What is a Fliege Filter?

Problem:
What is a Fliege Filter?

Solution:
A Fliege Filter is a two pole filter topology. It is available in low pass, high pass, bandpass, and notch versions. The lowpass, highpass, and bandpass versions have a fixed gain of 2 (6 dB), while the notch version has unity gain. The Q of the notch version cannot be adjusted. This is the lowest component count 2 opamp topology, and offers good controllability over frequency and type (Butterworth, Chebyshev, and Bessel).

The Fiege topology is well suited to operation from a single supply. Current feedback amplifiers cannot be used, because a capacitor is conneected from the op amp output to inverting input. The Fliege topology cannot be used with fully differential amplifiers.