How can we derive gain and phase margin for a ciruit?

raja.cedt wrote on Jan 21^st, 2012, 5:00am:


Gain and phase margin are defined and can be identified for one single transfer function only: The transfer function of the (open) loop ("loop gain").

i have to check the stability of circuit in closed loop configuration. should i consider the transfer function in closed loop configuration to find the gain and phase margins?

sapna wrote on Jan 21^st, 2012, 5:37am:


Of course, you can chec0k if the system is stable or not in a closed-loop configuration - for example by applying a step at he input and watch the output. For a stable system the ouput must within a limited time period return to the quiescent condition. However, in general it is not possible to assess the phase or gain margin. For this purpose you must perform open-loop measurements resp. simulations.
Exception: For a 2nd order system the gain peaking in the frequency domain or the overshoot in the time domain is a measure of the phase margin. For this purpose, some curves for evaluation are available.

sapna wrote on Jan 22^nd, 2012, 2:36am:


It is the correct PRINCIPLE.
If the applied method is correct depends....
You should know that opening of a feedback loop may cause problems (bias point lost, loading condtions altered). For this purpose, some specific methods have been proposed (Middlebrook method, series injection,..)

hello sapna,
You are wrong, any system can be unstable weather it could be open loop or closed loop. While calculating you have to take loop gain (means simply open loop gain*feedback factor). To prove your system unstable phase has to tough 180 in theory (but if it is near to 135 or 140, practically it is useless)

Thanks,
Raj.

i get phase angle for open loop transfer function at gain cross over freq as 57 degree .. so that means my phase margin will be 180+57 ?

this is the gain and phase plot i am looking at