Is this a complete active filter design and stability tool?

No. This is a first-pass designer meant to pick practical starting values (E-series parts), preview the transfer function, and flag common pitfalls (excessive op-amp GBW/slew requirements, extreme resistor values). Always validate with the op-amp datasheet and (ideally) a SPICE simulation.

What does Q mean in a 2nd-order filter?

Q controls damping. Higher Q means more peaking and more overshoot/ringing in the step response. Butterworth (Q≈0.707) is a common “flat magnitude” choice; Bessel (Q≈0.577) gives a smoother transient; critical damping is Q=0.5.

Why does Sallen-Key often need gain > 1 for Butterworth?

In the equal-component Sallen-Key approximation, Q is set by the op-amp’s closed-loop gain K (Q = 1/(3 − K)). With K=1 (unity gain) the maximum Q is 0.5, so Butterworth (0.707) requires K≈1.586. The tool can optionally add a simple output attenuator to keep overall gain at 1.

Which capacitor types are best for active filters?

For predictable filters, use stable capacitors (C0G/NP0 ceramics for small values, or film). Avoid high-K ceramics (X7R/X5R) when accuracy matters, because capacitance changes with voltage, temperature, and time.

Sallen-Key · E-series rounding · Bode + step response · SPICE export

Active Filter Designer (Sallen-Key + Bode Plot)

Design practical Sallen-Key active low-pass and high-pass filters: quick sizing from f0 or from fp/fs specs (Butterworth / Chebyshev I), E-series rounding, op-amp sanity checks, Bode/step plots, and SPICE export.

Quick start (2 minutes)

Pick Low-pass or High-pass, then choose Quick (f0) or From specs (fp/fs).
Quick: choose order + preset and enter f0. From specs: pick Butterworth/Chebyshev I and enter fp, fs, Ap, As.
Pick a realistic C (C0G/film recommended), then enable E-series rounding.
(Optional) Fill in op-amp GBW / slew to see quick feasibility warnings.
Use Export to download a SPICE netlist / PDF summary for review.

Designer

First-pass Sallen-Key sizing with E-series rounding, quick op-amp feasibility checks, and plots to sanity-check magnitude/phase/step response.

Email this export (PDF + share link).

Start here

Quick: you already know the order/f0 or want to explore fast.

From specs: you only know fp/fs/Ap/As and need an order.

Targets

Response

Workflow

From specs converts fp/fs/Ap/As into order plus per-stage f0/Q targets.

Order

Each 2nd-order stage uses one op-amp channel. 4th order = 2 stages.

Alignment / preset

Q (2nd order)

Normalize overall gain Target 1× (add output divider)

For Butterworth, each stage usually needs K > 1 to reach the target Q. Normalizing keeps your overall passband gain sane.

Component constraints

C (C1 = C2)

Cap rounding

R rounding

Gain R series

Rule of thumb: keep filter resistors roughly 1 kΩ → 200 kΩ to avoid excessive noise, bias-current errors, and parasitics. If you see out-of-range warnings, increase C to lower R (or decrease C to raise R).

Advanced: Op-amp sanity check

GBW

MHz

Slew rate

V/µs

Target output swing (peak-to-peak)

Vpp

What this checks

This tool treats each 2nd-order stage as one op-amp channel (or one section of a dual/quad). The checks are a quick per-stage sanity check (not “between stages”).

GBW recommended (worst stage): ---

GBW entered: ---

Slew recommended: ---

Slew entered: ---

These are heuristics. Always confirm stability, output swing/current, noise, and phase margin with the real op-amp + layout (and simulate when in doubt).

Advanced: Tolerance band (Monte Carlo)

Enable tolerance band

Tolerance

Runs

Disabled.

OK

Design summary

Response:

Workflow:

Alignment:

Order:

Target:

Q max:

Higher Q means more peaking/ringing. Use it as a quick sanity check, then validate with simulation.

Inspector: Hover anywhere in a plot to inspect

Dashed = ideal, solid = achieved. Shaded regions show pass/stop bands. Hover anywhere in a plot to inspect.

Magnitude (dB)

Phase (deg)

Step response (normalized)

Tip: step response is the easiest way to “feel” Q (overshoot/ringing). For high-pass, the output eventually returns to 0.

Results

Circuit sketch (Sallen-Key stage)

One stage at a time. Cascade the stages in order (Vout → Vin).

Stage

This is the classic VCVS Sallen-Key cell used by the equations in this tool (equal-component workflow).

Stage order matters: start with lower-Q stages and place higher-Q stages later. Add the output divider only after the final stage.

BOM

Stage	R1=R2	C1=C2	K	Rg	Rf	Notes

What this tool designs (and what it doesn’t)

Design model: equal-component Sallen-Key stages (R1=R2, C1=C2).
Goal: first-pass values + plots (ideal transfer function), with E-series rounding and quick feasibility warnings.
Not included: real op-amp open-loop gain/phase, noise, output swing/current, PCB parasitics, and stability margin checks.
Recommended workflow: use the BOM + plots here, then validate with an op-amp datasheet and a SPICE run (export included).

Key formulas (equal-component Sallen-Key)

Natural frequency: f0 = 1 / (2πRC)

Q from op-amp closed-loop gain: Q = 1 / (3 − K) (so Butterworth Q≈0.707 typically needs K≈1.586)

Non-inverting gain resistors: K = 1 + Rf/Rg

If you need Q < 0.5 or want high-Q with strict unity gain, consider a different topology (e.g. MFB).

Practical notes

Pick C first

Choose 1 nF → 100 nF to keep R in a sane range and reduce bias/noise issues. Use C0G/NP0 or film when the corner needs to be stable.

Mind the op-amp

High-Q stages demand more GBW and phase margin. Treat the built-in checks as heuristics and verify with your chosen amplifier.

Higher order = cascading

4th/6th/8th order filters are cascades of 2nd-order stages. In practice, place the highest-Q stage later in the chain to reduce internal clipping risk.