closed loop fully OPM problem

[shanmei]

I design a cascode_fully opm with cmfb, in the closed loop condition with gain equal to 1, the input is a sinewave of 1v, so the output signal should be 1v , for the sample reason that the gain=1, however, the output is 1.003v, why?
anyone know ?pls!
thank you!

 

[transbrother]

Your CMFB ckt must not have a high gain.

 

[caosl]

The phenomena in your description is natural because of opamp's finite dc gain which define the static error, for example, if you opamp's dc gain is 60dB, this will be bring out 1mv error, and some other nonideal characteristics will also generate some errors!

 

[surianova]

The phenomena in your description is natural because of opamp's finite dc gain which define the static error, for example, if you opamp's dc gain is 60dB, this will be bring out 1mv error, and some other nonideal characteristics will also generate some errors!

yes. it is normal to get dc error and it depend on the open loop gain as below formula

Vout/Vin= G/ 1+ GH

G = open loop gain, H= feedbk factor

for unity feebk and if G is high.. Vout almost equal to Vin.