[Help]About the CMFB simulation.

First, the fully diff amp is simulated by stb in a closed loop.

The CMFB phase margin is as below.

Second, the fully diff amp is simulated like below.

And, the CMFB phase margin is as below.

I'm not sure which one is correct, the 1st or the 2nd smulation. In my opinion, the first simulation is correct. 

Could anyone help me, please?

Thanks a lot. 

Dear xxgenerall,

> I'm not sure which one is correct, the 1st or the 2nd smulation.

I am not sure if others are experiencing the same problem, but It is very difficult to see any detail in your various JPEG files. I cannot make out the axes of your stability plots and I also can not tell how you are simulating the stability. From what I can tell, it appears you are doing two stability analyses - one with the opamp outputs open (i.e. no connects) and one with the opamp outputs feeding back to the inputs through a feedback factor. I can barely make out the DC operating point values annotated on each JPEG file, but it appears in the file "2.sch_closed", the opamp input values are the same whereas in file "3.sch_open", the two DC input values are signficantly different. This suggests that the opamp input devices in the analysis referred to by "3.sch_open" does not have the opamp devices biased at or near their expected operating points. Hence, I would not expect any results from a stability analysis using this topology to necessarily correspond to the stability of the opamp in its closed loop configuration (such as shown in 2.sch_closed).

Shawn