i have designed a two stage opamp (used for a buffer, fck=15M, CL=12pF), the first stage is folded cascode, the second is common source, and i use cascode compensation to ensure frequency stability, now, i get the following simulation curve, when the input signal from 0.4 to 1.6, it settles well, but when the input signal from 1.6 to 0.4, it has a very big overshoot, why? the PM is 70 deg or so, and it is not like normal underdamped oscilation, what is the reason of this big overshoot? please give me some advice, thanks all.

Hi Lhlbluesky_lhl,

It is not unusual that for large input signals, the transfer function can be different for input signal going low to high and high to low.

- can you plot similar curves when input varies from say 0.4 to 0.6?
- also please provide plots when input varies from 1.4 to 1.6
- is your 1.6V close to supply voltage (if yes, it is possible that the impedance of the current loads are reduced, pushing one of the poles further out and causing the underdamped behaviour)