What is a Sallen Key Filter?

Problem:
What is a Sallen Key Filter?

Solution:
A Sallen Key Filter is a two pole filter topology. It is sometimes called VCVS, which stands for Voltage Controlled Voltage Source. It is available in low pass, high pass, bandpass, and notch versions, although the notch variety owes more to the Twin T topology. The bandpass version version of the Sallen Key topology is not recommended, because the input resistor tends to be a very low value. The Sallen Key topology is often seen in a unity gain version, although the addition of gain resistors is an obvious enhancement. The designer is cautioned, however, that changing the gain also changes other filter characteristics. Like the other single opamp topologies, changing gain will also affect frequency and filter type (Butterworth, Chebyshev, and Bessel).

The Sallen Key topology is suited to operation from a single supply. Current feedback amplifiers can be used, with the restriction that the connection from the op amp output to inverting input must be the recommended feedback resistor value. This is true whether or not this resistor is part of a gain stage. The Sallen Key topology cannot be used with fully differential amplifiers.

Schematics for Sallen Key filters can be found at:

• The Split Supply Expert
• The Single Supply Expert
• The Current Feedback Expert