a question about the CMFB of a full differential amplifier

[lily1981216]

how to design cmfb differential amp

hello, I am designing a two stage operational amplifier.I always see the CMFB to control the out of the first differential input stage. How about the comon source stage? Why not use the CMFB to control the output of the common source stage?
Thanks!

 

[dick_freebird]

I'm sure there are multpile points where you can inject
an offset current / voltage to push the output CM sum,
but practicalities probably favor one.

For example, you probably do not need or want a high
V/V gain because the VCM generally needs only small
correction if the circuit is balanced and mismatch-
desensitized (large devices, etc.) and any excess gain
probably only increases noise coupling. A common-
source amplifier's gate would be a bad place to work
then, as gain is high.

Also the output of the common source stage is where
you pick up most of your gain (gm*Rout) and any
element you add will degrade Rout hence the -desired-
forward gain.

But nothing should stop you from trying out alternative
points of offset-injection, just to satisfy yourself that
you have in the end selected the best for the use.