The following is the schematic of my design.
![](http://e.imagehost.org/0802/OTA.png)
The left one is the differential difference OTA with gain enhanced, the right one is the CMFB circuit for pmos.
I breaked the cmfb, and replaced the Vcmfb in OTA with a dc bias and an AC signal(Vc).
When I measured the AC responce of the CMFB(Vcmfb/Vc), the phase is positive, which is quite strange. The simulation wave is as follows;
![](http://e.imagehost.org/0990/Simulation.png)
When I checked the ac response of (Vo+)/Vc , it is also postive in magnitude and phase.
while as Vc increases, the Vo+ should decrease actually.
![Embarrassed Embarrassed](https://designers-guide.org/forum/Templates/Forum/default/embarassed.gif)
Thanks for any advice.