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