Sallen–Key topology

A Sallen- key filter, in English also called a Sallen and Key filter or as a Voltage Controlled Voltage Source filter ( VCVS filter), is an active electronic filter consisting of an operational amplifier and a plurality of electrical resistors and capacitors. The name is derived from the name of the two developers RP Sallen and EL Key, which in 1955 developed this filter at the Massachusetts Institute of Technology (MIT).

The advantage of the Sallen-Key filter is that with minimal technical effort an analog filter circuit - as shown in left figure - can be realized. It can be realized by adjusting the filter structure of both low-pass filters, high pass filters and band-pass filter. Since the transfer function can not have complex zeros due to the branching structure, so that only so-called all-pole filter can be realized without transmission zeros. Bandstop filters and filters with elliptical elements as Cauer filters can not be implemented with a Sallen -Key filter.

Sallen-Key filter always have in this case a filter order of two. This means that a low-pass or high-pass filter the magnitude frequency response of a change of 12 dB per octave undergoes. Higher filter orders require a series connection of several Sallen-Key filter, which only just filter orders can be achieved. At Sallen-Key filters, the operational amplifier is usually operated with a positive gain, also referred to as Mitkopplungsstruktur. As can be the complex conjugate poles of the transfer function realized with a negative gain value, Sallen-Key filters can also be in a feedback structure, but with a higher component cost can be realized. Due to the higher complexity components with identical transfer function play Sallen-Key filter in feedback structure are not essential. The operational amplifier is operated in both cases in negative feedback.

The filter structure is also relatively stable with respect to component tolerances, thus justifying the practical relevance in addition to the simple structure. These advantages are gained by the disadvantage that in order to achieve the highest possible quality factor Q extremely high and not practical component values ​​are needed. Sallen -Key filters are therefore used there in the analog circuitry preferred where space is crucial component count with large tolerance scheme and where low quality factors are acceptable.

Examples of applications in the form of a low-pass filter can be found on the analog side directly against aliasing before analog-to- digital converter and as a filter to image spectrums (anti- imaging filter) immediately after the digital to analog converter in communication engineering systems.

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