Hi,

I designed a 2 stage amplifier (with compensation) and when simulated (setup in attachment) using tran, it shows that the output common-mode of the amplifier is unstable.
I then used STB (substituting the SC-CMFB for an equivalent RC circuit) and PSTB, using the actual SC-CMFB, and placed the cmdmprobe (cmdm=1) at the output of the amplifier, thus breaking all loops in the circuit. STB and PSTB analyses always return a PM > 65º.
The DC OP point has been preserved, and the circuit is also unstable for Vin=vss (without stimulation).

Why can't I detect this instability? Is there something wrong with the setup?

Thanks in advance.