Design of active R filters using only resistors and operational amplifiers

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Abstract
Active filters can be designed using only resistors and amplifiers as the circuit elements. The reactance for these filters is obtained from the parasitic capacitances associated with the amplifiers. The resonant frequency and Q of the poles and the location of the zeros can be controlled over a wide range by resistor feedback and feedforward circuits. General configuration for the first-, second- and third-order filter building blocks may be used in cascade to achieve any desired filtering function. The basic building block is constructed from only two amplifiers and three resistors. This circuit simultaneously exhibits low-pass, high-pass, and band-pass functions, respectively, at its three outputs. A simple summing network allows these outputs to be summed to obtain any second- or third-order filter function. In addition to simplifying the circuit configurations, the active R filter technique results in filters which may be operated at much higher frequencies than classical active RC filters. In fact, experimental results confirm several orders of magnitude improvement in the high frequency response of such filters.

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