active filter

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, or solid-state (transistor or operational amplifier).
Active filters have three main advantages over passive filters:
Inductors can be avoided. Passive filters without inductors cannot obtain a high Q (low damping), but with them are often large and expensive (at low frequencies), may have significant internal resistance, and may pick up surrounding electromagnetic signals.
The shape of the response, the Q (Quality factor), and the tuned frequency can often be set easily by varying resistors, in some filters one parameter can be adjusted without affecting the others. Variable inductances for low frequency filters are not practical.
The amplifier powering the filter can be used to buffer the filter from the electronic components it drives or is fed from, variations in which could otherwise significantly affect the shape of the frequency response.
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)
DABP Dual Amplifier Bandpass
Wien notch
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)
All the varieties of passive filters can also be found in active filters. Some of them 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.
Combinations are possible, such as notch and high-pass (for example, in a rumble filter where most of the offending rumble comes from a particular frequency), e.g.Elliptic filters.