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Active Filter Calculations

Since http://captain.at has apparently gone awry, I've copied his code here and added some enhancements.

A simple tool for designing active filters using voltage-feedback opamps. All calculations refer to the values in the Picture (C1, C2, R1, R2, R3)

Type in the center frequency (fc), bandwidth (B), gain (A) and choose a value for the capacitors (C1, C2 = C; 10nF is a good value to start) and click on "Calculate".

Light-grey fields are input fields - white fields are output fields for the calculated values.

Active Filter - Bandpass

Input Filter Requirements Output Circuit Values


Reverse Engineer an Active Filter

Use the "Resistor Series lookup" tool to get values for available resistors and calculate the actual center frequency, bandwidth, Q factor and gain.

Input Circuit Values Output Filter Qualities

Resistor Series lookup

Provide your Target Resistance, R in Ohms.

Select the resistor series (E6, E12, E24, or E48) and click "Suggest" to obtain the closest match within that resistor series.


Reference: http://www.logwell.com/tech/components/resistor_values.html

Worst Case Frequencies

Type in your real values of the components, select the tolerance of the capacitors and resistors and click "Calculate". The calculation will reveal the worst case frequency (fc), bandwidth (B), gain (A).

Input Circuit Values
Values @ min. frequency: Values @ max. frequency: