I would ask you verify your assumptions.
Hi, Tony, thanks for your reply so far and I finally figured out the reason why I got this plot. It may be caused by the simulator (Spectre) or the PDK I used rather than the circuit design.
The simulation I did these days is measuring the common-mode feedback performance of an Op-amp in a modulator. Normally, its frequency response should be something similar to that of an OTA, i.e. the dominant pole is an LHP rather than an RHP I showed you before.
This has troubled me a lot recently and when I made a small change to the Op-amp I measured, which technically shouldn't have any significant impact on the frequency response, something
magical happened: the phase starts from 180 degrees and coincides with the previous curve at high frequency (the
purple curve shown below). That matches our theory.
As for the change I made, I just changed the connection of the bulk of the input differential pair. Before, I connected it to the source, and when I changed it to the power rail, the simulation plot was normal.