Low-power, low-noise, active-RC band-pass filters using a 'lossy' LP-BP transformation

A new and straightforward design procedure for simple canonical topologies of allpole, active-RC, low-selectivity band-pass (BP) filters, with low sensitivity to component tolerances is presented. The procedure is primarily intended for discrete-component, low-power filter applications using just one amplifier for relatively high-order filters. The design procedure starts out with an 'optimized' low-pass (LP) prototype filter, yielding an 'optimized' BP filter, whereby the wealth of 'optimized' single-amplifier LP filter designs can be exploited. Using a so-called 'lossy' LP–BP transformation, closed-form design equations for the design of second- to eighth-order, single-amplifier BP filters are presented. The low sensitivity, low power consumption, and low noise features of the resulting circuits, as well as the influence of the finite gain-bandwidth product and component spread, are demonstrated for the case of a fourth-order filter example. The optimized single-opamp fourth-order filter is compared with other designs, such as the cascade of optimized Biquads. Using PSpice with a TL081 opamp model, the filter performance is simulated and the results compared and verified with measurements of a discrete-component breadboard filter using 1% resistors, 1% capacitors, and a TL081 opamp.

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