Hi, all

Recently I met a problem, I have designed a two stage fully differential opamp with cmfb, supply is 1.2V. The open loop simulation is ok, 40db gain. The input voltage bias voltage is 0.6V, cm reference is 0.6v as well. However, when I add four 40k resistors between out+ and input- , out- and input+ to  create a unity gain, I found that the dc voltage at the gate of the input pair transistors are 0.8V, and the output dc level is 1.18V. Did anybody have this problem before ? How did you solve this problem? Any suggestions would be appreciated.

Best Regards
James

Hi,

May be loading effect due to the 40K resistor? You might no have enough current driving capability in your output stage?

Regards
tosei