The design of second-order band-pass (BP) active RC filters using a modified low-pass to band-pass (LP-BP) frequency transformation is presented. The transformation is applied to a first-order low-pass (LP) filter as the (odd-order-)prototype, from which a single-amplifier second-order BP filter is constructed. The operational amplifier is added to the first-order LP circuit in order to provide a low output impedance and supply a positive feedback loop to enable a pole shifting process needed in the realization. It is shown that a BP filter can be realized by substitution of resistor and capacitor in the low-pass prototype filter, by serial parallel RC circuits in the resulting band-pass structure. Schoeffler sensitivity is used as a measure of the magnitude sensitivity to component tolerances. A step-by-step design procedure is verified for several second-order band-pass filter circuits, using different impedance scaling ratios, resulting by different sensitivities. It is shown that the circuit with equal impedance scaling ratios yields the best results. Obtained results are double-checked using PSPICE.
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