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phase margin calculation

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akbarza

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hi
i have a question and thanks if answer to me.
in wikipedia if you search for phase margin, you find this statment:
More generally, PM is defined as that of the amplifier and its feedback network combined (the "loop", normally opened at the amplifier input), measured at a frequency where the loop gain is unity, and prior to the closing of the loop, through tying the output of the open loop to the input source, in such a way as to subtract from it.
can any one explain it to me?

i always did for measuring phase margin of an opamp, after simulation in open_loop case , i calculated phase margin of frequency where gain of open -loop amplifier was equal 1 .
thanks
 

If you find the Wikipedia phase margin article hard to read, why don't you refer to a better understandable explanation?

Unity gain frequency and phase margin are characteristic parameters of the loop gain function, the problem is how to measure it in a real circuit. Middlebrook's method is an elegant approach, read about it.
 

hi fvm
i have only one question : for determining phase margin. if i must use opamp in unity gain configuration ?
i know only that i don't know.
 

Phase margin has to be determined for the actual OP configuration whatever if is.
 

akbarza said:
hi fvm
i have only one question : for determining phase margin. if i must use opamp in unity gain configuration ?
i know only that i don't know.
Click to expand...

The phase margin is a measure of system stability for the actual circuit with feedback (as stated - with other words - by FvM).
 

sorry. i don't receive my answer.
we design an opamp with special characteristic . then we apply it in different circuits each with related feedback factor.
i have this question that to determine phase margin of this opamp with simulation, if we must use opamp in unity gain configuration OR ONLY connect the input of opamp with an ac signal and get gain and phase and then calculate phase margin?
thanks for your times
 

akbarza - one simple question: Do you know the difference between "closed-loop gain" and "loop gain" ?
This is of great importance for understanding the basics of the term "stability margin".
 

Phase margin is a parameter of the feedback system achieved with a specific feedback factor, not a generic OP parameter. You'll use unity gain configuration if it's the intended application circuit.

To measure loop gain, you must somehow open the loop.

connect the input of opamp with an ac signal and get gain and phase and then calculate phase margin
Click to expand...
If you put the known feedback factor into the calculation, it's one option to determine loop gain and phase margin. But the method has some prerequisites, the OP must be biased to an operation gain in linear operation range, e.g. output at mid-supply. And the OP output must be loaded similarly as by the real feedback network. Correct bias is often provided by a DC feedback network.

Another option is to use the actual feedback circuit and cut the loop virtually by using Middlebrooks method.
 

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