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RobertZ

Community Member

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the art of analog

Posts: 55
San Diego, CA

loop stability question
May 26^th, 2006, 10:14pm

Hi,

I designed a fully differential OTA, and there is a negative feedback (resistor divider) from output to input.
There is also a CMFB inside this OTA.

In CM sense, the first path(resistor divider) is postive, while the CMFB is negative feedback.
As I know, the first one should be weak, so that in overall, the system is stable.

When I checked the loop gain of the CMFB loop with the other feedback path (resistor divider) closed, I got a 0 degree phase at DC, which means a positve feedback?? If I break the resistor divider feedback path and check the CMFB loop gain, I got 180 degrees at DC.

but I also did transient simulation, eg, step vdd, the CMFB loop looks stable.

Is this circuit stable? or any concern?

Thanks!!

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Thanks,
Robert

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Andrew Beckett

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Life, don't talk to
me about Life...

Posts: 1742
Bracknell, UK

Re: loop stability question
Reply #1 - May 29^th, 2006, 5:08am

Without considering the specifics of your circuit, spectre has a stability analysis (called "stb") which can be used to analyse both single-ended and differential loops - did you try using that? With the differential probe (called cmdmprobe or dmcmprobe, I forget), you can look at either the common mode or differential mode stability.

Regards,

Andrew.

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RobertZ Community Member Offline the art of analog Posts: 55 San Diego, CA	Re: loop stability question Reply #2 - May 30^th, 2006, 7:20pm I didn't know that analysis before. I will try that. Thx!
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raf New Member Offline Posts: 5	Re: loop stability question Reply #3 - Jun 1^st, 2006, 2:41pm I don't really understand your question, but the step response is directly related to your phase margin. So if your step response has no ringing you are fine.
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