Bias Circuit

etween the drain and source of the transistor, but the circuit as given is equivalent, as the
source is at ground. Since the output voltage is Vo = –IRDRD, the new gain result is
Equation 5.4
Note that this form evolves from ideal transistor current, gmVgs, flowing through the parallel
combination of the output resistance and RD.
To facilitate an intuitive grasp of the magnitude of the effect of gds, we use the expression for
gds (4.13) in (5.4), to obtain
Equation 5.5
Note that IDRD is the voltage drop across RD. For example, for a –10-V power supply, we
choose IDRD 5 V. A measurement of λp for our devices will show that λp 1/20 V, which
results in λpIDRD
1/4. Thus, the effect of gds (= λpID) for this case is significant.
Finally, we can get an overall current dependence for av with the elimination of gm, using(4.5)
with kp, which results in
Equation 5.6
Using an alternative form for gm (= 2ID/Veffp), also (4.5), the gain expression is
Equation 5.7
where
For simplicity, approximate forms of (4.5) and (4.13) of gm and gds are used here, which are
independent of VSD. For reference, the "exact" and approximate forms of (4.5) and (4.13),
respectively, are repeated here: