I am into the design of a two stage opamp with differential output. The first satge is a telescopic opamp and second is a common source stage. I have designed a SC CMFB circuit. Since my design is two stage, I cannot directly control the first stage bias currents with the inputs to CMFB being second stage output voltages. So I gave a little thought and introduced a common source stage (which acts as inverting stage) between the output of SC CMFB and the gate controlling transistor of the first stage.

Infact this kind of  arrangement worked but the common mode level tends to vary with different corners (SS, FF, FS, SF) and it completely failed for SF corner when the supply voltage is made 10% (I was testing for all 64 corners) above the normal (3.3V).

Now, can anyone tell me how can I make my common mode voltage independent of corner variations and at the same time suggest whether I am treading the correct path in the design or I am trying to do any inherent mistakes by inverting the SC CMFB output and giving it to first stage controlling current source.