8.5 FREQUENCY RESPONSE OF AMPLIFIERS 409
RS
+
−
iS v
S
+
−
vL
RSS
v 1
Input
gmvGS ro RD RL
+
−
vGS
GDiL
S
Output
(b)
Figure 8.4.3Continued
On comparing the CS, CD, and CG configurations one can make the following observations:
- For CS and CD configurationsRin=R 1 ‖R 2 , which can be selected to be large during
the bias design. For the CG configuration, however,Rinis not very large, on the order of
a few hundred ohms.
- For the CD configuration the voltage gain is generally less than unity or near unity, while
it can exceed unity in the other configurations. The voltage gain of the CG configuration
is slightly larger than that of the CS configuration.
- For the CG configuration the current gain cannot be larger than unity. But for the CS and
CD configurations it can be large by the choice ofRin.
MOSFET Amplifiers
Because the same small-signal equivalent circuits apply to both the JFET and the MOSFETs, all
the equations developed for the JFET amplifiers hold good for the MOSFET amplifiers, so long
asgmandroare computed properly. For the depletion MOSFET the equations forgmandro, given
by Equations (7.4.4) and (7.4.5), are the same as for the JFET. For the enhancement MOSFET,
they are given by Equations (7.4.9) and (7.4.10).
8.5 Frequency Response of Amplifiers
All amplifiers exhibit variations of performance as the signal frequency is changed. The frequency
response of an amplifier may be defined as the functional dependence of output amplitude and
phase upon frequency, for all frequencies. Invariably there is a maximum frequency above which
amplification does not occur. Depending on the design of the circuit, there may also be a lower
frequency limit below which amplification disappears. The range of frequencies over which
significant amplification of the magnitude is obtained is known as thepassband, which is bounded
by the upper cutoff frequency and the lower cutoff frequency (if one exists).Dc-coupledamplifiers,
in which the stages are coupled together for all frequencies down to zero, have no low-frequency
limit. On the other hand, circuits containing coupling capacitors that couple stages together for
