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There is a fixed definition of phase margin, which is to be applied to all circuits and systems with feedback. That means that there is no specialized procedure for some types of amplifiers.
There is a fixed definition of phase margin, which is to be applied to all circuits and systems with feedback. That means that there is no specialized procedure for some types of amplifiers.
The simulation, of course, has to follow the definition - that means to simulate the loop gain and to determine the phase at the unity gain frequency.
You are right, that the bias voltage may cause some problems by opening the loop.
However, there are some specific methods which can be used to avoid these problems (LC-method, ac-series excitation, Middlebrook procedure,....).
It depends on your circuitry.
The simulation, of course, has to follow the definition - that means to simulate the loop gain and to determine the phase at the unity gain frequency.
You are right, that the bias voltage may cause some problems by opening the loop.
However, there are some specific methods which can be used to avoid these problems (LC-method, ac-series excitation, Middlebrook procedure,....).
It depends on your circuitry.
thanks very much!
I am sorry I am a beginner in circuit design, and I did not find any informatin about the methods you mentioned(LC-method, ac-series excitation, Middlebrook procedure,....) by google. Could you recommand me some paper, books or WEB about that?
ps: My circuit is a folded-cascode op amp used as unit gain buffer.
I am sorry I am a beginner in circuit design, and I did not find any informatin about the methods you mentioned(LC-method, ac-series excitation, Middlebrook procedure,....) by google. Could you recommand me some paper, books or WEB about that?
ps: My circuit is a folded-cascode op amp used as unit gain buffer.
OK, you speak about an opamp with a rather low output impedance, right ?
And the opamp is connected as a unity gain buffer with 100% feedback.
In this case, I propose to open the feedback loop and to place an ac voltage source of 1V between the negative opamp input and the opamp output. The positive input has to be grounded. After the ac analysis the voltage ratio V(out)/V(in) is identical to the open loop gain. That the method of ac series excitation.
In addition, you can try the second method: close the feedback loop with a very large inductor (100H) to hold a proper bias point. Ground the positive input and inject an ac test signal of 1V via a very large capacitor (100F) into the neg. opamp input.
The ac signal at the opamp output is identical to the loop gain.
Finally, determine the phase at the loop gain unity gain bandwidth and check how far it is from 360 deg. Thus you have the phase margin. OK ?
I am sorry I am a beginner in circuit design, and I did not find any informatin about the methods you mentioned(LC-method, ac-series excitation, Middlebrook procedure,....) by google. Could you recommand me some paper, books or WEB about that?
ps: My circuit is a folded-cascode op amp used as unit gain buffer.
OK, you speak about an opamp with a rather low output impedance, right ?
And the opamp is connected as a unity gain buffer with 100% feedback.
In this case, I propose to open the feedback loop and to place an ac voltage source of 1V between the negative opamp input and the opamp output. The positive input has to be grounded. After the ac analysis the voltage ratio V(out)/V(in) is identical to the open loop gain. That the method of ac series excitation.
In addition, you can try the second method: close the feedback loop with a very large inductor (100H) to hold a proper bias point. Ground the positive input and inject an ac test signal of 1V via a very large capacitor (100F) into the neg. opamp input.
The ac signal at the opamp output is identical to the loop gain.
Finally, determine the phase at the loop gain unity gain bandwidth and check how far it is from 360 deg. Thus you have the phase margin. OK ?
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