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The technique of the conformal transformation is used in the design of RC bandpass filters. An elliptic-function transformation that transforms the complex-frequency plane into a rectangle is used. In the transformed plane, poles and zeros can be located to produce several useful bandpass characteristics. Key quantities, such as the passband ripple, the equal-ripple bandwidth, and the half-power bandwidth, of these filters can be calculated in terms of a few parameters in closed form. Some design charts are presented. Several design examples and experimental filters are given.