Bias Circuit

With a 1 - V drop across RS and a 5 - V drop across RD, and with λn = 1/50 V, the new Gm is
gm/5.12, compared with gm/5 when neglecting gds [(4.8)].
In general, the complete circuit includes, in addition, the body-effect transconductance current
source of Figs. 4.3 and 4.4. The omission of this current source in Fig. 4.6 implies that Vsb = 0
because the source and body are connected. This connection is possible to implement in
special cases such as in some of our MOSFET lab projects where only one transistor on the
chip is used or for the case of a differential stage where the source of two transistors is at the
same node. It is also possible to eliminate the body-effect current source by bypassing the
source resistor with a bypass capacitor. The capacitor places the source at signal ground.
However, in this case, the dc threshold voltage is still affected by VSB = IDRS.
In Unit 8, the circuit transconductance equivalent to (4.8) and (4.18), but which includes the
body effect [(8.49)], is given as
where η is defined in (4.11). For gds = 0 and η = 0, (4.19) reduces to (4.8). Note that
including the body effect will in general have more effect on Gm than including gds, as η can be
on the order of 0.2. In all cases where we can calculate the circuit transconductance, Gm, the
magnitude of the voltage gain is obtainable from –GmRD (common-source stage) and
GmRS(source-follower stage).