Op amp notch filter
the circuit and design considerations for a notch filter using an
operational amplifier, 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.
Active operational amplifier notch filter circuit
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:
fnotch = 1 / (2 pi R C)
R = R3 = R4
C = C1 = C2
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
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.