Phase-Gain plot, will this oscillate? - diyAudio
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Old 6th January 2004, 12:49 AM   #1
azira is offline azira  United States
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Default Phase-Gain plot, will this oscillate?

This is the phase/gain plot of something I'm working on. At 0dB gain, the phase is about -106 deg. Technically according to my electronics textbooks, this should mean that I have 94 degrees of phase margin. But I thought that 90 degrees was sort of the ideal phase margin because for some reason I couldn't remember, you automatically lose 90 degrees for like capacitive loading or something.
Thanks.
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Old 6th January 2004, 01:26 AM   #2
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I really don't know what is considered a good or sufficient phase
margin although the more the better, of course. I have done
simulations on a number of amps by Slone and other and they
all had much less than 90 degrees phase margin, often they
are rather around 40 to 45 degrees.

Zobel filters and/or inductors on the output will help to reduce
the effect of reactive loads too.
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Old 6th January 2004, 02:38 AM   #3
sam9 is offline sam9  United States
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Based on my limited experience, the odds are good it won't oscillate. However, assuming those plots were made by the typical AC analysis feature of some simulation program, there is room for uncertainty. AC analysis is strictly a static computation. It is not as if a true sweep against frequency was made. My experience has been that sometimes a circuit when constructed will oscilate even when a simulation says it won't. I've also constructed simulations of a circuit that in real life is quite stable but the sim says there should be a problem.

Nonetheless, it doesn't do to ignore the sim since they are usually more or less right. Just go ahead and build the thing but watch it carefully as you power it up the first time and don't frwak out too much if the it doesn't act quite as predicted.
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Old 6th January 2004, 02:49 AM   #4
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Default phase margin and stability

Actually, you have 74 degrees of phase margin. Phase margin is the difference between the actual phase angle at the 0 dB crossing freq, -106 deg in this case, and -180 deg. This is more than adequate for stability. Typically, 45 deg is considered adequate, and 60 deg is plenty. Of course, capacitive loading will decrease the phase margin. I would recommend recomputing the gain and phase plots with a capacitive load similar to what would be expected in actual use. Extending the plot out to the frequency where the phase crosses -180 deg would display the gain margin present. For gain margin, 6 dB is considered the minimum needed, while 12 dB or more is recommended. I hope this helps.
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Old 6th January 2004, 04:32 AM   #5
azira is offline azira  United States
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Sorry, I was absent minded when I did my math, yes, 74 degrees is what I meant. Thanks for the help.

I checked the step response of my circuit and it does not have any ringing and very little overshoot so I was pretty sure I was safe.
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Old 6th January 2004, 05:12 AM   #6
sonnya is offline sonnya  Denmark
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Try different values of capacitive load....

like 10n,20n,30n,40n,50n,60n ....

With a single pair of output transistors uou will most likely it worst phasemarging around 10n - 60n...

Higher values could be a problem but not as bad as 10n - 60n..

Try run some sims...
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Old 6th January 2004, 05:17 AM   #7
azira is offline azira  United States
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I'm using a pair of darlingtons for my output. I tried some values and it still looks stable. Thanks for the tips. Time to go build this thing.
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Old 6th January 2004, 02:10 PM   #8
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Default measuring phase

I was able to purchase a phase-gain meter (HP3575A) pretty cheaply back in the tech-bust -- came with the manual too. The HP3575A can be modified pretty easily to simultaneously read gain and phase, or output to a chart-recorder etc. (HP had two field installable options which would be easy to implement with a single microcontroller today.) Pricing of HP3575's is peculiar with some going for as little as $25 or as high as $300.


It lead me onto one of those quixotic bursts of design enthusiasm resulting in having some boards done for a phase-gain meter which was cribbed off Intersil's site. (Sorry to say that I had to use TI op-amps as ISIL discontinued the ones used in the apnote.) It's all SMT, but if anyone needs boards let me know.
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Old 6th January 2004, 09:40 PM   #9
sam9 is offline sam9  United States
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"a phase-gain meter which was cribbed off Intersil's site."

Wouldn't recall the app note or document number or url off hand, would you?
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Old 6th January 2004, 11:52 PM   #10
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Quote:
Originally posted by sam9
"a phase-gain meter which was cribbed off Intersil's site."

Wouldn't recall the app note or document number or url off hand, would you?
http://www.intersil.com/data/an/an9637.pdf

as I said, I had some boards made up...the device needs ALC before the amplifiers which feed the comparators, elsewise they overload and you can't measure anything.
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