amplifier stability
Amplifier stability is the honest reality that a negative-feedback amplifier, the very thing meant to be calm and self-correcting, can instead burst into oscillation — like a public-address system that howls when the microphone hears its own speaker. The correction signal travels around the loop, and if it comes back at the wrong moment it stops cancelling the disturbance and starts feeding it. A stable amplifier settles after a nudge; an unstable one rings or oscillates forever.
The mechanism comes down to phase. Negative feedback already inverts the signal by 180 degrees at the input. Every pole in the loop (the op-amp's roll-off, an output pole, a capacitive load) adds extra phase lag. If the total extra lag reaches another 180 degrees at a frequency where the loop gain is still 1 or more, the feedback that was negative has effectively turned positive: the signal comes back in phase and reinforces itself, and the circuit oscillates. This is the Barkhausen condition — loop gain of 1 with 360 degrees total phase. We judge how close we are using a Bode plot of the loop gain, reading off the phase margin and the gain margin.
Why this matters: a schematic that is perfectly correct on paper can oscillate in real life — from a capacitive load, from layout parasitics, from too little gain for a decompensated part, or from a too-fast op-amp in a too-low gain. The fixes are the craft of this field: frequency compensation, a series isolation resistor, a feedback capacitor, or simply choosing a slower or differently-compensated op-amp. Stability is something you design in, not something you hope for.
A unity-gain buffer oscillates at 8 MHz the instant a 470 pF load is connected. A Bode plot of the loop shows the extra pole has pushed the phase to nearly 180 degrees right at the crossover frequency — phase margin near zero — so the loop is on the brink and rings.
Too much loop phase shift at crossover turns negative feedback positive and the amp oscillates.
Not all oscillation is obvious. An amplifier can be marginally stable — clean on a slow signal yet ringing on fast edges, or breaking into bursts only under certain loads or temperatures. Marginal stability is a latent failure waiting for the field.