Does Monte carlo variation depends on input common mode of amplifier?

[pmr9]

Hi All,
I have a question regarding MC variation (offset of amplifier), Does Monte carlo variation depends on input common mode of amplifier??

Thanks

 

[crutschow]

Input common-mode (voltage) does not typically have much affect on input offset voltage of a well designed differential amplifier.

 

[pmr9]

Thanks crutschow, Just thinking that reduced common mode will reduce gm, that increases the mismatch contribution from the load devices. Considering the relation: (gm_diff/gm_load)*Vth_mismatch_loads

 

[dick_freebird]

When common mode position drives the front end into a
linear region (MOS) or saturation or near-cutoff (BJT)
the gain will be reduced. The common mode position at
which this rolls on, is process / temp / voltage variable.
So at extreme common mode positions you may see
"enhanced" process sensitivity (like, if you're 0.5V from
the lower rail with a NMOS front end, and VTN varies
0.3 - 0.7V, half of the distribution might be into cutoff
of the diff pair tail current and gain tanks, bandwidth
pulls in, etc.).

This is why rail-rail input stages are interesting.