This filter is used to reject or block a frequency or band of frequencies. These filters are often designed into audio and instrumentation systems to eliminate a single frequency, such as 60Hz. Commercial grade components with 5%-10% tolerance produce a null depth of at least 30 to 40 dB. When a twinT network is combined with a TL081 op amp in a circuit, an active filter can be implemented. The added resistor capacitor network, R2 and C2, work effectively in parallel with the original twin-T network, on the input of the filter.
Twin-t-notch-filter - schematic

These networks set the Q of the filter. The op amp is basically connected as a unity-gain voltage follower. For a 60-Hz notch filter with a Q of 5, it is usually best to pick the C1 capacitor value and calculate the resistor Rl. Let C1 ~ 0.22 JlF. Then: R 1 ~ 12 KO - R 2 ~ 120 KO - C2 ~ 0.047 JlF Standard 5% resistors and 10% capacitors produce a notch depth of about 40 dB, as shown in the frequency response curve.

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