Is gm (and thus device fT) independent of overdrive voltage (Eq. [9.57])
in nanometer (or long-channel) CMOS?
Both gm and fT vary greatly with the overdrive voltage. See Eq. (9.55)
(or, for long-channels, Eq. [9.36]) and Fig. 9.33.
The most important aspect of this dependence is that to increase CMOS
device speeds (fT) you need to increase the overdrive voltage. This is
one of the reasons why we bias with the BMR (it's easy to set overdrive
voltage with changes in temperature and process).
The confusion comes from claims that short channel devices' fT
does not vary with overdrive voltage because of saturation velocity effects
(as seen in Eq. (9.57) if vsat is constant). vsat isn't constant and does
depend on both VGS and VDS as indicated on page 299 (clearly gm does
increase with overdrive as seen in Fig. 9.33). The reasons for the
variations in vsat are outside the scope of the book (e.g. velocity
Biasing is often a neglected topic when learning to design CMOS analog ICs.
This is unfortunate because it is the MOST IMPORTANT topic in analog IC
design. When I look at someone else's design the first things I look at
are the threshold voltage of the process and then VGS so I can determine
overdrive. If the VGS is close to the threshold voltage (so overdrive
is close to zero) then I know that the design will be inherently slow (and
operating in weak inversion).
If the overdrive voltages vary from device to device then the design isn't
optimized for speed (it's that simple). If I need more speed, in a two stage
op-amp, I simply decrease the compensation capacitor (to push the unity gain
frequency, fun = gm/2πCc, out to a higher frequency) and increase the widths
of the output buffer (keeping the overdrives constant, with an increase in
current). This pushes f2 (see Eq. [24.24]) to a higher frequency (because
of the increase in gm2 with the increase in device widths) so that f2 > fun.
Again, for general design set overdrives to 5% of VDD. For high-speed designs
set overdrive voltages to 10% of VDD or more.