Operational amplifier / op amp notch filter circuit

- the circuit and design considerations for a notch filter using an operational amplifier or op amp, four resistors and two capacitors

This operational amplifier notch filter circuit is simple yet effective, providing a notch on a specific fixed frequency. It can be used to notch out or remove a particular frequency that may need to be removed.

Having a fixed frequency, this operational amplifier, op amp, notch filter circuit may find applications such as removing fixed frequency interference like mains hum, from audio circuits.

The circuit is quite straightforward to build. It employs both negative and positive feedback around the operational amplifier chip and in this way it is able to provide a high degree of performance.

Calculation of the value for the circuit is very straightforward. The formula to calculate the resistor and capacitor values for the notch filter circuit is:

Where:
fnotch = centre frequency of the notch in Hertz
pi = 3.142
R and C are the values of the resistors and capacitors in Ohms and Farads

When building the circuit, high tolerance components must be used to obtain the best performance. Typically they should be 1% or better. A notch depth of 45 dB can be obtained using 1% components, although in theory it is possible for the notch to be of the order of 60 dB using ideal components. R1 and R2 should be matched to within 0.5% or they may be trimmed using parallel resistors.

A further item to ensure the optimum operation of the circuit is to ensure that the source impedance is less than about 100 ohms. Additionally the load impedance should be greater than about 2 M Ohms.

The circuit is often used to remove unwanted hum from circuits. Values for a 50 Hz notch would be: C1, C2 = 47 nF, R1, R2 = 10 k, R3, R4 = 68 k.