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Sallen–Key low-pass filter

Op-amps and analog ICs · Advanced · about 20 minutes · part of Zen’s Electronics Design Mastery

Two RC stages and an op-amp follower make a second-order filter: flat in the passband, then falling 40 dB per decade, twice as steep as a single RC. Capacitor C1 to the output sets how sharp the corner is (Q).

Sallen–Key low-pass filter: the circuit on the simulator's bench

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How it works

f0 = 1 / (2π√(R1R2C1C2)) = 232.2 Hz, Q = √(R1R2C1C2) / (C2(R1+R2)) = 0.729

H = 1 / (1 − (f/f0)² + j(f/f0)/Q): at 100 Hz, |H| = 0.994 (-0.1 dB), φ = -35.9°

Try it on the bench

  1. Set the source to the corner frequency: the gain is about Q and the phase −90°.
  2. Try 2.3 kHz, a decade above the corner: the output falls to about 1 %, where a single RC would still pass 10 %.
  3. Open Tools… → Bode plotter and run a sweep: read the −3 dB corner off the curve and compare it with fc in the theory. Change a part and run again; the two curves overlay.

Parts on the bench

In the course

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

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