Filter design inputs
Formula used
How to use this calculator
- Select the physical topology and desired response family.
- Choose whether cutoff frequencies, center and bandwidth, or center and Q are known.
- Enter frequency values, filter order, gain, ripple, and input amplitude.
- Select a component-solving mode and provide practical seed values.
- Choose a preferred-number series and expected component tolerance.
- For active filters, enter the amplifier bandwidth and supply voltage.
- Calculate, review warnings, inspect graphs, and export the design.
Example design data
| Application | Lower cutoff | Upper cutoff | Suggested topology | Typical order |
|---|---|---|---|---|
| Voice communications | 300 Hz | 3.4 kHz | Active cascaded filter | 4 |
| Audio crossover band | 500 Hz | 5 kHz | Linkwitz–Riley cascade | 4 |
| Sensor noise isolation | 10 Hz | 100 Hz | Butterworth active filter | 4 |
| Narrow resonant detector | 9.5 kHz | 10.5 kHz | Series RLC or MFB | 2 |
Understanding bandpass filter design
A bandpass filter passes frequencies between lower and upper cutoff points. Frequencies outside that region are attenuated according to filter order. The center frequency often uses the geometric mean of both cutoffs.
Passive RC filters are simple, inexpensive, and suitable for broad bands. RLC filters provide resonance and can achieve selective narrowband behavior. Active filters add gain, buffering, and flexible response shaping without inductors.
Butterworth responses prioritize flat passband magnitude with smooth transitions. Bessel responses favor phase behavior and transient preservation over sharp rejection. Chebyshev responses trade ripple for faster attenuation near cutoff frequencies.
Real resistors, capacitors, and inductors differ from their marked values. Preferred-number rounding changes center frequency, bandwidth, and quality factor. Tolerance estimates help identify designs needing trimming or precision components.
High-Q filters are especially sensitive to component losses and amplifier limitations. Active stages require adequate gain-bandwidth product, output swing, and stable operation. Simulation and prototype measurements remain important before production use.
The response graph gives an engineering estimate across a logarithmic frequency range. Component tables compare ideal values with practical preferred values. Exported data supports documentation, simulation setup, and design review workflows.
Frequently asked questions
What is a bandpass filter?
It is a circuit that passes a selected frequency band while reducing frequencies below and above that band.
Why is the center frequency a geometric mean?
For logarithmic frequency spacing, the geometric mean lies equally between the two cutoff frequencies.
What does Q represent?
Q describes selectivity. A larger Q indicates a narrower bandwidth around the center frequency.
Which topology suits a narrow band?
Series RLC, parallel RLC, and multiple-feedback active filters are common starting choices for narrowband designs.
What is filter order?
Order indicates the number of poles and largely controls how rapidly attenuation increases outside the passband.
Why round components to E-series values?
Preferred-number series represent commonly manufactured values, making the calculated design easier to build.
How much op-amp bandwidth is required?
The amplifier should provide substantial gain-bandwidth margin above the highest cutoff, stage gain, and Q requirement.
Does tolerance affect Q?
Yes. Component variation changes pole locations, bandwidth, and peak response, especially in high-Q filters.
Can the graph replace circuit simulation?
No. It is an analytical estimate and does not fully model parasitics, loading, noise, slew rate, or device nonlinearities.
Why can a calculated active filter clip?
The expected output may exceed the amplifier output swing available from the selected supply voltage.