RobertZ
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the art of analog
Posts: 55
San Diego, CA
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Hi,
I designed a fully differential OTA, and there is a negative feedback (resistor divider) from output to input. There is also a CMFB inside this OTA.
In CM sense, the first path(resistor divider) is postive, while the CMFB is negative feedback. As I know, the first one should be weak, so that in overall, the system is stable.
When I checked the loop gain of the CMFB loop with the other feedback path (resistor divider) closed, I got a 0 degree phase at DC, which means a positve feedback?? If I break the resistor divider feedback path and check the CMFB loop gain, I got 180 degrees at DC.
but I also did transient simulation, eg, step vdd, the CMFB loop looks stable.
Is this circuit stable? or any concern?
Thanks!!
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