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Concept
Active filter
Summary
An active filter is a type of analog circuit implementing an electronic filter using active components, typically an amplifier. Amplifiers included in a filter design can be used to improve the cost, performance and predictability of a filter.
An amplifier prevents the load impedance of the following stage from affecting the characteristics of the filter. An active filter can have complex poles and zeros without using a bulky or expensive inductor. The shape of the response, the Q (quality factor), and the tuned frequency can often be set with inexpensive variable resistors. In some active filter circuits, one parameter can be adjusted without affecting the others.
Using active elements has some limitations. Basic filter design equations neglect the finite bandwidth of amplifiers. Available active devices have limited bandwidth, so they are often impractical at high frequencies. Amplifiers consume power and inject noise into a system. Certain circuit topologies may be impractical if no DC path is provided for bias current to the amplifier elements. Power handling capability is limited by the amplifier stages.
Active filter circuit configurations (electronic filter topology) include:
Sallen-Key, and VCVS filters (low sensitivity to component tolerance)
State variable filters and biquadratic or biquad filters
Dual amplifier bandpass (DABP)
Wien notch
Multiple feedback filters
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)
Active filters can implement the same transfer functions as passive filters. Common transfer functions are:
High-pass filter – attenuation of frequencies below their cut-off points.
Low-pass filter – attenuation of frequencies above their cut-off points.
Band-pass filter – attenuation of frequencies both above and below those they allow to pass.
Band-stop filter (Notch filter) – attenuation of certain frequencies while allowing all others to pass.
Official source
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