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# Multiple Feedback Bandpass Filter (CD4001)

2015-01-13 07:08
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Introduction

The multiple feedback bandpass filter is a simple looking
design, but it is difficult to calculate the values for a given
set of parameters. These filters are useful for equalisation,
analysis and other tasks such as the Sound to Light converter
(Project 62) or even a fully functional Vocoder. For those
who have not heard of the vocoder, it is a device that takes a
music source as one input and vocals as the other, allowing a
guitar, keyboard or complete ensemble to be made to speak or
sing. The “speech” from a good vocoder is quite intelligible, and
is “ear candy” of the very best kind for experimental musicians.

This is the first in a series of projects using this filter
type, and I have included a small calculator programme to make it
easier to determine the component values for different filter
parameters.

Description

A schematic for the filter is shown in Figure 1. The source
impedance must be low with respect to the input resistance, and
normally these filters are driven from an opamp buffer. If a high
impedance is used, it adds to the total input resistance,
causing unpredictable centre frequency and response.

Figure 1 – Multiple Feedback Bandpass Flter

The resistor and capacitor values not shown are calculated
from the formulae below, or by using the calculator program (see
below for details). The opamp shown is a single device, but most
commonly dual or quad opamps will be used for this kind of
application. A resistance from the ve input of the opamp is
optional. If used, the resistor should be the same value as R3 to
obtain minimum DC offset from the opamp output. A 100nF
capacitor is highly recommended to bypass the non inverting input
to earth for AC, and helps to reduce noise. If offset is not a
problem for you, simply connect the non inverting input to the
earth (GND) rail as shown. C4 and C5 are supply bypass
capacitors, and should be used at each IC package. Ceramic
capacitors are recommended for the most effective high frequency
bypass. The opamp can be any common device for low frequencies,
but at high frequencies (above about 2kHz) a high speed unit is
required for best performance.