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Op-amp relaxation oscillator

Oscillators · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

A Schmitt trigger whose own output charges a capacitor on IN− through R. When the capacitor reaches a threshold the output flips and starts charging it the other way, giving a square wave: f = 1 / (2RC · ln((1+β)/(1−β))).

Op-amp relaxation oscillator: the circuit on the simulator's bench

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How it works

β = R1/(R1+R2) = 0.500

T = 2RC · ln((1+β)/(1−β)) = 2 × 1 ms × 1.099 = 2.2 ms

f = 455.1 Hz, about 50 % duty

Try it on the bench

  1. Double C to 220 nF: the frequency halves.
  2. Change R2 to 47 kΩ: the thresholds close in and it speeds up.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Op-Amps & Analog ICs”.

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