Practical universal active filter using generalised-immittance-convertor biquads

A general GIC (generalised-immittance-convertor) section, capable of realising any 2nd-order biquadratic functions, is presented. It employs, at most, three operational amplifiers and a few passive elements. Using this approach, a universal active filter(UAF) building block has been obtained, and thus any filtering functions, such as lowpass (LP), highpass(HP), bandpass (BP), allpass (AP) and notch (N) functions, can be easily constructed. The resulting filter realisations are shown to have low active and passive sensitivities to element variations. The high-frequency performance limitations imposed by the finite bandwidth of an operational amplifier have also been examined. The proposed scheme has been compared with a number of popular multiple amplifier RC-active biquad filters and was found to offer several attractive features as compared to these realisations. The design procedure is illustrated by a 6th-order elliptic bandpass filter, together with theoretical and experimental results. Further, temperature tests reveal that these realisations are insensitive to temperature variations.