The reason is that many practical factors can reduce the phase margin below the theoretical minimum.

Often amplifiers are designed to achieve a typical phase margin of 60 degrees.

See the discussion of gain margin and phase margin.

Therefore, operational amplifiers are usually compensated to achieve a minimum phase margin of 45 or so.

(Frequency "f" is needed later to find the phase margin.)

The result is a phase margin of 45 , depending on the proximity of still higher poles.

Loop parameters commonly examined for this are the loop's gain margin and phase margin.

This is because FDAs use internal frequency compensation to increase the phase margin.

Topics include gain and phase margin and amplitude margin.

In a controls sense, the oscillation is the result of reduced phase margin induced by the lag of the pilot's response.

Phase margin indicates relative stability, the tendency to oscillate during its damped response to an input change such as a step function.

That makes the phase margin:

Phase margin and its important companion concept, gain margin, are measures of stability in closed-loop, dynamic-control systems.

Figures 8 and 9 illustrate the gain margin and phase margin for a different amount of feedback β.

The latter appears often in feedback amplifiers (such as operational amplifiers) that do not have sufficient gain or phase margins.

Next, the choice of pole ratio τ/τ is related to the phase margin of the feedback amplifier.

In these cases, the feedback circuit can be stabilized by means of frequency compensation, which increases the gain or phase margin of the open-loop circuit.

The Open-Loop response characteristics applied in a specification are typically Gain and Phase margin and bandwidth.

If the typical phase margin is around 60 degrees then the minimum phase margin will typically be greater than 45 degrees.

An easier assessment, but less general, is based upon gain margin and phase margin using Bode plots (contributed by Hendrik Bode).

A phase margin of 60 degrees is also a magic number because it allows for the fastest settling time when attempting to follow a voltage step input (a Butterworth design).

While phase margin comes from the phase where the loop gain equals one, the gain margin is based upon the gain where the phase equals -180 degrees.

This is due to the fact that the phase margin and the maximum overshoot are defined by one parameter only (the fractional power of ), and are independent of open-loop gain.

Alternatively, the degree of stability can be expressed by the phase margin,, which is the phase separation from at the frequency at which the magnitude of the loop gain is unity.

He pioneered the field of robust control with the solution of the gain margin and phase margin problems using techniques from Nevanlinna-Pick interpolation theory, which was the first H-infinity type control problem solved.