Hi,

I tryied to simulate the gain and banswidth of my folded cascode with SC-CMFB. Nothing is special in my circuit.

The first method I used the way ywguo proposed on the forum, which replace the SC with equivalent resisotors and then AC simulation. I got about 69dB DC gain.

The second method I used is to  use a ideal CMFB composed of VCVS to run AC simulation and I got similar results as that in the first method.

However, when I used PSS and PAC to simulate the circuit, I got only 53dB DC gain. I checked the DC voltages of the differential output, they are about what I want I think the CMFB works.

I have no idea why the DC gain from PAC is so low, hope someone can give me a hand.

I guess it is due to some loading of my CMFB to the opamp. When I inserted a buffer (VCVS) between my opamp output and CMFB, I could get 69dB with PAC simulation. So how can I get around this problem? I use only 100f and 200f Caps in my CMFB. They are pretty small, aren't they?

Btw, how can I check DC operating points, such as gm, working region, etc. after running PSS analysis.

thanks a lot,
jimmy