Does offset in an OTA reduce its open loop gain?

[diarmuid]

Hello all,

I understand that the larger the gain the less the effect of offset since the more its impact on the output is supressed.

But does offset itself reduce gain?

Say my input diff pair contributes significant Vt mismatch leading to offset. Would this reduce my Aol?

Thanks,

Diarmuid

 

[dick_freebird]

Not offset per se. But if you operate a dead-balanced
front end away from null (like for example, putting a
Vdiff=0 input to a Vio=2mV op amp sample) you will
reduce the observed AVOL about Vdiff=0 (it will be
better at Vdiff=2mV).

This is why op amp ATE setups tend to be closed loop
with substantial gain elsewhere, so that the input can
be forced to wherever works best.

If you ran a fine grained DC analysis and looked at
deriv(Vout)/derive(Vin) you should see the gain peak
right at Vio.

So what you get at Vin=0 will be less. But AVOL may
or may not be measured with Vin pinned that way,
depending on who's doing the testing.

 

[diarmuid]

Thanks that makes sense.

Just to make sure Im am understanding clearly:

Say I have an NMOS diff pair. The -ve terminal NMOS has a Vt shifted by +2mV.
=> Avol is max when Vin(-ve) is +2mV above Vin(+ve).

If both Vts were the same (and ignoring other offset sources), Avol would be max when Vin(-ve) = Vin(+ve).

Is this what you mean?

 

[LvW]

Hello all,
I understand that the larger the gain the less the effect of offset since the more its impact on the output is supressed.
But does offset itself reduce gain?
Say my input diff pair contributes significant Vt mismatch leading to offset. Would this reduce my Aol?

I must admit that I don`t understand your question.
At first sight, both amplifier properties (offset, gain) are not coupled to each other.
However, the situation changes when FEEDBACK comes into play.
Are you speaking of feedback?
Feedback reduces the gain and, hence, the dc offset at the output caused by an input offset.

 

[ravindragudi]

There is no relation between gain and the input offset voltage. However in closed loop, if there is a high input voltage then it will reflect into the output current as increased error.

 

[crutschow]

Offset appears at the output multiplied by the closed-loop gain.
The open loop gain has little effect on that other than whatever effect it has on the closed-loop gain.