Resistive CMFB circuit question - capacitors

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fcfusion

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Hi everyone

I have a simple question about resisitive CMFB circuits. I often see in literature two capacitors in parallel with the CMFB resistors. What's their function?

Thanks
 

I think it's either
  • to speed up the CMFB control after fast changes
  • or it's actually a capacitive CMFB coupling, the resistors providing initial states and preventing unusual potential - just for simulation purpose; not necessary in real silicon.
 
I'm aware of only your first suggestion, especially if it's a continous time circuit.

Two resistors across the output will require a current if the differential output is nonzero, and, in an OTA, they will lower the voltage gain; therefore, they're typically taken as large as possible. The large R, combined with the input capacitance of the commom-mode amplifier (typically a gate capacitance), adds a pole to the CMFB loop. Adding a capacitor across the large R introduces a zero that can be used to prevent instability / have higher ugbw (= speed up). At high frequencies the transfer from circuit outputs to CM amp gate is just a capacitive divider now..

Hope this helps,
 
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