Design problem for CMFB

[gunpla]

Dear All:

i need to design common mode feedback(CMFB) for fully differential OPAmp

but i don't know to determine the value of R(SC) and C

how to design these two value in SC base circuit?

thanks for your read

 

[liuyonggen_1]

you could see the last chapter of gray's book.

 

[jonashat]

The CMFB circuit has a time constant representing the time constant of the common mode, it should be faster than the differential path. Cs is typically 1/3 to 1/5 Cc and some even say you can use the minimum size unit cap for Cs.

You can read more about it in:

Senderowicz, D.; Dreyer, S.F.; Huggins, J.H.; Rahim, C.F.; Laber, C.A., "A family of differential NMOS analog circuits for a PCM codec filter chip," Solid-State Circuits, IEEE Journal of , vol.17, no.6pp. 1014- 1023, Dec 1982