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Measuring Phase Margin
Measuring Phase Margin
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Old 18th December 2007, 03:41 PM   #1
davidallancole is offline davidallancole  Canada
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Default Measuring Phase Margin


I was wondering if some of the smarter or more experienced people here could help me with phase margin.

Is there a way to measure it with a physical amplifier and an oscilloscope?

Is there a way to measure it in PSPICE with a simulated amp?

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Old 18th December 2007, 04:34 PM   #2
KSTR is offline KSTR  Germany
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You can apply a small-signal square wave and look at the overshoot, there are diagrams**) from which one can read approximate phase margin from the amount of overshoot. This is tricky sometimes and can lead to wrong conclusions, e.g. when you run into hard slew-limiting (which actually opens the feedback loop for that moment).

**) see e.g. http://eportal.apexmicrotech.com/mainsite/pdf/an19u.pdf
(p.16, fig.40)

In Spice, with a typical op-amp style topology (voltage I/O), you can bode-plot Vout/Vdiff (Vdiff: differntial input voltage) using an .AC analysis, or use some of the more sophisticated loopgain probe techniques (in LTSpice, see loopgain.asc & loopgain2.asc in the examples section).

- Klaus
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Old 18th December 2007, 07:18 PM   #3
jackinnj is offline jackinnj  United States
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Measuring Phase Margin
There was a time at which you could purchase an HP 3575A Phase-Gain meter for a couple Jeffersons. The one I purchased had an analog output proportionate to one of the two variables plus pen lift so you could output the data to a data-logger.

Intersil has a neat method on their website using some CFB opamps -- I have done this and it works very well -- the HFA3102 dual long tail pair is a little difficult to obtain however -- you can use one of the fast comparators from Analog Devices or Linear Technologies, or make a discrete fast comparator, the results will be good enough for phase margin/ phase gain measurement for audio applications. The output signal is a DC value linearly proportionate to phase.

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Old 19th December 2007, 12:09 AM   #4
davidallancole is offline davidallancole  Canada
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Thanks guys. I may have to try that circuit in the future.
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Old 19th December 2007, 07:18 AM   #5
Cauhtemoc is offline Cauhtemoc  Sweden
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See here:

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