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1. Look for the point where the CMFB circuit is controlling the diff amp common mode. It's probably at the gate of either the NMOS or PMOS current mirros (most likely NMOS).
2. That's the point where you should open the loop. You can use a switch that lets the dc pass but not ac, (don't forget to put before the switch the Cgg of the current mirrors, such that the ac signal sees the Cload).
3. Put an ac signal at that point and look at the response when it comes back. In that way you can see the bode plot of the CMFB loop. Try to have simmilar DC gain and UGBW as in diff mode and enough phase margin.
PS. You should also cut with the ideal switches the ac signal through the diff path (feedback) but this is probably not going to have such big impact.
PS2. The switch can be found in the library analogLib and it is called sp1tswitch
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