Operational Amplifier Question in CMFB stability

[amr.walid]

Hi All
I have a problem that I want to design Op-Amp and its bias set as shown in the attached figures so When I vary the resistors of the bias the CMFB loop vary also so I can't get a defined Phase Margin across all values of resistor.
Please I want an answer to this effect that I can't understand it.
Thanks in advance

 

[FvM]

I guess the problem is created by an unrealistic high common mode source impedance which won't be found in a real world amplifier setup.

You should add a small amount of common mode capacitance, e.g. from transformer center tap to ground.

 

[amr.walid]

Thanks for your help but mainly my problem is that I can't understand how these common mode bias resistors can affect my CMFB loop also this loop hasn't any relation with these two resistors.

 

[FvM]

Surely they are related.

Please consider what happens if the input stage is driven by a high common mode impedance. Also remember that MOSFETs have finite input impedance at higher frequencies.

How about sketching an equivalent circuit of the common mode amplifier and feedback path?