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.
There are two principal reasons for the use of active 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 (topology) include:
- [[Sallen Key filter|Sallen and Key, and VCVS filters (low dependency on inaccuracy of the components)
- State variable filter|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.
- Band-stop filter|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)
- Chebyshev filter
- Butterworth filter
- Bessel filter
- Elliptic filter