RL 2 CD

(8.5.7)

ωL 3 =

1

RL 3 CG

(8.5.8)

RL 1 =

RSS

1 +gmRSS

(8.5.9)