Active Filter Calculator

Active Filter Calculator. An active filter uses op-amps alongside resistors and capacitors to pass or block specific frequency ranges in audio, signal processing, and electronics circuits. Select your filter type (Low Pass, High Pass, Band Pass, or Notch) and topology, then enter your cutoff frequency, gain, quality factor, resistor value, and supply voltage to get the required Capacitor C1 value. Secondary outputs include R2 and C2 component values, gain in dB, and phase shift at the cutoff frequency. Also try the Bessel Filter Calculator.

Filter Type *

Topology *

Cutoff Frequency *

Gain *

V/V

Quality Factor (Q) *

Q = 0.707 for Butterworth response

Resistor Value (R1) *

kΩ

Supply Voltage *

Results

Capacitor Value (C1)

Resistor R2 Value

Capacitor C2 Value

Gain (dB)

Phase Shift at fc

Results Table

What is an active filter and how does it differ from a passive filter?

An active filter uses active components like op-amps along with resistors and capacitors to provide signal filtering with gain. Unlike passive filters, active filters can amplify signals and don't suffer from loading effects. See also our calculate Resistor Value, Capacitor Value & Inductor Value — Band Stop / Notch Filter.

What is the Sallen-Key topology?

The Sallen-Key topology is a popular active filter design using an op-amp in a non-inverting configuration. It provides good performance with minimal components and is widely used for second-order filters.

What does the Quality Factor (Q) represent in filter design?

Quality Factor (Q) determines the sharpness of the filter's response. A Q of 0.707 gives a Butterworth (maximally flat) response, while higher Q values create a sharper cutoff but may introduce peaking.

How do I choose the resistor and capacitor values?

Start with a practical resistor value (1kΩ to 100kΩ) and calculate the required capacitor value. Use standard component values closest to calculated values for best results.

What is the cutoff frequency and why is it important?

The cutoff frequency (fc) is where the filter's output drops by -3dB (about 70.7% of the input). It defines the transition point between the passband and stopband of the filter.

Can I use this calculator for band-pass and notch filters?

Yes, the calculator supports multiple filter types including low-pass, high-pass, band-pass, and notch filters with various topologies and component calculations.

What op-amp should I use for my active filter?

Choose an op-amp with bandwidth at least 10x your cutoff frequency, low noise, and sufficient slew rate. Popular choices include TL072, LM358, or OPA2134 depending on performance requirements. You might also find our Band Pass Filter Calculator useful.