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Design >> Analog Design >> Nested loop stability requirement
https://designers-guide.org/forum/YaBB.pl?num=1471606584

Message started by thomasross20 on Aug 19^th, 2016, 4:36am

Title: Nested loop stability requirement
Post by thomasross20 on Aug 19^th, 2016, 4:36am

I know there are other topics touching on this but I never saw a definitive answer.

If I have an amplifier with one master outer loop which is stable... and one inner loop... does the inner loop need to be stable or not?

References and hand-waving explanation much appreciated!

Thanks..

Title: Re: Nested loop stability requirement
Post by Frank Wiedmann on Aug 19^th, 2016, 5:37am

No, the inner loop does not need to be stable. You can have a system that is open-loop unstable but closed-loop stable. See http://www.designers-guide.org/Forum/YaBB.pl?num=1294178255 for an example.

Title: Re: Nested loop stability requirement
Post by loose-electron on Aug 23^rd, 2016, 10:42am

Last I checked, stability is only a question for those systems in which feedback exists.

No?

Think about it.

Title: Re: Nested loop stability requirement
Post by Frank Wiedmann on Aug 24^th, 2016, 12:18am

"Open-loop" refers to the outer loop. Of course, the inner (unstable) loop is still closed.

Title: Re: Nested loop stability requirement
Post by thomasross20 on Aug 24^th, 2016, 10:59am

Thanks - I will check out the link you posted.

FYI I have an inner loop and an outer loop which is closed.
So you can have a system which is closed-loop stable (main outer loop) and have internal loops that are unstable... I never did manage to find a good paper on this. I'll check out the link..