when do we need to use CMFB?

[knack]

Hi there,
When designing a Diff. Pair amp., when should one use a CMFB ?
Thanks,
--Knack

 

[eng_Semi]

You will need a CMFB circuit if u r designing a fully differential OpAmp, this is because you will two high impedance nodes and thus the dc level couldn't be maintained at a certain level

 

[knack]

You will need a CMFB circuit if u r designing a fully differential OpAmp, this is because you will two high impedance nodes and thus the dc level couldn't be maintained at a certain level

Sorry, I couldn't get what you said.
Please give more explanation,

Cheers,
--Knack

 

[eng_Semi]

You will need a CMFB circuit if u r designing a fully differential OpAmp, this is because you will two high impedance nodes and thus the dc level couldn't be maintained at a certain level

Sorry, I couldn't get what you said.
Please give more explanation,

Cheers,
--Knack

You can check the chapter of the fully differential amplifier from Razavi, it is very well explained

 

[iamxo]

CMFB is used to fix the output dc voltage level which comes from up and down currents mismatch.

and it can't be fixed through amplifier's negative feedback. so another feedback(CMFB) is used.