Op amp Common Mode Feedback Design

[Shady Ahmed]

I am designing a 2 stage fully differential amplifier, however I don't fully understand the CMFB circuits and how should I design it.

Also, the relation between the gain of the CMFB circuit versus the gain of the op-amp.
Same for the gain bandwidth product, should the CMFB gain & GBW > op amp gain & GBW? or the opposite?

 

[kenambo]

hi take a look at this

https://www.ece.tamu.edu/~spalermo/ecen474/lecture17_ee474_fully_diff_amps_cmfb.pdf

 

[erikl]

... the relation between the gain of the CMFB circuit versus the gain of the op-amp.
Same for the gain bandwidth product, should the CMFB gain & GBW > op amp gain & GBW? or the opposite?

1. Gain: The CMFB's DC gain has no direct relation to the opAmp's DC gain. It should be just large enough to compensate for the common mode output offset (which is created by the DC amplified input offset, as well as circuit generated offset by the opAmp itself).

If an uncompensated output offset of, e.g., 1V would exist, it would probably be sufficient to reduce it to 10mV. This requires a CMFB DC gain (actually: of the CMFB offset compensation loop) of 1V/10mV = 40dB .


2. GBW: In order not to loose bandwidth of the main opAmp due to failing offset compensation in due time, the CMFB's GBW should be at least as high as the main opAmp's GBW.

This goal should easily be attainable, because the CMFB's DC gain usually is (much) less than the main opAmp's gain.