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Design >> Analog Design >> loop stability question
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Message started by driveforce on May 26^th, 2006, 10:14pm

Title: loop stability question
Post by driveforce on 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!!

Title: Re: loop stability question
Post by Andrew Beckett on 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.

Title: Re: loop stability question
Post by driveforce on May 30^th, 2006, 7:20pm

I didn't know that analysis before. I will try that.
Thx!

Title: Re: loop stability question
Post by raf on 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.