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CHAP. 12]

FREQUENCY RESPONSE, FILTERS, AND RESONANCE

281

Fig. 12-14

and stop others (the stop-band). Ideally, in the pass-band, Hð j!Þ ¼ 1 and in the stop-band, Hð j!Þ ¼ 0. We therefore recognize the following classes of filters: low-pass [Fig. 12-15(a)], high-pass [Fig. 12-15(b)], bandpass [Fig. 12-15(c)], and bandstop [Fig. 12-15(d)]. Ideal filters are not physically realizable, but we can design and build practical filters as close to the ideal one as desired. The closer to the ideal characteristic, the more complex the circuit of a practical filter will be. The RC or RL circuits of Section 12.2 are first-order filters. They are far from ideal filters. As illustrated in the following example, the frequency response can approach that of the ideal filters if we increase the order of the filter.

Fig. 12-15

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