A Sallen-Key filter is a type of active electronic filter commonly used in analog signal processing and audio applications. It is designed to provide low-pass, high-pass, band-pass, or band-reject (notch) filtering of electrical signals. The filter was invented by R.P. Sallen and E.L. Key in the late 1950s.
Sallen-Key filters are known for their simplicity, ease of design, and relatively low component count compared to other filter designs. They use operational amplifiers (op-amps) as the active components to achieve the desired filtering characteristics.
The basic Sallen-Key filter consists of two cascaded stages, each composed of an op-amp and a combination of resistors and capacitors. The arrangement and values of these components determine the filter's frequency response and filtering behavior.
The most common Sallen-Key filter configurations are:
Low-pass filter (LPF): Allows low-frequency signals to pass while attenuating higher frequencies.
High-pass filter (HPF): Allows high-frequency signals to pass while attenuating lower frequencies.
Band-pass filter (BPF): Allows a specific range of frequencies to pass while attenuating frequencies outside that range.
Band-reject filter (BRF) or notch filter: Attenuates a specific frequency band while passing frequencies above and below it.
One of the advantages of the Sallen-Key filter is its ability to achieve a higher order (steeper roll-off) by cascading multiple stages together. Additionally, the filter's characteristics can be adjusted by varying the resistor and capacitor values, allowing for a wide range of design flexibility.
Sallen-Key filters find applications in various areas, such as audio processing, telecommunications, instrumentation, and control systems. They are particularly useful in scenarios where precise signal conditioning and filtering are required.