8.4 FET AMPLIFIERS 405
On comparing the CE, CC, and CB configurations one can come up with the following
observations:
- RiandRinare largest for the CC configuration, smallest for the CB, and in between those
extremes for the CE.
- While there is a sign inversion in the voltage-gain expression with the CE amplifier, about
the same magnitude of gain (which can be greater than unity) results for the CE and CB
configurations. For the CC amplifier, however, the voltage gain cannot exceed unity.
- Both CE and CB configurations can yield large current-gain magnitudes; the CB amplifier
has a current gain less than unity.
8.4 FET Amplifiers
Just like the BJT amplifiers, FET amplifiers are constructed in common-source (CS, analogous
to CE), common-drain (CD, analogous to CC), and common-gate (CG, analogous to CB)
configuration. First we shall consider JFET amplifiers, then we show how the results are modified
with MOSFETs.
Common-Source (CS) JFET Amplifier
Figure 8.4.1(a) shows a CS JFET amplifier in which resistorsR 1 ,R 2 ,RD,andRSSare selected
by the bias design, and capacitorsCG,CD, andCSare chosen to be large enough that they act as
short circuits at the lowest frequency of interest in the input signalvS. Figure 8.4.1(b) gives its
small-signal equivalent circuit. Noting that the input impedance of a JFET is very large, we have
Ri∼=∞ (8.4.1)
Rin=R 1 ‖R 2 =
R 1 R 2
R 1 +R 2
(8.4.2)
R 1
R 2
(a)
RS
+
−
vS
Input ac source
CG
Output
voltage
Rin
+
−
VDD
Ri
RSS CS
G
S
D
+
−
vL
CD
Ground
RL
n - Channel
JFET
iS
RD
iL
Figure 8.4.1Common-source (CS) JFET amplifier.(a)Circuit.(b)Small-signal ac equivalent circuit.
