Phase margin problem as circuit is unstable at 100MHz

hi every body

here i have the phase margin curve , could some one help me and till me if the out stable at 100 MHz.


Thanx in advanced

Re: Phase margin problem

Could you please attach the complete Bode plot? I need to take a look at the gain plot as well.

Phase margin problem

Yes , it is stable at 100 Mhz, as at 100M, the phase margin is around +80deg. So immaterial of the value of the gain, the output will be stable at 100Mhz.

Re: Phase margin problem

wael_wael said:

here i have the phase margin curve , could some one help me and till me if the out stable at 100 MHz.

Click to expand...

Hi
Is it Phase Margin curve or Phase curve?
I think phase curve.
because you have a negative gain, your phase was began from 180 deg.

PM=Phase(100M)=-100
so, is not stable.
regards

Re: Phase margin problem

Why is not stable ? i dont under stand the answers. could some one explain in more details
regds

Re: Phase margin problem

CANNOT BE DECIDED. Whether the system is stable or not, cannot be decided on the basis of phase plot only. You must provide the gain plot as well.

If the gain plot cuts the 0 dB line BEFORE the phase difference (as compared to 0 Hz) is greater than 180 degree, the system is stable. Otherwise, it is unstable.
This is b/c the system becomes a positive feedback system for those frequencies for which gain > 0 dB and phase difference > 180 degrees.

For your system, let's assume that the gain plot crosses the 0 dB line at 1 MHz. Then you system will be stable, because the phase difference (as compared to the phase at 0 Hz) is less than 180 degrees.

Now assume that the gain plot crosses the 0 dB line at 20 Mhz. Now the system will be unstable. This is because if somehow, a frequency component for which gain > 0 dB and phase > 180 degree were to enter the system (for example, due to ambient noise)..the system would behave as a positive feedback system for that frequency component, causing it to remain increasing in amplitude, until the whole system either saturates or oscillator. Either way, it's unstable.

Re: Phase margin problem

zeeshanzia84 said:

Now assume that the gain plot crosses the 0 dB line at 20 Mhz. Now the system will be unstable. This is because if somehow, a frequency component for which gain > 0 dB and phase > 180 degree were to enter the system (for example, due to ambient noise)..the system would behave as a positive feedback system for that frequency component, causing it to remain increasing in amplitude, until the whole system either saturates or oscillator. Either way, it's unstable.

Click to expand...

I have meet the same problem.
my question is:
1. usually the phase is 0 at low frequency, but why is it 180 at low frequency?
2. why "assume that the gain plot crosses the 0 dB line at 20 Mhz. Now the system will be unstable. "? the phase-margin is defined as 180+phase[A(s)], then the phase margin at 20MHz is about 130, why is is unstable?

thanks

Re: Phase margin problem

zeeshanzia84 said:

CANNOT BE DECIDED. Whether the system is stable or not, cannot be decided on the basis of phase plot only. You must provide the gain plot as well.

Click to expand...

Hi
Yes, it is true.
there is a positive feedback. but positive feedback can be stable.

flysnows said:

my question is:
1. usually the phase is 0 at low frequency, but why is it 180 at low frequency?
2. why "assume that the gain plot crosses the 0 dB line at 20 Mhz. Now the system will be unstable. "? the phase-margin is defined as 180+phase[A(s)], then the phase margin at 20MHz is about 130, why is is unstable?

Click to expand...

1- because of negative and positive gain
2- 180+phase[A(s)] is true if phase(low freq)=0 and is phase[A(s)] if phase(low freq)=180

regards

Re: Phase margin problem

the Phase margin can be got with the curve of gain curve,it is the phase at the gain of 0dB.I think.

Re: Phase margin problem

Hi
Yes, it is true.
there is a positive feedback. but positive feedback can be stable.

Click to expand...

Yes. I agree. you are right. For eg. I've seen many filter ckts. which employ both negative and positive feedback (the amount of negative feedback (1+Aβ) > the amount of +ve f/b; and thus +ve feedback is counteracted by the negative f/b).

However, IN MOST SYSTEMS, if your negative feedback path becomes a positive feedback path, due to the phase shift of the output signal....the system DOES become unstable, b/c there is no reduction in the ever-increasing input signal.

Your comments plz!!! :|