Phase margin is more for feedback systems. The stability of an RF amplifier is usually drawn on a smith chart showing input and output terminations that cause or not oscillations. This is done for a wide range of frequencies, not just the one you are to use. Then you examine (or design) the input and output networks to be in the stable impedance range for each frequency.
for springf2000, how do u automatically check the stability of the LNA using @ds.
i designed an LNA and i doubt that it oscillates at 2 GHz, how can i be sure of that?!
THANKS IN ADVANCE.