about cmfb and cm gain- explanation needed

[lhlbluesky]

about cmfb and cm gain

i have a simple question:in fully differential opamp, why is cmfb needed?what's the function of cmfb?
besides,a commonly used cmfb is sc-cmfb,i want to know what's the advantage and disadvantage of sc-cmfb? some says sc-cmfb has high linearity,but why?can anyone explain to me?

another question:how to calculate cm gain of a fully differential opamp?
Acm=(Vo1-Vo2)/Vcm or Acm=Vo1/Vcm,Vcm is common mode voltage,and Vi1=Vi2=Vcm;that is,differential output voltage/Vcm or single output voltage/Vcm(calculate by half common mode circuit)?which way is correct?pls tell me.thanks.

 

[arunkumar446]

Re: about cmfb and cm gain

cmfb is needed in fully differential OTA's because a little difference in the currents between the top pmos stack and the bottom nmos stack will bring the output node to either of the supply levels depending on the current difference. so a CMFB is must for a fully differential amplifying stage.

 

[BackerShu]

about cmfb and cm gain

some addition for arunkumar446~~


why the SC-CMFB has high linearity?It's mainly because the SC-CMFB has no additonal poles in the CM loop as a CMFB method which include complex circuit(like amp) does.


As to the CM gain calculation, I think (Von+Vop)/Vcm should be right for fully differential amp.

 

[shobhit]

Re: about cmfb and cm gain

when a pmos and and a nmos current source are connected in series then due to difference in current driving capabilities the difference of current Ip-In flows through Ro effective thereby changing the dc level of output voltage. The CMFB circuit track the variation of this dc level and hence the output of cmfb circiut has linear relationship with vo. The output voltage of CMFB is used to regulate the gate voltage of cascode transistors thereby adjusting the current supplied by the pmos transistors.

Hope this helps!!

 

[lhlbluesky]

about cmfb and cm gain

thanks all for reply.