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Frank Wiedmann

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Re: stb and ac analysis do not match?
Reply #15 - Nov 9^th, 2012, 1:57am

The method that injects only a series ac voltage simulates what I like to call the "voltage loop gain". The conditions under which this is a good approximation for the "real" loop gain are given in http://www.omicron-lab.com/fileadmin/assets/customer_examples/Bode_Info_LoopGain....

I compare the results of the different loop gain simulation methods at the end of http://groups.yahoo.com/group/Design-Oriented_Analysis_D-OA/message/41. Additional information about loop gain simulation is available on my webpage https://sites.google.com/site/frankwiedmann/loopgain (where you can also find my definition of the Y parameters used in the comparison, see https://sites.google.com/site/frankwiedmann/loopgain#TOC-What-about-other-defini...).

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« Last Edit: Nov 09^th, 2012, 6:26am by Frank Wiedmann »

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buddypoor

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Re: stb and ac analysis do not match?
Reply #16 - Nov 9^th, 2012, 3:16am

Frank, thank you for clarification (and for the 1st referenced document, which I didn`t know yet).
Of course, I agree with everything and - if I understood you and the linked documents right - you are supporting my doubts regarding the strict and general statement that an "ac analysis approach should never be used" for loop gain determination, right?
Thank you.
B.

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LvW (buddypoor: In memory of the great late Buddy Rich)

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Frank Wiedmann

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Re: stb and ac analysis do not match?
Reply #17 - Nov 9^th, 2012, 4:32am

In my opinion, simulation of voltage loop gain can be adequate in many situations, but you should know what you are doing. The other methods presented on my webpage have the advantage that they always give the same result, no matter where in the loop you put the probe.

For Spectre users, I see no reason to use ac analysis to simulate voltage loop gain instead of using stb analysis (which corresponds to Tian's Method as described on my webpage), because the results of stb analysis are more accurate and it is at least as easy to set up as ac analysis.

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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: stb and ac analysis do not match? Reply #18 - Nov 9^th, 2012, 4:57am Fine - thank you. Your statements - and , in particular, your sentence "...you should know what you are doing" - are in full agreement with my way of "engineering thinking".
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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: stb and ac analysis do not match? Reply #19 - Nov 9^th, 2012, 11:00pm Buddypoor, I'm with you on the 'correct' method, but not on the 'exact enough'. In my experience attempting to open the loop always changes the loading effects, which changes the measured loop gain at the higher frequencies (near the unity gain frequency), making the results largely useless. -Ken
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buddypoor

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Re: stb and ac analysis do not match?
Reply #20 - Nov 10^th, 2012, 1:55am

Hi Ken,

I am afraid, I have expressed myself not clear enough.
I think the precondition "I should know what I am doing and why/if it is allowed to do" covers everything.
And - as I have said - I can accept an error if it "remains within the limits set by the aforementioned uncertainties (other error sources)."

To be specific, I am afraid that in many cases the error introduced by the two-pole opamp model (neglecting higher poles as well as parasitic influences) can be in the same order or even larger if compared with the error introduced by the voltage-loop gain method. But, of course, it depends on the particular case and cannot be generalized. Therefore, I have to know what I am doing....
B.

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« Last Edit: Nov 10^th, 2012, 3:57am by buddypoor »

LvW (buddypoor: In memory of the great late Buddy Rich)

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Frank Wiedmann

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Re: stb and ac analysis do not match?
Reply #21 - Nov 10^th, 2012, 4:23am

Ken Kundert wrote on Nov 9^th, 2012, 11:00pm:

Buddypoor,
� � I'm with you on the 'correct' method, but not on the 'exact enough'. In my experience attempting to open the loop always changes the loading effects, which changes the measured loop gain at the higher frequencies (near the unity gain frequency), making the results largely useless.

-Ken

The setup for voltage loop gain simulation does not open the loop. You put the ac voltage source in the same place where you put the loop gain probe for an stb analysis. I agree that opening the loop with large inductors and large capacitors is generally a bad idea.

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