how to calculate phase margin of fully diff folded cascade opamp

[bhaveshsoni]

I have got the phase value at 0db gain is -74deg so my phase margin is 180-74=106 or 180+74=254 which one is correct or is there any error in design ?

 

[Milad-D]

106 is correct. But I am wondering, how do you get this big phase margin. For single pole system, the phase margin is 90 degree. If you add a zero, deliberately, you might increase the phase margin more than 90.

if you the simulation right(?), this means that you have dominant zeros in your amplifer, which is approximately impossible in simple folded cascode differential amplifier. Therefore, something is wrong. Do you have CMFB circuit for your amplifier.

 

[bhaveshsoni]

thanx
I am not use CMFB simply Folded Cascode Input and Common Source Output Stage with 4 pf load capacitor my gain is 73 db. Now what is the error ? CMFB is require ? now I want to design 4.3MHz LPF using these opamp but it is not working can you solve these problem.