Multiple Feedback Bandpass Filter

Posted by Author makecircuits

Multiple Feedback Bandpass Filter

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.