What is a Biquad Filter?
Problem:
What is a Biquad Filter?
Solution:
A Biquad filter is a two pole filter topology. It is available in low pass, high pass, bandpass, and notch versions. The highpass and notch versions are not recommended, because they require 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 Biquad filter has the unique characteristic of
producing two polarities of lowpass output.
The Biquad 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 Biquad topology can be used with fully differential amplifiers, with the additional advantage that the number
of opamps required is reduced from 3 to 2.
Schematics for Biquad filters can be found at: