What is a State Variable Filter?
Problem:
What is a State Variable Filter?
Solution:
A State Variable filter is a two pole filter topology. It is available in low pass, high pass, bandpass, and notch versions. The notch version is not recommended, because it requires 4 opamps. It is
one of three topologies that offer complete and independent control over gain, frequency, and type (Butterworth, Chebyshev, and
Bessel). Signal input is at one place, and the output is taken from different places. The State Variable filter has the unique characteristic of
producing low pass, high pass, band pass (and notch) outputs simultaneously.
The State Variable topology is suited to operation from a single supply. Current feedback amplifiers cannot be used, because a capacitor is connected from the
op amp output to inverting input. The State Variable topology cannot be used with fully differential amplifiers.
Schematics for State Variable filters can be found at: