Active filter

Encyclopedia : A : AC : ACT : Active filter

An example of high-pass active filter (an operational amplifier is designated U1)

An active filter is a type of analog electronic filter, distinguished by the use of one or more active component i.e. one which provides some form of voltage amplification. 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 stop band. (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.

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.

• Sallen and Key, and VCVS filters
• State variable, and biquadratic filters

See also

• Chebyshev filter
• Butterworth filter
• Bessel filter
• Elliptic filter

External links

• [Split-Supply Analog Filter Expert]
• [Single-Supply Analog Filter Expert]
• [Introduction to active filters]
• National Semiconductor's [AN-779] application note

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