What is a filter?
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.
Another practical application of filter
circuits is in the "conditioning" of non-sinusoidal voltage waveforms in
power circuits. Some electronic devices are sensitive to the presence of
harmonics in the power supply voltage, and so require power conditioning
for proper operation. If a distorted sine-wave voltage behaves like a
series of harmonic waveforms added to the fundamental frequency, then it
should be possible to construct a filter circuit that only allows the
fundamental waveform frequency to pass through, blocking all
(higher-frequency) harmonics.
We will be studying the design of several
elementary filter circuits in this lesson. To reduce the load of math on
the reader, I will make extensive use of SPICE as an analysis tool,
displaying Bode plots (amplitude versus frequency) for the various kinds
of filters. Bear in mind, though, that these circuits can be analyzed
over several points of frequency by repeated series-parallel analysis,
much like the previous example with two sources (60 and 90 Hz), if the
student is willing to invest a lot of time working and re-working
circuit calculations for each frequency.
- REVIEW:
- A filter is an AC circuit that
separates some frequencies from others in within mixed-frequency
signals.
- Audio equalizers and
crossover networks are two well-known applications of filter
circuits.
- A Bode plot is a graph plotting
waveform amplitude or phase on one axis and frequency on the other.
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