I was reading datasheet of MCP6072. I saw a parameter namely "phase margin" (table 1-2, page 4). As far as I know, "phase margin" is a control engineering term, and implies the phase difference between input and output when the gain is unity. I don't understand the meaning of this term in opamp terminology. The typical phase margin of this opamp is given to be 57o. That what does it mean?
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The phase margin PM is a measure for the stability of a system with feedback. And, thus, it also applies to operational amplifiers. The PM is defined for the LOOP GAIN of the system - that means: open the loop at a suitable node and measure/simulate the gain and the phase around the complete loop. Then ,the PM is the DIFFERENCE between the measured phase and -360 deg (that means: The "distance" to the oscillation condition, positive feedback) at the frequency which gives unity loop gain. Without taking the phase inversion at the inverting input into account, the PM is the "distance" to -180 deg.
Now, for an opamp the most critical situation arises for 100% feedback (unity gain operation). In this case, the feedback factor is unity and the Loop gain is identical to the open-loop gain Ao of the opamp. Normally, only this condition is used to specify the PM in the opamp´s data sheet.
Summary: The PM as given for an opamp is the DIFFERENCE between the opamp´s phase shift and -180 deg at the unity-gain frequency.
An op-amp is a "control system" and the phase margin is defined as the difference from -180 degrees of the phase of the open-loop transfer function when the magnitude is unity. This allows you to predict the stability and response of the system when you close the loop with a given amount of feedback.
An op-amp is typically compensated with a dominant pole, so the transfer function is approximately Ao/(1+s/wo) where wo is the dominant pole frequency and Ao is the DC gain- This implies a phase margin of 90 degrees. In practice, the Ft of the transistors causes additional phase shift and the dominant pole is set so that the 0dB crossover occurs with reasonable phase margin.
Phase Margin is the amount of phase shift margin at unity gain which could cause instability or Oscillation.
90 deg is theoretical ideal, 0 is NG , 45 deg will have some overshoot, 60 deg is practical solution. Phase margin shows tradeoff between rise time and overshoot.
Op-amps are used with feedback, this makes it possible for them to oscillate, which is bad unless you are designing an oscillator!
The phase margin basically states how stable the op-amp is, i.e. phase angle distance from the point of oscillation, in the worst-case configuration of unity gain.
The addition of stray input & output capacitance, but especially load capacitance, can cause a phase shift that reduces the stability margin.
Typically a small resistor is added to the output to compensate for a "large" capacitive load, but this of course reduces the effective gain. Input capacitance is usually less of a problem, but it too can be compensated.
Many op-amps are internally compensated for typical usage conditions, but not all are.