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CMOS opamp diff pair design

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The first stage of a CMOS opamp consists of NFET tail current NFET, NFET diff pairs and two PFETs for the current mirror load.

The NFET tail current and PFET current mirror load are should be in saturation.

Is the NFET diff pair also in saturation for all but the highest Vcm or is it better if it is in triode region all the time?
 

Hi,

Not sure, I thought - using terminology I've read about - it goes from weak inversion (not doing much, but can be useful for certain designs), to strong inversion (useful region), then at higher Vin reaches saturation (nothing changes with further increases in Vin).

Sorry if that's not what you're asking about.
 

Gain is maximized if everything is in saturation, but
you don't always get to say.

You say with certainty how a diff pair is designed,
but what you say is not always so. Sometimes a
P diff pair is better and very often a rail-rail input
will consist of both N- and P- diff pairs woven
together so when one quits working well, the other
has come around.

An N diff pair will go from cutoff (bad) to saturation
(good) to triode / linear (not so good) as VCM rises
from 0.
 

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