why Vin+ is not equal to Vin- for opamp in closed loop

[shineqi]

Hi friends:

what may cause the probelm about Vin+ is not equal to Vin- for a fully differential ota in a closed loop application?

Thanks

 

[LvW]

Hi friends:

what may cause the probelm about Vin+ is not equal to Vin- for a fully differential ota in a closed loop application?
Thanks

Why do you expect both being equal ?
The difference between both inputs is never zero - however, for high gain opamps it approaches zero !
But for an OTA you have a situation which is completely different since the transconductance has a finite value which is rather low. Thus, the input differential voltage has a finite value.

 

[liusupeng]

you can calculate the input voltage difference if you know the DC gain of your amplifier