Sallen-Key Active Filter
Filter design
Choose type, design mode, cutoff, C1 and R1 — the remaining values are computed.
Component values
How to use this calculator
The Sallen‑Key is the most common 2nd‑order active filter. For an equal‑resistor design (R1 = R2) the cutoff is fc = 1 / (2π√(R1R2C1C2)). This tool lets you fix fc, C1 and R1, then pick a response shape — Butterworth (maximally flat, Q ≈ 0.707), Bessel (flat group delay, Q ≈ 0.667), Chebyshev (0.5 or 3 dB ripple, steeper roll‑off), or a custom Q — and returns C2, the feedback gain K, and Rf/Rg. It supports low‑pass and high‑pass; the component placement (series C vs series R) swaps between the two, with the same equations.
Assumptions, limitations & real‑circuit use
Assumptions: ideal op‑amp (infinite gain/bandwidth), equal resistors R1 = R2, and the standard 2nd‑order topology. For the amplitude‑set modes the gain is fixed by the response (K = 3 − 1/Q); for custom Q the tool solves C2 from the Q relation C2 = C1(4Q − 1/K). Limitations: real op‑amp finite bandwidth, resistor/capacitor tolerances, and the fact that Q is very sensitive to component values mean the actual response should be trimmed and verified in simulation. Use this for the first‑cut design, then fine‑tune with a network analyzer or SPICE.