Common mode and Differential Mode stability Test

[Ali263]

I have designed a two stage differential Amplifier with CMFB as shown in second picture of schematic for using it as Bipolar shaper . For the sake of stability, first i check phase of OTA which is 90 around and i understand, it is quite good.

Now i have to check for common mode and differential mode stability test and do stb analysis in cadence. I have some queries if some one can kindly comment/confirm/correct me on it :

1: For common mode stability i use cmdmprobe between CMFB and output of op AMP as shown in second picture and connect both terminals of shaper, Inp, Inm in first picture, to CM voltage.
Is it right?
2: For differential mode, i break the feedback path and connect diffstrobe, as shown in first picture and connect the input, Inp & Inm in first picture, to some differential input voltages. Is it correct?

3: Is there any link between Phase margin of OTA and Loop gain Phase Margin?

4: Is there any link between Unity Gain Frequency of OTA and cross over frequency which we get in stability analysis?

Thanks in advance

 

[Dominik Przyborowski]

1: For common mode stability i use cmdmprobe between CMFB and output of op AMP as shown in second picture and connect both terminals of shaper, Inp, Inm in first picture, to CM voltage.
Is it right?

No, it is not.
The differential feedback is part of CMFB and has to be taken into account.
Differential amplifier is also biased correctly only by feedback.

2: For differential mode, i break the feedback path and connect diffstrobe, as shown in first picture and connect the input, Inp & Inm in first picture, to some differential input voltages. Is it correct?

Looks OK.

 

[Ali263]

No, it is not.
The differential feedback is part of CMFB and has to be taken into account.
Differential amplifier is also biased correctly only by feedback.
Thankyou for tour reply. Just to clear that i am using feedback components and input components as the are in picture but the cmdmprobe is attached between output of amp and CMFB network as shown in picture 2. Sounds ok now?
Looks OK.

 

[frankrose]

Hi,
I think you can use diffstbprobe for differential and common mode stability analysis of fully differential amplifier, cmdmprobe is not necessary. You should rather set stb analysis, here is a great guide for stability simulation, go to page 12: https://www.eecis.udel.edu/~vsaxena/courses/ece614/Handouts/Loop Stability Analysis.pdf