The circuit in Fig. 12.49a is called a notch filter. From a sketch of its Bode plot in Fig. 12.49b, we see that at the notch frequency, f_{n} , the transfer function gain is zero, while at frequencies above and below f_{n} the gain is unity. Let us design a notch filter to remove an annoying 60-Hz hum from the output voltage of a cassette tape player and generate its Bode plot.