common mode feedback malfunction?

[jimito13]

Hi,

I have designed a fully differential opamp.It's a folded cascode archtecture with gain boosters.Each module has it's own cmfb circuit.After adding the cmfb and after substistuting all ideal things of the schematic (dc voltages sources) with the respective biases from the biasing circuitry,i notice that my output node's common mode potential (output dc voltage) is about 80mv larger that the value that had initially(Vdd/2).Shouldn't be made the proper correction from the cmfb's?This means that my cmfbs don't work or it is another cause for this difference?I should say that cmfbs excibit 20dB of loop gain and above 90deg phase margin.

Thanks in advance for any helpful idea.

 

[Wu_uW]

Hi,

You checked the Gain of your CMFB is still normal?

 

[jimito13]

yes it remains at 20db and the PM is the same...

 

[Wu_uW]

Well, I dont quite know how your circuit of CMFB works. however, 20dB seems not a lot. say you want to keep a output voltage of 800mV then you need a input signal difference of 80mV for 20dB.

 

[jimito13]

it is the classical resistor averaged cmfb with the diff pair after that to make the amplification of the difference Vref-VCM,sensed.my desired output dc potential is at Vdd/2=1.2V/2=600mV.Do you have any idea about typical loop gain values?I suppose that what you said about the 80mv is referred to the input of the cmfb and not the whole amplifier,right??