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Phase margin and gain margin

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chang830

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Hi
I have a question on the phase margin and gain margin.

We know both the phase matgin and gain margin are needed to describe the stability.

In design the OSC, we always give the phase margin and gain margin to evaluate its capabilty to buildup the oscillation.

But in the stabilty analysis of the amplifier, we always only gives its phase margin and seldom concern the gain margin.

Anyone tell me why?

I think if the gain is always too small when phase chage is 0, so we ingore it.

Pls. comments.

Thanks
 

What do you mean "the gain is too small"?
 

Hello chang830

"Instability" of a system means simply oscilation of the system.

if amplification is les than 1 (atenuation in fact) your system is allways stable.

Problem is when you have amplification greater than 1 : system may oscilate because at some frequencies negative feedback beckome positive....

So Amplification=1 is an important number.

This is why "phase margin" is defined exactly at Amplification=1.

Note that some analog guys use Decibels to express amplification factor, without clearly mentionning this.

"Amplification=1" is the same like "Amplification=0 dB" or by language abuse "Amplification=0 "(in dB without specifyng it)

See the "trap"?
 

For stability, one of the two is enough
 

i think both are important and we shouldn't neglect either
 

Phase margin and gain margin are very important for AC simulation, so you need check PM and GM is enougth for your system.
 

In fact there is another about the same subject:
Wide-Bandwidth High Dynamic Range D/A Converters
By Domine M. W. Leenaerts, Arthur H. M. van Roermund,
Konstantinos Doris
 

I think phase margin is more intuitive for the stability. Cause we can easily know that the loop gain at the cross over frequency(f1) is:
T(j*f1)=-exp(j P) where P is the phase margin here.
the error function at f1 then be:
1/(1+1/T(j*f1))=1/(1--exp(j P))
We can easily calculated that only P<60 will induce a error greater than one at f1, that is the peaking begining. In fact, the peaking occurs for P<65 and ringing happen when P<75.
However, from Gain margin, it is not that easily to know the peaking and ringing that intuitive.
So I think that is why they always use the Phase marging here.
 

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