Oscillation occurs due to signals being fed back from the output to the input.
By positively feeding back an output signal to the input when the gain is ≥1 (0 dB in the graph on the left), the signal amplifies itself and oscillation occurs, even without an external signal.
In a negative feedback amplifier, the input and output phases are inverted (by 180 degrees). Therefore, the whole phase is delayed by 360 degrees if the phase of the negative feedback loop is delayed by 180 degrees, making it the same as positive feedback.
In this state, if amount of feedback × gain is higher than 1, oscillation inadvertently occurs at this frequency, in the same way as positive feedback.
Attaching a capacitor to the output pin causes the phase to be delayed, making it easier for oscillation to occur compared with a resistance load.
As shown on the graph on the left, the degree to which oscillation does not occur is indicated by the phase differential of the frequency at which the gain is 1 (0 dB) (known as the phase margin) and by the gain at the point where the phase is 180 degrees (known as the gain margin).
In the ideal amplifier, there is no effect from floating capacitance or GND wiring; but in reality, feedback occurs due to these factors.
This is why oscillation occurs.
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