What does the phase margin=110 degrees mean?

[electronicuser]

Hello
I design a four-quadrant analog multiplier and plot it's gain and phase bode plot.
then I try to determine phase margin and analysis stability, but I figure out that phase margin=110 degrees.
I don,t have an idea what does the phase margin=110 degrees mean?:shock:
Is it stable?:roll:
please help me
thanks


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Of course, I know that minimum phase margin=45 degrees and best phase margin=63 degrees

 

[sutapanaki]

Is this multiplier a feedback system?

 

[electronicuser]

hi, multiplier includes a PMOS cross-coupled pair (positive feedback) connected to differential output as an active load.

 

[andre_luis]

Where did you see -110 degrees in the above graph ?

 

[electronicuser]

Phase margin=phase in f(0dB)+180=-70+180=+110 degrees, it's true?

 

[andre_luis]

Once you are seemingly showing the graph of the closed-loop transfer function of the system, there is no need to make any additional math, just use usual phase/gain margin criteria to determine overall stability.

 

[electronicuser]

How do I determine phase or gain margin or stability?
Look at it this way, is that right for the closed-loop system?

 

[andre_luis]

What is preventing you from assessing this by yourself? There are just 2 rules related to the gain and phase margin; tell us your conclusion so that we can confirm or correct it.

 

[electronicuser]

In my circuit:
phase margin=110, gain margin=100dB
Is my circuit stable?

 

[sutapanaki]

OK, so you have cross coupled PMOS transistors but you still don't show the feedback loop for which you simulate the phase margin. Can you please show that loop?

 

[electronicuser]

Unfortunately, I can't show the whole circuit, but there is no feedback in multiplier except the positive feedback in the PMOS cross-coupled pair that I already showed it (the following figure).

Do you still need the figure of the whole circuit?

 

[sutapanaki]

So, it is an open loop system then. That crossed-coupled PMOS pair is local feedback and it is usually pretty stable, plus it doesn't seem that you are breaking that cross-coupled pair loop to assess its stability. And as far as I understood from the plots in your first post, you are looking there at the input-output gain of the entire circuit. In this case, since you don't have a feedback system, why are you talking about phase margin at all ? Phase margin is a metric that is relevant to the loop gain of a feedback circuit, not an open loop one.

 

[electronicuser]

When completing your training, consider the circuit shown below. The multiplier has been published in a Q1 journal.

In the results section of this article, only the gain curve is plotted and the phase curve is not plotted.
So is it correct to conclude that in such circuits there is no need to draw a phase curve and these circuits are stable?

 

[sutapanaki]

Again, phase response is something different than phase margin. Every circuit has some phase response, even a simple RC circuit will have a phase response. However, phase margin is only relevant for feedback circuits where you look at the magnitude and phase responses of the loop gain. So, in your case input to output frequency response has magnitude and phase characteristics, but there is no feedback and hence irrelevant to talk about phase margin. But now that we see the whole circuit, there is a feedback loop involving that local differential pair and the followers going to the outputs. This is a positive feedback with relatively small gain, probably in the order of 1. You should check the stability of that local loop in frequency.