Low active-sensitivity allpole active-RC filters using impedance tapering

To reduce the low-power allpole active-RC filter´s output magnitude sensitivity to passive components, the "impedance tapering" design method was used. This design procedure has been applied to the design of 2nd- and 3rd-order low-pass (LP) (class 4) Sallen-and-Key filters and has already been published in G. S. Moschytz (1999). In this paper the active sensitivity analysis using Mathematica was performed on the same "impedance tapered" filter sections. The analysis was performed by calculation of the real filter´s transfer-function magnitude which was denormalized to various cut-off frequencies from low to high, using a single-pole model of a real operational amplifier. The amount of active and passive sensitivity reduction was compared using the same degree of impedance tapering. It was demonstrated that both sensitivities, i.e. active and passive, were corresponding reduced for the same filter. Thus, a judicious selection of component values accounts for a considerable decrease in both passive and active sensitivities.