AmplifierStability question

[tomph]

Hi all,
I'm studying the stability of the feedback amplifier. I 'm confusing
the term " phase-magin" Could you please help me to ex[lain what exactly phase-margin is ?
and why choosing criteria : 1) Phase shift between input,output < 180°
2) Unity Gain ( gain=1)
For an amplifier to be stable.

Thank you very much.
Tomph

 

[noiseless]

on the amplitude plot, find the frequency where gain=1. then at that frequency, find the phase value. Use 180-that phase value, you will get phase margin.

 

[tomph]

"on the amplitude plot, find the frequency where gain=1. then at that frequency, find the phase value. Use 180-that phase value, you will get phase margin."

I knew the things you 've just mentioned ...
But what the phase-magin really means ? it says somthing about the stability ?

 

[Btrend]

if gain >1 @ phase=180 in a negative feedback system, the feedback will change to positive feedback which is unstable(i.e. it will oscillated), otherwise it will be stable (i.e. it will converge).

 

[a_aziz]

Hi.
To obtain stability for amplifier
you may be use nagative feedback.

 

[VVV]

Phase margin is the difference in degrees from -180 deg, when the gain of the amplifier is 1, or 0dB. Similarly, the gain margin is the difference in dB from 0dB, when the phase is -180deg.

It all has to do with the characteristic avoiding the critical point (-1,0). Both of these parameters tell you how well you avoid the critical point. So both are a measure of stability. The greater the phase margin, the farther from the critical point the characteristic passes, so the more stable the system. Similarly, the greater the gain margin, the more stable the system, because again, the characteristic passes farther from the critical point .

Take a look at the picture. It's not accurately drawn, but it should help get the idea. A perfect picture can only be drawn using some math program, but I was in a hurry.
The top figure shows the Bode diagrams. The phase margin is the difference from actual phase and -180 deg when the gain (red) crosses 0dB. The gain margin is the amount of (negative) gain when the phase crosses -180 deg.

The bottom picture is the Nyquist plot (I wish I could say for the same system, but I do not draw so accurately). Anyway, as you can see, a gain of 1 is at the intersection of the characteristic with the unity circle. Then the phase margin is the angle difference to -180 deg, when the gain is 1 (equiv. to 0dB above).
When the phase is -180 deg, you get the gain margin as the "difference" from actual characteristic and a gain of 1. (Many will not agree with this, strictly speaking, but I find it very easy to picture in my mind).

I hope this helps.

 

[tomph]

Hi VVV,
Sorry ,for some reasons I could not see your drawing. Please re-post it.
Thanks,
Tomph

 

[VVV]

The drawing is visible.
Perhaps you did not log in? You can only see attachments if you log in.

 

[mukeshk]

he he he..VVV if he is not logged in then how the hell his username is been displayed..
may be his browser doesnt support the pictures..
bbye

 

[ezt]

Hi.

About the phase margin I think you know enough. I agree with Btrend, but I think if gain > 1 @ phase=180, the output will be saturated not oscillate (i.e. it will stick to one of supply rails), just if when gain=1 phase=180, the output will oscillate. afterall, in most of applications designers try to have a phase margin about 60 degree, because it has a very good time response (at least in amplifiers). And about criteria I should say, usually the feedback factor is less than unity, so if your circuit is stable for unity-feedback case it is stable for other cases.

Regards,
EZT