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).