<P> In the above loop - gain definition, it is assumed that the amplifier input presents zero load . To make this work for non-zero - load input, the output of the feedback network needs to be loaded with an equivalent load for the purpose of determining the frequency response of the loop gain . </P> <P> It is also assumed that the graph of gain vs. frequency crosses unity gain with a negative slope and does so only once . This consideration matters only with reactive and active feedback networks, as may be the case with active filters . </P> <P> Phase margin and its important companion concept, gain margin, are measures of stability in closed - loop, dynamic - control systems . Phase margin indicates relative stability, the tendency to oscillate during its damped response to an input change such as a step function . Gain margin indicates absolute stability and the degree to which the system will oscillate, without limit, given any disturbance . </P> <P> The output signals of all amplifiers exhibit a time delay when compared to their input signals . This delay causes a phase difference between the amplifier's input and output signals . If there are enough stages in the amplifier, at some frequency, the output signal will lag behind the input signal by one cycle period at that frequency . In this situation, the amplifier's output signal will be in phase with its input signal though lagging behind it by 360 °, i.e., the output will have a phase angle of − 360 ° . This lag is of great consequence in amplifiers that use feedback . The reason: the amplifier will oscillate if the fed - back output signal is in phase with the input signal at the frequency at which its open - loop voltage gain equals its closed - loop voltage gain and the open - loop voltage gain is one or greater . The oscillation will occur because the fed - back output signal will then reinforce the input signal at that frequency . In conventional operational amplifiers, the critical output phase angle is − 180 ° because the output is fed back to the input through an inverting input which adds an additional − 180 ° . </P>

Definition for gain margin and phase margin of control system
find me the text answering this question