Figure 1-18B.Components of a simple high-pass filter.
Figure 1-18C.Components of a simple high-pass filter.
RESONANT CIRCUITS AS FILTERS
Resonant circuits can be made to serve as filters in a manner similar to the action of individual
capacitors and inductors. As you know, the series-LC circuit offers minimum opposition to currents that
have frequencies at or near the resonant frequency, and maximum opposition to currents of all other
You also know that a parallel-LC circuit offers a very high impedance to currents that have
frequencies at or near the resonant frequency, and a relatively low impedance to currents of all other
If you use these two basic concepts, the BANDPASS and BAND-REJECT filters can be constructed.
The bandpass filter and the band-reject filter are two common types of filters that use resonant circuits.
A bandpass filter passes a narrow band of frequencies through a circuit and attenuates all other
frequencies that are higher or lower than the desired band of frequencies. This is shown in figure 1-19
where the greatest current exists at the center frequency (fr). Frequencies below resonance (f1) and
frequencies above resonance (f2) drop off rapidly and are rejected.