Active filter

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

An active filter is a type of analog electronic filter that uses active components such as an amplifier. Amplifiers included in a filter design can be used to improve the performance and predictability of a filter,[1] while avoiding the need for inductors (which are typically expensive compared to other components). 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. [1]

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.[2]

Active filter circuit configurations (electronic filter topology) include:

Active filters can implement the same transfer functions as passive filters. Common transfer functions are:

Combinations are possible, such as notch and high-pass (in a rumble filter where most of the offending rumble comes from a particular frequency). Another example is an elliptic filter.

Design of active filters

To design filters, the specifications that need to be established include:

References

  1. 1 2 Don Lancaster, Active-Filter Cookbook, Howard W. Sams and Co., 1975 ISBN 0-672-21168-8 pages 8-10
  2. Muhammad H. Rashid, Microelectronic Circuits: Analysis and Design, Cengage Learning, 2010 ISBN 0-495-66772-2, page 804

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