Help request for Closed-loop OTA with CMFB

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mirror_pole

Joined Nov 14, 2020
8
Hello guys,

I have a question concerning the design on an operational transconductance amplifier in closed loop.
Im relativly new to this topic. As already posted in another forum section im trying to design a fully differential Class AB OTA in cadence, which provides high slew rate due to the fact that the maximum possible output current is not fixed by a tail current source (therefore the input transconductor is also in class AB).

The problem with this OTA is that it provides high common mode gain at the output, and it is not possible to get rid of it by matching the devices.
Therefore a common mode feedback circuit is needed to supress A,cm.

I have a question about CMFB circuits because i think i still dont fully understand the need of it.
From my understanding so far i need CMFB to make sure that my Dc output voltages, hence my bias points are fixed (is this correct?). The idea is that the CMFB circuit "adjusts" the output dc currents in a way that the common mode output voltage stays constant and doesnt change.

When i do simulation of my OTA in open loop configuration everything looks fine but when i put it in closed loop the bias points become a total mess. I think it is because of the fact that this OTA has a high common mode gain and that i need CMFB in first place to avoid this problem when i close the loop.
 
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