Op-Amp Active Filter Calculator

Active filters use op-amps to achieve steeper roll-off than passive RC circuits. Calculate resistor values for 1st-order and 2nd-order Sallen-Key Butterworth low-pass, high-pass, and band-pass designs.

Formulas

1st-order: fc = 1 / (2π × R × C)

2nd-order Sallen-Key (R1=R2=R, C1=C2=C):

fc = 1/(2πRC); Q = 0.7071

Filter Comparison

Type	Roll-off	Typical Use
1st-order	−20 dB/dec	Simple anti-aliasing
2nd-order Butterworth	−40 dB/dec	Audio, sensors
Band-pass	±20 dB/dec	Frequency selection

Design Tips

Choose a standard capacitor value (10 nF, 100 nF) first, then calculate R.
Op-amp GBW must be ≥ 10–100× the cutoff frequency.
Use 1% resistors (E96) for accurate Q in 2nd-order designs.

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