Common-mode feedback in the full differential opamp?

[wangh19]

common mode feedback

I design a full differentiall opamp with cmfb. It works well as an open loop amplifier, and there is no problem with PM and stability. But when I use it in a closeloop way to constitute a resisters feedback opamp, it can not work. I check the dc point of the input which gose to VDD, so it seems the cmfb can not work in a reasonable way to force dc voltage of the input to follow the input common mode reference voltage . what is wrong with my circuit? thanks

 

[chippendale]

common-mode feedback

Hi,
a schematic would be nice

 

[wangh19]

miller opamp common mode feedback

the opam circuit is just copyed from Razavi book, there is not any tricky points relevant to my circuit. The opam is differential common two stage amplifier with miller cap compensation, and the cmfb use a error amplifier senses the dc point of opam differential output votage and feedback to control the bias voltage of current load transistors of the first stage opamp amplifer. May any one give me some sugesstions to check the problem?thanks

 

[jerryzhao]

differential and common mode operation of opam

Could the opamp diver the resistor loading? Maybe the resistor effect the output resistance.
You'd better upload the schematic.

 

[kundahari]

common mode feedback

Hallo, I designed same config..Fully differential with cmfb.
Can you please upload the schematic..just screenshot..
so that i can help you,

Thanks