Phase and gain margin (control)

[LM741]

hey guys.

this has to do with determining the phase and gain margins of a system.
i've read and understood the follwing:

"To find GM, first find the phase crossover frequency, wGM. This is the frequency where the phase curve passes through -180. Get the magnitude at that frequency [looking at a bode plot]

To find FM, first find the gain crossover frequency, wFM. This is the frequency where M = 1. Recall at M = 1, 20 log M = 20 log 1 = 0."

what i don't understand is:
in the bode plot - what if the phase plot does not go up to 180 degrees - does this mean there is no gain margin?also - I've only seen examples where the gain margin is from a negative dB number to 0dB. what if the the -180 degrees has a corresponding gain greater then 0dB , then will i have a negative gain margin?

in the magnitude plot - what if the graph does not cut the w axis , does this mean there is no phase margin?

thanks
John

 

[SGT]

If the graphs don't cross the limits, the gain and phase margins are meaninless. You could say they are infinite.

 

[LM741]

......thanks