Op amp twin T notch filter
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.
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