Active filter

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An example of high-pass active filter of the Sallen Key topology. The operational amplifier, U1, is used as a buffer amplifier.

An active filter is a type of analog electronic filter, distinguished by the use of one or more active components i.e. voltage amplifiers or buffer amplifiers. Typically this will be a vacuum tube, transistor or operational amplifier.

Active filters have two main advantages over passive filters. The first is that the amplifier powering the filter can be used to shape the filter's response, e.g., how quickly and how steeply it moves from its passband into its stopband. (To do this passively, one must use inductors, which tend to pick up surrounding electromagnetic signals and are often quite physically large.) The second is that the amplifier powering the filter can be used to buffer the filter from the electronic components it drives. This is often necessary so that they do not affect the filter's actions.

Active filter circuit configurations (electronic filter topology) include:

Sallen and Key, and VCVS filters (low dependency on accuracy of the components)
State variable, and biquadratic filters
Twin T filter (fully passive)
Multiple Feedback Filter
Fliege (lowest component count for 2 opamp but with good controllability over frequency and type)
Akerberg Mossberg (one of the topologies that offer complete and independent control over gain, frequency, and type)

There are several varieties of active filter. Some of them, also available in passive form, are:

High-pass filters – attenuation of frequencies below their cut-off points.
Low-pass filters – attenuation of frequencies above their cut-off points.
Band-pass filters – attenuation of frequencies both above and below those they allow to pass.
Notch filters – attenuation of certain frequencies while allowing all others to pass.

To design filters, different types are available to set the component value based on mathematical properties (which define "shape" of the frequency bands)