 Phase margin

In electronic amplifiers, the phase margin (PM) is the difference between the phase lag (< 0) and -180°, for an amplifier's output signal (relative to its input) at zero dB gain or output is same as of input.For example, if the amplifier's open-loop gain crosses 0 dB at a frequency where the phase lag is -135°, then the phase margin of this feedback system is -135° - (-180°) = 45°.