Page 230 - Op Amps Design, Applications, and Troubleshooting

CHAPTER FIVE

Active Filters

5.1 FILTER FUNDAMENTALS

A filter, be it an oil filter, a lint filter, a furnace filter, or an active filter, accepts a
wide spectrum of inputs, but only passes certain of these inputs through to the
output. In some cases, it may pass through the "good stuff" while it catches the
"bad stuff." An oil filter in your car is one example. Other applications require a
filter to catch the "good stuff" and let the "bad stuff" pass through. A gold
prospector's sieve is an example of this type of filter action. In both of the preced-
ing examples, the filter discriminates between "good" and "bad" on the basis of
physical size (i.e., size of the dirt particle). In the filters discussed in this chapter,
the "good" and "bad" signals will be classified on the basis of their frequency. The
input will be a broad range of signal frequencies. The filter will allow a certain
range of them to pass and will reject others.
Electronic niters designed to discriminate as a function of frequency can be
broadly grouped into five classes:

1. Low pass Allows frequencies below a specified frequency to pass
through the filter circuit.
2. High pass Allows frequencies above a specified frequency to pass
through the filter circuit.
3. Bandpass Allows a range or band of frequencies to pass through the
filter circuit while rejecting frequencies higher or lower than
the desired band.
4. Band reject Rejects all frequencies within a certain band, but passes
frequencies higher or lower than the specified band. Also
called a band-stop filter.
5. Notch Essentially a band-stop filter with a very narrow range of
frequencies that are rejected.

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