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Stability analysis of a continuous time CMFB (Read 944 times)
polyam
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Re: Stability analysis of a continuous time CMFB
Reply #15 - Apr 18th, 2017, 8:44am
 
ULPAnalog,

Thank you for your reply. Yes! your understanding is absolutely right.

To answer your first question, I am using a differential difference amplifier (DDA) for the firs OTA (gm1) and a so-called 5-transistor amplifier for others (gmp2 and gmp3). Please see the screenshot in CMFB123.pdf.

And about your second question, as a matter of fact, that's exactly what I was asking in my previous posts. I want to check the stability of each gm-cell (OTA) with its corresponding CMFB individually. BUT I don't know how to set up a test bench to do that  :'(. That's my problem.

I attached my top-level schematic of the op-amp with "diffstbprobe" to check the stability of each loop. Can you please verify if my test bench to check the stability of the CMFB is right?

Thank you






ULPAnalog wrote on Apr 17th, 2017, 2:26am:
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

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polyam
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Re: Stability analysis of a continuous time CMFB
Reply #16 - Apr 18th, 2017, 8:46am
 
That's the way I placed "diffstbprobe" on my op-amp to examine the stability of CMFBs. Is that a right TB?


ULPAnalog wrote on Apr 17th, 2017, 2:26am:
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

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diffstbprobe.png
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Ken Kundert
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Re: Stability analysis of a continuous time CMFB
Reply #17 - Apr 18th, 2017, 10:29am
 
I would recommend that as long as the simulation time is not unbearable you run three stb analyses with the amplifiers in place. Then if you want see loop gain, use the loop gain computed by the stb analysis -- do not compute it yourself as you did in your figures.

Finally, the diffstbprobe is not going to work for you. You are interested in checking the stability of the common mode loop, the diffstbprobe ignores common mode signals. Instead you use back to back ideal baluns to decompose the feedback signal into differential and common mode components, and then inject the test source into the common mode path.

-Ken
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ULPAnalog
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Re: Stability analysis of a continuous time CMFB
Reply #18 - Apr 19th, 2017, 2:14am
 
Quote:
Finally, the diffstbprobe is not going to work for you. You are interested in checking the stability of the common mode loop, the diffstbprobe ignores common mode signals. Instead you use back to back ideal baluns to decompose the feedback signal into differential and common mode components, and then inject the test source into the common mode path.


Dear Ken

Isn't diffstbprobe actually made from back to back ideal baluns? My understanding was that the diffstbprobe, based on the choice of common mode stb analysis or diff mode stb analysis, will automatically perform the injection of test source into appropriate signal path. Is that not true?
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Ken Kundert
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Re: Stability analysis of a continuous time CMFB
Reply #19 - Apr 20th, 2017, 12:40am
 
Perhaps it is. It was my understanding that diffstbprobe only provided differential stability. My mistake. I'm afraid I never use the diffstbprobe. Cadence has encrypted it, so I am not really sure what is in there. And since they the last version they provided was inaccurate, I figure it is safer simply to role my own.

-Ken
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Andrew Beckett
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Re: Stability analysis of a continuous time CMFB
Reply #20 - Apr 21st, 2017, 1:44pm
 
Ken,

It's just back to back baluns (if my memory is correct) and then the choice on the ADE stb analysis from controls whether you are analysing the differential mode or common mode stability by picking the appropriate iprobe.

The earlier cmdmprobe (which wasn't encrypted) was inaccurate - you're right, but diffstbprobe should always have been OK, AFAIK.

Not entirely sure why we encrypted it - that seems more trouble than it's probably worth...

Regards,

Andrew.
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Frank Wiedmann
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Re: Stability analysis of a continuous time CMFB
Reply #21 - Apr 25th, 2017, 6:41am
 
Andrew Beckett wrote on Apr 21st, 2017, 1:44pm:
It's just back to back baluns (if my memory is correct) and then the choice on the ADE stb analysis from controls whether you are analysing the differential mode or common mode stability by picking the appropriate iprobe.

As far as I can tell, your memory is correct (see https://community.cadence.com/cadence_technology_forums/f/33/t/27989).
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