In
the indirect compensation scheme
used in the book you use 1 capacitor which is traditionally
the
way I have used it as well. I have,
however, seen people use two capacitors when doing the
compensation,
one to the NMOS cascode
and one for PMOS cascode but I have never heard of
a
satisfactory answer for why they do
this. Is it to make the circuit symmetrical so that the PSRR
from
N-side and P-side are they
same?
There
is no need for two capacitors for
small-signals, a single capacitor will work fine and is
preferred
for power-supply rejection
reasons as discussed in the book. However for larger signals
it is
possible for one side to start
shutting off or simply have a considerably different small-signal
equivalent
behavior. Using two paths
helps to ensure that the op-amp is always compensated for
large
signals.
A
simple example of when one cap alone
for compensation can fail is when you have a step input
and
the side of the diff-amp that is
connected to the compensation capacitor shuts off (no compensation
while
the output changes, and you will
get ringing and a recovery time).