some questions about CMFB capacitor, too

[lhlbluesky]

for a fully differential opamp, SC-CMFB is commonly used. if Gain-Boosted telescopic opamp is used, then both main opamp and auxiliary amplifier need SC-CMFB circuit. i want to ask:

how to choose the value of the two cmfb capacitors C1 and C2 (assume C2 is cmfb capacitor) for both main opamp and auxiliary amplifier?

someone says, according to the bandwidth requirement of DM loop and CM loop, that is, BW-CM>BW-DM. is that right? and how to realize it? and how to calculate the feedback factor for both DM loop and CM loop and for both main opamp and auxiliary amplifier? for auxiliary amplifier, there is no DM loop, then how to get the DM feddback factor of it?

another question: how to decide the current ratio of main opamp and auxiliary amplifier? someone tell me it is about 5:1 to 10:1, but why? are there some reasons?

finally, why PMOS transistor has smaller 1/f noise than NMOS transistor?

i'm terribly confused about these problems, pls help me.
thanks everyone for reply.

 

[lhlbluesky]

can everyone help me,pls?

 

[arunkumar446]

i think cmfb is needed only for the OTA not for the auxiliary amp. the capacitors ratio should be chosen depending on the common mode gain and the smoothness you need at the output of the amplifier