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A filter circuit is like
a sieve. It allows some
things through and holds
back others. In this
case we are talking
about AC frequencies.
Some frequencies pass
through the filter while
others are rejected.
The characteristics of a
filter can be shown on a
graph called a FREQUENCY
RESPONSE CURVE.
VOLTAGE OUT is plotted
against FREQUENCY.
Figure 2 shows a LOW
PASS filter response
curve giving output at
low frequencies but none
at higher frequencies.
Figure 3 shows a
selection of filter
characteristics.
Simple filters can be
made from capacitors and
resistors
Filters have many
applications.
In audio frequency
amplifiers, CROSSOVER
filters to direct low
frequencies to the
WOOFER and high
frequencies to the
TWEETER speakers.
In SCRATCH filters to
remove unwanted high
frequency noise.
In NOTCH filters to
remove whistles due to
two radio stations being
too close together in
frequency.
In Hum filters to remove
low frequency noise due
to the mains supply.
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