Hi, rajasekhar

It´s very simple.
At the frequency where the loop gain is unity: The phase margin is the amount of phase to be introduced into the loop in order to get a total phase of 360 deg. (which is the oscillation condition regarding the phase)

At the frequency where the loop phase is 360 deg.: The gain margin is the amount of gain to be introduced into the loop to get a total gain of unity (osc. condition for gain).
(Comment: In the Nyquist diagram the critical phase is 180 deg instead of 360 deg. because the neg. feedback sign is not included).

Raja,

  The same effect of insufficient gain margin is the same as
not having enough phase margin.  They are two measurements
with two different reference points of the same thing, how close
you are to oscillation. Phase margin tells you how close you
are in phase and gain margin tells you how close you are in
gain.

                                                             Best Regards,

                                                                Sheldon

hi all,
       thanks for your reply,lastly i want to know what is the reasonable value of gain margin like 60 deg phase margin is good and above is good from stability point of view but slow.
  As Sheldon said phase margin and gain margin are same indications means if PM is high means GM is also good but i think other way around is wrong because even though you have less PM there may be some poles beyond UGB which can give high GM, so now oscillation condition for magnitude is not satisfied and it may not oscillate.

thanks,
Rajasekhar.

hi buddypoor,
                     i am very happy to see your answer with a new interesting term (vector margin), but if you have time can you please direct me a good reference regarding Vector margin or give little introduction about that.

thanks,
Rajasekhar.

And in any case, you should always do a transient simulation of the step response, which will tell you the final truth. Also examine the effect of variations (as mentioned by Vivek) on the step response.

As responded in the earlier posts PM and GM both are two different measurement of same thing i.e. instability of the system.

From PM alone one can decisively say that whether the system has potential threat of instability or not but on seeing GM it is not so.
If PM = 35 deg, the condition of oscillation is at the border
(considering the Supply, Voltage & Temperature variations).

But if GM is <5dB or so, Can we still say that the system may potentially go to instability?

Even if GM is <5dB but PM is 92 deg (say), there is no real threat of instability because though the pole is outside UGB but very close to UGB (which may be causing low GM), 92 deg of PM is good enough to take care even if the pole comes inside the UGB in different Voltage, Process and Temperature conditions

So from PM alone one comment about the stability of the system BUT from GM alone it is hard to make same comment.

Thanks

In my opinion, phase margin is more important since it is calculated at GBW.  Those poles and zeros that affect transfer function below GBW  also have impact on settling. Gain margin is calculated beyond GBW and poles and zeros there have little impact on settling, but still affect transfer function.
We want to know the impact on settling, therefore, PM can be viewed a direct index of poles and zeros that has such impact.  As for GM, some outband poles and zeros might affect it but not settling, making it less meaningful.
As a extreme case, I once designed a feedforward opamp which gives PM of 30 but no GM below GBW*10! The phase shift returns from 150 to 120 above GBW and begins to drop beyond GBW*10.