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Lesson 7
Hang a capacitor on an op-amp's output and its own output resistance puts a new pole inside the loop. The phase margin shrinks, the step rings and the response peaks, all from one loop gain you can read. Commit to predictions, then fire up the scope and see why.
Predict · 1 of 2 · the ring
Lesson 6's op-amp (GBW 1 MHz, a second pole at 3 MHz, 50 Ω of output resistance) as a unity-gain follower, driving a 1 nF load: a short cable, say. Unloaded it overshoots 0.4 %. Give it a 10 mV step. By how much does the load overshoot? The loop gain on the Bode is yours to read.
Result
Waiting on your call.
Make your mark, then lock it in. The answer, your error and the reason behind it land here, and the scope and every view below fire up.
Engine: the in-browser MNA solver with lesson 6's op-amp as controlled sources: the step in 4,000 timesteps, the loop gain by an AC solve with the loop broken at the inverting input. The closed-loop poles are the roots of the closed form A₀/D(s); the step from them agrees with the solver to 0.0006 % of the step. Checked by scripts/opamp-check.mjs. SPICE: its numbers show once you're in the lab (they'd give the answers away).