An active RC filter capable of easy lowpass-highpass transformation

An RC active filter, whose transfer characteristics can easily be changed from a given lowpass to its highpass form only by interchanging one capacitor and one resistor regardless of the order of the filter, is proposed. The transfer function realizable by this method are limited to odd-order Butterworth and a class of elliptic functions which have their poles on a circle in the complex s-plane. A novel all-pass active RC structure, which has the transfer level of 1.0 independent of the quality factor of the transfer function and is quite suitable for the proposed realization, is also given. The experimental results on the fifth-order Butterworth filter show quite good agreement with theory.<>