Calculator Inputs
Example Data Table
| Use Case | Lower Cutoff | Upper Cutoff | Center Frequency | Bandwidth | Estimated Q |
|---|---|---|---|---|---|
| Audio tone band | 300 Hz | 3,400 Hz | 1,010 Hz | 3,100 Hz | 0.33 |
| Sensor signal cleanup | 1 kHz | 5 kHz | 2.236 kHz | 4 kHz | 0.56 |
| Narrow RF preselect | 98 kHz | 102 kHz | 99.98 kHz | 4 kHz | 25 |
Formula Used
The calculator uses standard second order bandpass relationships.
Bandwidth = fH - fL
Center frequency = √(fL × fH)
Quality factor = f0 / Bandwidth
Series RLC: L = 1 / ((2πf0)²C)
Series RLC: R = 2πL × Bandwidth
Parallel RLC: R = Q / (2πf0C)
Gain linear = 10^(Gain dB / 20)
For center frequency and Q mode, lower and upper cutoffs are derived from the bandwidth equation and the center frequency relationship.
How to Use This Calculator
Choose the design method first. Use cutoff mode when you already know the lower and upper passband limits. Use center and Q mode when selectivity is the main design target.
Select a topology. Series RLC is useful for passive bandpass examples. Parallel RLC is useful for tuned circuits. Active RC gives a normalized resistor estimate for op amp based work.
Enter the capacitor value and unit. The calculator will estimate the matching resistor and inductor values. Add gain, tolerance, stage count, source resistance, and load resistance for a deeper practical estimate.
Press the calculate button. The result appears above the form and below the header. Review the graph, export the CSV file, or save a PDF report.
Bandpass Filter Design Guide
What a Bandpass Filter Does
A bandpass filter allows one selected frequency range to pass. It reduces lower frequencies and higher frequencies. This makes it useful when a circuit must isolate a signal from noise. Audio systems use it for tone shaping. Communication circuits use it for channel selection. Sensor circuits use it for removing drift and high frequency interference.
Key Design Values
The lower cutoff is the first edge of the passband. The upper cutoff is the second edge. The bandwidth is the distance between both limits. The center frequency is the geometric middle of the band. Quality factor shows how narrow or sharp the filter is. A high Q gives a narrow band. A low Q gives a wide band.
Choosing a Topology
A series RLC filter is simple and clear. It is often used for learning, matching, and passive selection. A parallel RLC filter is common in tank circuits. It can create strong selectivity near resonance. An active RC filter avoids inductors. It is useful when inductors are large, costly, or less accurate.
Practical Accuracy
Real parts are not ideal. Capacitor tolerance changes the final center frequency. Inductor resistance lowers the peak response. Source and load resistance also change the loaded Q. For this reason, the calculator includes tolerance and loading estimates. These values help you judge the first design before simulation or lab testing.
Using the Output
Start with the calculated values. Then choose the nearest standard part values. After that, simulate the circuit. Check the actual response with the selected components. For narrow filters, use tighter tolerance parts. For high frequency filters, include parasitic capacitance and layout effects. Keep leads short. Use a clean ground path.
Design Workflow
Define the passband first. Select the topology next. Pick a capacitor value that creates practical resistor and inductor values. Check Q and bandwidth. Review the response curve. Export the data for notes. Then refine the circuit with standard components and measured results.
FAQs
What is a bandpass filter?
A bandpass filter passes a selected frequency range. It reduces frequencies below and above that range. It is used in audio, radio, sensors, and measurement circuits.
What is center frequency?
Center frequency is the middle of the passband on a logarithmic scale. It is calculated as the square root of lower cutoff times upper cutoff.
What does Q mean?
Q means quality factor. It compares center frequency with bandwidth. Higher Q creates a narrower and more selective bandpass response.
Which topology should I choose?
Use series RLC for simple passive design. Use parallel RLC for tuned tank circuits. Use active RC when you want to avoid inductors.
Why does tolerance matter?
Part tolerance changes resistance, capacitance, and inductance values. These changes shift center frequency, bandwidth, and Q from the ideal calculated result.
Can I use standard component values?
Yes. Use the calculator result as a target. Then choose the closest standard values and recheck the final response with those selected parts.
Is this suitable for RF circuits?
It can estimate early RF values. For final RF work, include parasitic capacitance, inductor self resonance, board layout, and impedance matching.
Why is the graph useful?
The graph shows estimated gain across frequency. It helps confirm the passband shape, peak point, and attenuation outside the target band.