Active filter designer
Component values for 12 dB/octave Sallen-Key low-pass, high-pass and band-pass filters.
nF
Pick a standard value; the tool computes the resistors.
Hz
Low-pass stage (Sallen-Key, unity gain)
- R1, R2 (series)
- 11.25 kΩ
- C1 (to output)
- 20 nF
- C2 (to ground)
- 10 nF
For comparison: first-order RC resistor with the same capacitor
15.92 kΩ
Second-order Butterworth (Q = 0.707) Sallen-Key stages with a unity-gain op-amp buffer. Use 1 % resistors and film capacitors; round to the nearest standard values and check the result. Keep resistors between about 1 kΩ and 100 kΩ by choosing C.
How it works
A Sallen-Key stage with a unity-gain op-amp gives a second-order (12 dB/octave) filter with only four passive parts. With the ratios used here it has a Butterworth response: maximally flat, −3 dB at the cutoff.
Formulas
- Low-pass: C1 = 2·C2, R = 1 / (2π f √(C1·C2))
- High-pass: C1 = C2 = C, R1 = 1 / (2π f C √2), R2 = 2·R1
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