CMFB loop stability in a 2 stage opamp

[mretsh91]

I am designing a two stage op amp with CMFB ,,,but the bode plots of my opamp CMFB loop is totally unstable although the differential bode plots is stable and the output is adjusted correctly by the CMFB to Vdd/2.


Thanks in advance

 

[pooh_bear]

Hi, I was wondering how the polarity of your CMFB works. I am assuming you are just sensing with resistors. If you sense the output of the second stage and feed that back to the first stage then it would cause positive feedback. Consider the output CM voltage from the second stage lowers a bit. This means your feedback voltage has to decrease. If your feedback voltage decreases then more current will be sourced from the first stage PMOS load. So Vout from first stage rises. Since your second stage is a common source, it will invert the first stage output voltage rise to a second stage voltage drop. So it looks like you do have positive feedback in your CMFB loop unless I missed something else from your schematic.

Try doing a common mode voltage step see if the output common mode voltage does settle. That way you can check for settling time and understhoot/overshoot behavior and match that with your bode plots.

There is useful discussion on CMFB here: https://www.edaboard.com/threads/133945/