Stability and Compensation
Negative feedback becomes dangerous if frequency-dependent phase shift turns it effectively positive while loop gain is still high.
Loop gain
Phase margin
Phase margin indicates how far the loop is from sustained oscillation at the frequency where loop gain reaches one.
Unity-gain stability
A unity-gain-stable amplifier is compensated to operate at noise gain one under specified loading. Some high-speed amplifiers require a minimum stable gain.
Capacitive loads
Cables, ADC inputs, MOSFET gates, and large capacitors can add phase shift at the output. A small isolation resistor often improves stability.
Transimpedance stability
Photodiodes and op-amp input capacitance create a pole at the summing node. A small feedback capacitor is often required.
Layout
Keep feedback paths short, place bypass capacitors close to supply pins, and keep high-current or fast output traces away from sensitive inputs.
Invisible oscillation
An audio or DC circuit can oscillate at megahertz frequencies. Symptoms may be heating, strange offset, distortion, or excessive current rather than an audible whistle.