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voltage gain of common mode feedback circuit

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jimito13

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Hi,

I am uploading the picture of a differential differences (DDA) common mode feedback circuit.Does anybody know the equation that gives the dc gain of this topology.I have gone through google and known analog design books and papers but i can't find it...I would appreciate any helpful answer.Thanks in advance.
 

Well, the transconductance gain is gm(Vo1+Vo2-2Vcm). You'll need to multiply this with the transresistance gain from CMC to Vo1/Vo2 to get the CMFB gain.
 

Thanks checkmate for your answer.I don't care about the transconductance gain for this circuit,but for the dc voltage gain : Vcmc/(V01-V02) .Can you provide me this equation?I would appreciate that.
 

jimeece13 said:
dc voltage gain : Vcmc/(V01-V02)
This is a common mode - not a differential mode amplifier, i.e. it amplifies the sum of the input voltages, not their difference! If all PFETs have the same transconductance (gm21), and all NFETs have the same transconductance (gm25), then - to a first order approximation - Vcmc/(V01+V02)= gm25/gm21 .
 

ok,thanks erikl for making it more clear to me!
 

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