Active filter designer
Design Butterworth, Bessel, Chebyshev, and Linkwitz–Riley op-amp filters — low-pass, high-pass, band-pass, and notch — as Sallen–Key or multiple-feedback stages with standard resistor and capacitor values, then check the Bode plot, group delay, step response, tolerance spread, and a SPICE netlist.
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How to use it
Choose the response, the family, the order, and the cutoff; the designer splits the filter into second-order stages (plus one first-order stage for odd orders), picks capacitors from the series you choose starting at the base capacitor, and solves the resistors, rounding them to standard values. Each stage card shows its schematic, part values, and how far the rounded parts move its f₀ and Q. Examples loads an anti-alias filter, a crossover, a hum notch, and others.
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Families — Butterworth is maximally flat; Bessel has nearly constant group delay, so pulses keep their shape (it is normalised to −3 dB at the cutoff); Chebyshev trades pass-band ripple for a steeper skirt (the cutoff is the ripple edge — an even order starts at 0 dB and ripples up by the ripple amount); Linkwitz–Riley is two cascaded Butterworth filters, −6 dB at the cutoff, so a low-pass and high-pass pair sum flat for loudspeaker crossovers.
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Sallen–Key stages are unity-gain, non-inverting, and need few parts; the second capacitor is chosen at least 4Q² times the first so that real resistor values exist. Multiple feedback stages invert, provide gain (spread evenly over the stages), and are less sensitive to the op-amp.
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Band-pass is a multiple-feedback stage with equal capacitors (Q = πf₀R3C, gain −R3/2R1). Notch subtracts that band-pass from the input in an inverting adder, so the depth depends on the three adder resistors matching.
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The plots show the ideal response, the response with standard parts, and the spread of 200 random builds with every part inside the tolerances you set. Group delay is −dφ/dω. The step response is integrated stage by stage and reports overshoot, rise time, and 1% settling.
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Op-amp: the "GBW needed" figure is a rule of thumb — pick an op-amp with a gain-bandwidth product at least 100 × f₀ × Q × gain for the worst stage, and use C0G/NP0 or film capacitors.
The SPICE netlist uses ideal op-amps (voltage-controlled sources with a gain of a million) and runs in ngspice or LTspice. Keep resistors between about 1 kΩ and 100 kΩ by changing the base capacitor.
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