Electrical Engineering Dictionary
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## Active Notch Filter

A Notch filter is a band-stop filter which attenuates frequencies within a narrow band and passes frequencies outside this narrow band. There are a few common configurations of notch filters using operational amplifiers, a dual Op Amp approach is shown below, but single amplifier designs also exist.

This design basically uses the general purpose 741 operational amplifier. In this case the LM1458 is a dual 741 Op Amp, or two of the same amplifiers in a 16-pin package. The dual package was selected because the circuit uses both devices, and the 741 style because it's so common. However the amplifier needs to be selected to insure it has the bandwidth required at the desired frequency of operation, as with any design. LM1458 Tuned Notch Filter

### Notch Filter Design

The equation for calculating the center frequency for this circuit is given by:
1 / (2 * 3.1415 * [(R1*R2*C1*C2)1/2]).
Any of the four components used in the equation can be varied to tune the circuit. In this example capacitor C1 is a trimmer, or Variable Capacitor. However there is nothing stopping the design from using a variable resistor for R1 or R2. One approach is to replace R1 and R2 with a potentiometer, with the wiper connected to C2. Regardless of R1 and R2 being resistors or the combination turned into a potentiometer, it's value [or values] should be equal to R3. So R1 + R2 = R3. In addition this resistor ratio should also be realized; R4 = R5 = 2R3.

For higher frequency operation reduce the value of R3, than reduce the other resistors by the same proportion. Using precision resistors help the circuit obtain a higher Q, so some of the data sheets indicate 0.1 percent resistors. The resistors could be hand selected if that tolerance resistor was not stocked.