Hi,

I am trying to check the stability of my CMFB. To do so, I am using the test bench (TB) shown in the screenshot. The gm cell is a telescopic OTA and the schematic of the CMFB is also shown. I have several problems and doubts.

1- Am I using a proper TB to examine the stability of the CMFB? I am using cmdmprobe in analoglibe to break the loop. (I tried to follow this tutorial:  https://pdfs.semanticscholar.org/c1dc/91d09c4112cf9aab8baefcc345bd2141fcdd.pdf ).
When CMDM -->1 it measures common mode response
When CMDM -->-1 it measures differential mode response.

2- I don't know how to set the value of the resistors. The way to choose the value of the resistors on the TB is not clear to me.

3- The results I get from the stb analysis is not rational to me. I cannot interpret the results. Please see the stb responses when CMDM is 1 and -1. Is this the way I have to follow to simulate the phase margin of the CMFB?

Thank you

The schematic of the CMFB.

STB response when CMDM is -1.

Hi Ken,

Thank you very much for replying my question. I checked "diffstbprobe" and it seems to be a nice solution. Let me be more clear on my problem and the reason behind my previous question. I am designing a multi-stage (3-stages in my case) amplifier with no capacitor feedforward compensation. Please see the screenshot. Three CMFBs are used to control the CM level. CMFB 1 for gm1, CMFB2 for gm2 and gmp2 and finally CMFB3 for gm3 and gmp3. The op-amp is going to be used in a CT delta-sigma modulator with the active-RC integrator. I performed ac analysis and the system satisfies my specs. I attached the ac response of the op-amp.
I also simulated my CT-DSM  with the op-amp and it didn't work. I suspected that there was something wrong with my CMFBs (CMFB1,2,3). That's why I performed transient analysis on my op-amp and the op-amp gets unstable. Please see the step response.

If I break my op-amp into three parts namely

gm1+CMFB1 ( I already showed this part in my last post)
gm2+gmp2+CMFB2
And gm3+gmp3+CMFB3

I would be able to check the stability of each part individually. (That's what's happening in my head, I don't know if I am truly right).


Having had above explanation, several questions came to me:
1- Am I thinking in a right way? (Checking the stability of individual parts)
If so, What would be a proper test bench and proper analysis?

2- Could you please let me know what I have to do in such a case?

3- Should I put my entire op-amp in a proper TB (in my case an active RC-integrator) and then check the stability of the CMFBs?

Sorry for a bunch of questions!
Many thanks

Step response of the op-amp

Ken,

I attached all screenshots of my op-amp in a single ZIP file.

"3Stage_opamp.png" shows a 3-stage no capacitor feedforward compensation op-amp. gm1, gm2, and gm3 all together make a 3rd order path. gmp2 is located in a 2nd order path and gmp3 is located in a first order path. It shows the topology of the designed op-amp without placing it in any feedback configuration (The transistor level schematics of all above gms are attached).

I also have three CMFBs (please see CMFB.png):

CMFB1 controls the CM level of gm1.

CMFB2 controls the CM level of gm2 and gmp2. (I also attached the schematic of gm2 and gmp2).

CMFB3 controls the CM level of gm3 and gmp3. (Please see the schematic of the gm3 and gmp3 and how they are related together)

Let me review some of my observations:

1- I performed an ac analysis on my op-amp. Please see "ac_response.png". It shows 82 dB DC-gain, 1.8GHz UGBW, and 65-degree phase margin with 3pF load capacitor.

2- I also applied a sin wave to the input of the op-amp. Input amplitude=100uV, input frequency=4MHz.

"in_tran.png" shows the input differential signal.

"gm1_tran.png" shows the tran response at the output of gm1.

"gm2_tran.png" shows the tran response at the output of gm2 (and also gmp2).

"gm3_tran.png" shows the tran response at the output of gm3 (and also gmp3)

The tran response looks logical until 1us and then it seems that the op-amp gets unstable. ( I think there is something wrong with my CMFBs).

Now, I have the following questions:

1- Is what I am guessing correct? CMFBs are not working properly?

2- I want to check the stability of the CMFB (to see the phase margin)  no matter what my previous question is right or wrong. To do so, obviously, I have to put the op-amp in a feedback test bench (TB). Could active RC-integrator be a proper test bench since I am going to use this op amp in a CT delta sigma modulator with active RC integrators?

  2-1 I think I have to use stb analysis to examine the stability of my CMFBs. So, I have to use three diffstbprobes to break the CM loops while op-amp is placed in a TB asked in question 2. Please put me right if I am wrong.

Many Thanks

I took a chance to check the content of the zip. My understanding is that the op-amp itself is feedforward compensated with local feedback around gm stages to set the common mode. I have a couple of questions to polyam.

1. Your CMFB has a structure by name single OTA but I do not see what is inside it. Is it a single stage OTA?
2. Did you verify that each OTA with its CMFB is stable? Instead of attacking the 3 stage opamp directly, you may want to first strip it down to a single OTA with CMFB and check if it stays stable. I wonder if the CMFB loop, itself being a two pole system (one from single OTA and other from the pmos device, the gate of which is being controlled by CMFB loop), is adequately compensated.

Thanks and regards

Ken,
I don't know if you open pdf files or not. But I've inserted all figures in a single pdf file.
Thank you