How does the following offset cancellation works?

[sys_eng]

The following diagram shows internal offset cancellation.

It looks alot like a common mode feedback.

Are they the same thing? How does the offset cancellation works in here?

 

[D.A.(Tony)Stewart]

Are M15 & M16 different geometry such that one has a slightly lower Vgs but lower gm so that there is a neutral point of uA current swing at some middle of the range of Vc input. Thus creating offset above and below this point? just a guess.

 

[Ata_sa16]

I think C1 and C2 are short for AC signal so circuit working normally.

But when there is an offset in VGA probably because of high gain output voltages will be 0 and VDD. (Vo+ =0 and Vo-=VDD if Vcm1 = Vcm2 +Voff )

This time M13 is on and M14 is off so M13 adds more current to that branch (M2 branch) and compensate this unbalance so again Vo+ and Vo- meats somewhere Vo+ = Vo- = VoDC

 

[FvM]

Won't talk about transistor being "on" or "off", the circuit should work in linear range normally. It's simply a negative feedback path with low pass characteristic.