Effects of CMFB question

[artho]

Hi,

I'm designing a fully diff amplifier. I already have designed the FD amplifier using a ideal CMFB and it works fine. The problem is when i replace the ideal CMFB for the real circuit. The gain with ideal circuito is about 48 dB, when i change for the real circuit its about 36 dB and its gain-badwidth has increase (too much!).
With this i would like to know what in the CMFB may be causing to the gain degrade. What should i have to consider to design de CMFB circuit to keep the same specifications as using the ideal CMFB model?
Thanks.

 

[erikl]

2 reasons may contribute to the gain reduction:

1. the CMFB inputs' impedance loads the FD amplifier outputs

2. the CMFB control mechanism might change the operation point of the amplifier into an awkward region