Op amp twin T notch filter circuit

- the circuit and design considerations for a twin T notch filter with variable Q using an operational amplifier

The twin T notch filter is a simple circuit that can provide a good level of rejection at the "notch" frequency. The simple RC notch filter can be placed within an operational amplifier circuit to provide an active filter. In the circuit shown below, the level of Q of the notch filter can be varied.

Active twin T notch filter circuit with variable Q

Calculation of the value for the circuit is very straightforward. The formula is the same as that used for the passive version of the twin T notch filter.

fc     =     1 / (2 pi R C)

Where:
fc = cut off frequency in Hertz
pi = 3.142
R and C are the values of the resistors and capacitors as in the circuit

The notch filter circuit can be very useful, and the adjustment facility for the Q can also be very handy. The main drawback of the notch filter circuit is that as the level of Q is increased, the depth of the null reduces. Despite this the notch filter circuit can be successfully incorporated into many circuit applications.