Phase Margin and Gain Margin

By examining the loop gain βA as a function of frequency, we can determine whether the feedback amplifier is stable or not. The simplest and most effective means for doing this is through the use of a Bode plot for βA.

The difference between unity and the value of |βA| at the frequency of -180° shift, called the gain margin, is usually expressed in dB.

The difference between the phase angle at the frequency where |βA| =0dB and -180° is termed the phase margin.


If the β is reduced (less feedback is applied) shown in the figure below, then the magnitude plot is shifted down, phase margin increases. Thus, the worst case stability corresponds to β=1. Phase margin in an op-amp datasheet describes the stability of a unity gain buffer; other gains will have better phase margin. Loop gain must drop to unity before the non-inverting input phase shift reaches -180°. In other words, the greater the phase margin, the more stable the feedback system. There are designs (e.g., photo-diode trans-impedance amplifier and heavy capacitive loads) that will need a special compensation network to achieve reasonable stability.


Let's use an example below to demonstrate gain and phase margin in op-amp stability. The figure below shows various phase margins (30°, 45°, 65°, 90°) vs frequency plot when the closed-loop gain is 1. At 30°, 45°, the frequency step response shows a large peaking and ringing indicate potential oscillations. At 65°, the step response is negligible. At 90°, the op-amp is the most stable with the trade-off of slow timing response. Phase margin needs to be specified at a closed-loop gain based on the the combination of the op-amp and its feedback components.

© 2024 Microchip Technology, Inc.
Notice: ARM and Cortex are the registered trademarks of ARM Limited in the EU and other countries.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.