Low-pass circuits derive their name from the fact that the output of these circuits is larger for lower frequencies and vice-versa. Figures 3.1(a) and (b) represent a low-pass *RC* circuit and a low-pass *RL* circuit, respectively.

In the *RC* circuit, shown in Fig. 3.1(a), at low frequencies, the reactance of *C* is large and decreases with increasing frequency. Hence, the output is smaller for higher frequencies and vice-versa. Similarly, in the *RL* circuit shown in Fig. 3.1(b), the inductive reactance is small for low frequencies and hence, the output is large at low frequencies. As the frequency increases, the inductive reactance increases; hence, the output decreases. Therefore, these circuits are called low-pass circuits. Let ...

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