# Feedback Band-Pass Filter

Posted on Dec 13, 2012

One application of this handy circuit is a graphic equalizer, created by feeding a signal to a number of parallel band-pass filters each tuned to a different frequency; typically octaves apart. Then, you can adjust the strength in each band via a front panel potentiometer. The outputs of all the filters are then summed to create overall frequency response of the equalizer.

There's a couple of advantages to this active band-pass filter. First, you don't need an inductor (bulky and expensive at low frequencies) to create the band-pass shape. And second, it only needs one op amp device. One disadvantage is the nature of adjusting the center frequency fo. Adjustments are not orthogonal (independent). You can tune fo with a resistor, but, the Q also changes. Still, the circuit is easy to implement and is useful for Q's up to about 20. But you've got to be aware how component tolerances alter your tuning frequency. And, you'll need an op amp with enough horsepower so it won't spoil your frequency response. You can visualize the band-pass nature of this circuit by inspecting its topology - R2 and C2 form a differentiator like circuit (high-pass), while C1 and R1A/B form an integrator like circuit (low-pass). Letting C1 = C2 makes the Multiple Feedback Band-pass filter straight forward to design. Just follow these simple steps. As an example, suppose R2 value differs by 5 % of the design value. What is the effect on tuning? From the equations, fo is a function of the square root of R2. So you can expect the center frequency to change by ?5% or about 2.4 %. Increase R2 to 15.9 k x 1.05 = 16.7k and see what happens to fo. Use a cursor to get an accurate measure of fo. Where exactly is the center frequency of this filter? There's two ways to find the tuned frequency of a band-pass filter: 1. Find the...

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