Stability

When the op-amp is connected in closed-loop configurations, under certain condition, the op-amp can go into oscillation causing stability issues.

Using the closed-loop transfer function below, as G = A(f)/[1+βA(f)], where f is the operating frequency. The closed-loop gain of the op-amp is frequency dependent.

LoopGain.png

Noted that if βA(f0) = -1, the gain goes to infinity, and the circuit is unstable and it can amplify its own noise and it eventually begins to oscillate. Keep in mind that we can construct an op-amp as an oscillator with a defined and controlled frequency. However, in this case, the op-amp is presumably used as a DC amplifier and any oscillation is undesirable. This condition can be expressed as |βA|=1 and ےβA = -180°, at f = f0. Note that the total phase shift around the loop at f = f0 is 360° because negative feedback itself introduce 180°of phase shift. In other words, if the overall controlled loop introduces a total of 360° phase shift, the op-amp can potentially oscillate. These conditions because the criteria of op-amp stability. There are two specific criteria we use to determine if the op-amp is stable or not at a specific frequency. They are Phase Margin and Gain Margin.

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