Multiple-feedback bandpass filter

  
The op amp is connected in the inverting mode. Resistor R3 from the output to the inverting input sets the gain and current through the frequency-determining capacitor, CI. Capacitor C2 provides feedback from the output to the junction of Rl and R2. CI and C2 are always equal in value. Resistor R2 may be made adjustable in order to adjust the center frequency which is determined from: 1 1 Rl + R2 a fo = 2TTC R3 R1 R2 When designing a filter of this type.it is best to select a value for CI and C2, keeping them equal.
Multiple-feedback bandpass filter - schematic

Typical audio filters have capacitor values from 0.01 µf to 0.1 /µf which will result in reasonable values for the resistors.




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