The
simplest high-pass filter one can create using
discrete circuit components is the first-order RC high-pass filter. It uses exactly one
resistor, and one
capacitor.
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Vin --| |----+--- Vout
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C \
R /
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Gnd
A high-pass filter,roughly, is characterized by two things: its corner frequency, and its order. The corner frequency of the above high-pass filter is fc = 1/(2πRC)
The corner frequency is where an input signal power will be cut down by a factor of 2. Typically, power is reported in decibels (dB); the corner frequency is the frequency at which the signal power is attenuated by 3 dB. The region above the corner frequency is called the passband, the region below is called the stopband.
The order of the filter determines how steeply the filter cuts off high frequencies; a first-order filter reduces signal power by 20 dB per decade once the frequencies are below the corner frequency; that is, if the input signal frequency goes down by a factor of 10, the input will be attenuated roughly by factor of 8 more than before. A second-order filter will attenuate at 40 dB per decade, and so on.
A perfect high-pass filter, one that removes all of the signal below the corner frequency, is impossible to create with analog circuitry. The sharper a filter one wants, the more complex it gets, and the more it tends to cost.
Higher-order filters can be made in various ways, which result in different frequency responses, suitable for various applications. A Butterworth filter has a very flat passband frequency response; that is, it doesn't attenuate the passband at all. However, it does not attenuate very steeply compared to the other types. A Bessel filter has a very sharp cutoff, but isn't flat in the passband-there is a ripple of a few dB, typically, attenuating some frequencies in the passband slightly. Many other types exist, tailored to specific applications.
High-pass filters are a very basic circuit element, and are used for numerous applications. Some basic uses are the removal of DC offsets, removal of low-frequency noise, as a part of a pass-band filter, among many others.