CMFB Stability issue

[mordak]

Hi,

I am using an opamp like the one attached, it's a simple current mirror amplifier. To set the common mode voltage of the output, I used a CMFB circuit like the one in the right hand side of the picture to control the VB2 voltage of the main opamp. In the CMFB circuit, Voc is the reference voltage, and Vcm is also average of the outputs of the main opamp using two resistors (in parallel with two capacitors).

My circuit works perfectly with an ideal CMFB, however, after adding the real CMFB circuit the pulse response of the opamp is not good anymore. By checking loop gain of the CMFB using cmdmprobe, I saw that it does not have good phase margin. Now my question is how I can make it better? I am not quite sure how one can improve phase margin for the CMFB, and not killing bandwidth of the CMFB.



Thanks

 

[erikl]

... how I can make it better? I am not quite sure how one can improve phase margin for the CMFB, and not killing bandwidth of the CMFB.

Perhaps somebody can help you if you show your current W/L, power supply, and process size values of your CMFB circuit. And tell which unity gain bandwidth and phase margin you have now.