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It is sometimes desirable to have circuits
capable of selectively filtering one frequency or range of
frequencies out of a mix of different frequencies in a
circuit. A circuit designed to perform this frequency
selection is called a filter circuit, or simply a
filter. A common need for filter circuits is in
high-performance stereo systems, where certain ranges of
audio frequencies need to be amplified or suppressed for
best sound quality and power efficiency. You may be familiar
with equalizers, which allow the amplitudes of
several frequency ranges to be adjusted to suit the
listener's taste and acoustic properties of the listening
area. You may also be familiar with crossover networks,
which block certain ranges of frequencies from reaching
speakers. A tweeter (high-frequency speaker) is inefficient
at reproducing low-frequency signals such as drum beats, so
a crossover circuit is connected between the tweeter and the
stereo's output terminals to block low-frequency signals,
only passing high-frequency signals to the speaker's
connection terminals. This gives better audio system
efficiency and thus better performance. Both equalizers and
crossover networks are examples of filters, designed to
accomplish filtering of certain frequencies.
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