Highpass and bandpass filters using a new single-amplifier simulated inductor

Abstract

A new single-amplifier single-capacitor circuit simulating a grounded inductor, for use in active RC highpass and bandpass filters, is discussed, and a method of compensating for the nonideal behaviour of simulated inductors of this type is presented. In contrast to known compensation methods, the compensation procedure proposed here does not seek to improve the nonideal simulated inductors, but deliberately designs the simulating circuit to have a specific biquadratic impedance function and then modifies the filter circuit so as to produce the required loss/frequency response using these biquadratic impedances instead of the original inductors. For bandpass filters the compensation is only approximate. However, for highpass filters, complete compensation can be achieved over the entire frequency range in which the gain of the amplifier can be adequately described by a single-pole model; the cost of achieving this is some increase in sensitivity. The new design procedure has been applied successfully to a 5th-order Cauer-type highpass filter; computed incremental sensitivities and some measured results are presented.