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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 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.[1]

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.[2] In some active filter circuits, one parameter can be adjusted without affecting the others.[1]


A 1974 KROHN-HITE model 3500 filter.

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.[3]

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

See also: Filter design

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

Comparison to passive filters

An active filter can have gain, increasing the power available in a signal compared to the input. Passive filters dissipate energy from a signal and cannot have a net power gain. For some ranges of frequencies, for example at audio frequencies and below, an active filter can realize a given transfer function without using inductors, which are relatively large and costly components compared to resistors and capacitors, and which are more expensive to make with the required high quality and accurate values. This advantage may not be as important for active filters entirely integrated on a chip because the available capacitors have relatively low values and so require high value resistors which take up area of the integrated circuit. Active filters have good isolation between stages, and can provide high input impedance and low output impedance; this makes their characteristics independent of the source and load impedances. Multiple stages can be cascaded when desired to improve characteristics. In contrast, design of multiple-stage passive filters must take into account each stage's frequency-dependent loading of the preceding stage. It is feasible to make active filters tunable over a wide range, compared with passive filters. Since inductors are not used, filters can be made in a very compact size and do not produce or interact with magnetic fields that may be present.

Compared with active filters, passive filters require no additional power supplies. The amplifying devices of an active filter must provide predictable gain and performance over the entire frequency range to be processed; the gain–bandwidth product of the amplifier will constrain the maximum frequency that can be used.[5][6]

See also


  1. ^ a b Don Lancaster, Active-Filter Cookbook, Howard W. Sams and Co., 1975 ISBN 0-672-21168-8 pages 8-10
  2. ^ "Op-amp Band Pass Filter". Basic Electronics Tutorials. 2013-08-14. Retrieved 2018-12-26.
  3. ^ Muhammad H. Rashid, Microelectronic Circuits: Analysis and Design, Cengage Learning, 2010 ISBN 0-495-66772-2, page 804
  4. ^ "Band Stop Filters are called Reject Filters". Basic Electronics Tutorials. 2015-10-20. Retrieved 2018-12-26.
  5. ^ Don Lancaster, Active-Filter Cookbook, Elsevier Science, 1996 ISBN 9780750629867
  6. ^ "Basic Introduction to Filters - Active, Passive, and Switched-Cap (Rev. A) Analog & Mixed-Signal SNOA224A -" (PDF). Retrieved 2020-02-03.