A notch filter is a circuit that rejects a very narrow band of frequency.  A common application of a notch filter is the removal of the 60-Hz hum from an AC line.  The twin-T filter, one of the most commonly used notch filters, is a passive filter composed of two T-networks.  One of these T networks has one resistor and two capacitors, while the other has two resistors and one capacitor.  For the twin-T notch filter in Figure 1, maximum attenuation occurs at fn = 1/(2πRC).

The Q factor of the circuit in Figure 1 may be significantly increased by feeding it into an operational amplifier configured as a voltage follower, as shown in Figure 2.  The values shown in Figure 2 were chosen to achieve a notch frequency of about 60 Hz.