Low-sensitivity, low-noise, band-rejection and all-pass active-RC filters

In this paper we introduce a new design procedure for low-sensitivity and low-noise filter biquads which have a symmetrical bridged-T passive-RC network. They are also low-power circuits, in that they use only one operational amplifier (opamp). The new design concept is based on the recently introduced "impedance tapering" method which was applied to allpole filters in [1], whereas in this paper it is extended to the design of filters with finite zeros. Thus, it is applied to two commonly used filter sections suitable for the realization of band-rejection and all-pass filters with low and medium pole-Q factors. In the new design procedure, the topology and component count remain the same; we just judiciously select the component values in order to reduce component tolerance sensitivity and improve noise performance. The sensitivity analysis is examined analytically and double-checked using PSpice Monte Carlo runs. In the PSpice noise analysis, a macro-model of the uA741 opamp is used. It is found that the minimum-sensitivity and minimum-noise filters coincide.