What is a Multiple Feedback Filter?
Problem:
What is a Multiple Feedback Filter?
Solution:
A Multiple Feedback Filter is a two pole filter topology. It is available in low pass, high pass, bandpass, and notch versions, although the bandpass and notch versions are not recommended due to low resistor values. It can
easily accomodate gain, although this knowledgebase only presents unity gain versions. The reason is that component ratios with only two variables (center freqency and
type) are much easier to present in a quick "Q and A" format than three variables. The filter types, Butterworth, Chebyshev,
and Bessel, are also easily accomodated. The component value calculations can be formidible. Modifications are not easy, because
gain, frequency, and type all interact. Although this topology has one more passive component than the Sallen-Key topology, the stopband rejection is better,
making this the topology of choice for low cost applications.
The MFB topology is suited to operation from a single supply, because the non-inverting input is left unused. Current feedback amplifiers cannot be used, because
there is always a capacitor connected from the output to the inverting input. The MFB topology can be used with fully differential amplifiers.
Schematics for Multiple Feedback filters can be found at: