driving a capacitive load

Hang a capacitor on an op-amp's output — a long cable, the input of an ADC, a piezo element, a MOSFET gate — and a perfectly stable amplifier can suddenly ring, overshoot, or break into outright oscillation. The capacitor is not doing anything malicious; it simply combines with the op-amp's own output resistance to add an extra lag in the feedback loop, and that lag eats away the safety margin that keeps the loop calm.

Here is the mechanism. The op-amp's output resistance Ro and the load capacitance CL form an extra low-pass pole at the frequency 1 / (2 times pi times Ro times CL). That pole adds phase lag inside the feedback loop, stealing phase margin. Example: an output resistance of 50 Ω with a 1 nF load makes a pole at about 1/(2 times pi times 50 times 1e-9) = 3.2 MHz; if that lands near the loop's crossover, the circuit rings or oscillates. A voltage follower is the worst case because its loop gain is highest. Cures: a small series isolation resistor (say 20 to 50 Ω) between the output and the cap, an RC snubber, feeding the feedback from before the resistor, or choosing an op-amp specified as stable into capacitive loads.

Why this matters: capacitive loads are everywhere — driving coax or twisted pair, the sample capacitor of an ADC, the gate of a power MOSFET, or just a long PCB trace. A schematic that simulates perfectly can oscillate on the bench purely because of a cable's capacitance. Always check the load capacitance an op-amp can tolerate, and reach for an isolation resistor the moment you must drive a real-world capacitive load.