ch8 active filter lec 1

Active filter
The basic concept of a filter was originally introduced by G. Campbell and
K. Wagner independently in 1915 in relation with their work on transmission
lines and vibrating systems. Since then, the development of filter knowledge
and filter technologies has been and is still expanding. Today, filters have
permeated the electronic technology so much that it is difficult to think of any
moderately complex system or device that does not employ a filter in one
form or another.

In the most general sense, a "filter" is a device or a system that alxers in a
prescribed way the input that passes through it. In essence, a filter converts
inputs into outputs in such a fashion that certain desirable features of the
inputs are retained in the outputs while undesirable features are suppressed.
There are many kinds of filters; only a few examples are given here. In
automobiles, the oil filter removes unwanted particles that are suspended in
the oil passing through the filter; the air filter passes air but prevents dirt
and dust from reaching the carburetor. Colored glass may be used as an
optical filter to absorb light of certain wavelengths, thus altering the light
that reaches the sensitized film in a camera.

An electrical filter is designed to separate and pass a desired signal from a
mixture of desired and undesired signals. Typical examples of complex
electrical filters are televisions and radios. More specifically, when a television is
turned to a particular channel, say Channel 2, it will pass those signals (audio
and visual) transmitted by Channel 2 and block out all other signals. On a
smaller scale, filters are basic electronic components in many
communication systems such as the telephone, television, radio, radar, and sonar.
Electrical filters can also be found in power conversion circuits and power systems
in general. In fact, electrical filters permeate modern technology so much
that it is difficult to think of any moderately complex electronic device that
does not employ a filter in one form or another.