0195136047.pdf

8.4 FET AMPLIFIERS 407

Rin=R 1 ‖R 2 =

300 ( 100 )

400

=75 k

Ai=

Rin

RL

Av 1 =

75

3

(− 2 )=− 50

Common-Drain (CD) JFET Amplifier

Figure 8.4.2(a) shows a CD JFET amplifier in which resistorsR 1 ,R 2 , andRSSare selected by the
bias design, and capacitorsCGandCSare chosen to be large enough to act as short circuits at
frequencies in the band of interest (known as the midband). Figure 8.4.2(b) gives its small-signal
ac equivalent circuit. Analysis of this circuit yields

Ri∼=∞ (8.4.6)

Rin=R 1 ‖R 2 =

R 1 R 2

R 1 +R 2

(8.4.7)

Av 1 =

vL

v 1

=

gmro(RSS‖RL)

ro+(RSS‖RL)( 1 +gmro)

(8.4.8)

Ai=

iL

iS

=

gmro(RSS‖RL)(R 1 ‖R 2 )

RL[ro+(RSS‖RL)( 1 +gmro)]

(8.4.9)

In many casesAv 1 ∼=1 andvL=v 1 ; that is to say the load voltage “follows” the input. Hence a
CD amplifier is often known as asource follower, which becomes an excellent buffer to couple a
high-resistance source to a low-resistance load with nearly no loss in signal voltage. The current
gain, however, can be very large, leading to significant power gain.

Common-Gate (CG) JFET Amplifier

Figure 8.4.3(a) shows a CG JFET amplifier in which resistorsR 1 ,R 2 ,RD,andRSSare selected by
the bias design, and capacitorsCDandCSare chosen to be large enough to act as short circuits at

R 1

R 2

(a)

RS

+

−

vS

Input ac source

CG

Output

voltage

Rin

+

−

VDD

Ri

RSS

G

S

D

+

−

vL

CS

Ground

RL

n- Channel

JFET

Figure 8.4.2Common-drain (CD) JFET amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.