Four-terminal network of adjustable transfer function

Abstract

A four-terminal network selectively convertible into a low-pass, high-pass, band-pass, all-pass or band-stop filter comprises three cascaded stages each including an operational amplifier with a grounded noninverting input, the amplifiers of the first two stages being of the integrating type provided with capacitive feedback circuits. Resistive feedback connections extend from a main output A of the third stage to the inverting inputs of the first and third stages and from a relatively inverted output A' of the third stage to the inverting input of the second stage, their respective resistances R.sub.u, R.sub.v, R.sub.w determining the coefficients of the denominator in a biquadratic equation ##EQU1## where H(p) is the transfer function of the network and p=jf/f.sub.n is given by the ratio of an operating frequency f to a reference frequency f.sub.o. The coefficients of the numerator, which could be of either sign and may also be zero, are determined by supply resistances R.sub.a, R.sub.b, R.sub.c connected to the inverting inputs of the several amplifiers, these resistances being switchable to either of two input terminals carrying two relatively inverted signal waves. The feedback capacitances of the first two stages lie in series with respective coupling input resistors of the immediately following stages and determine therewith the reference frequency f.sub.o which can therefore be changed by varying the relative magnitudes of these parameters, either directly or through interposition of adjustable ancillary amplifiers. The supply and feedback resistances may be part of interchangeable modular inserts establishing different signal-transmission modes.