Structure generation and performance comparison of elliptic Gm‐C filters

Starting from a set of matrices describing a general Gm-C filter topology, a procedure is developed for generating structures of lowpass filters. As the matrices and the filter topologies have a one-to-one correspondence, an algebraic method is used to identify filter topologies with desired properties, here, transfer functions with finite jω-axis transmission zeros, specifically elliptic filters. Sensitivity expressions for these structures are derived and a performance comparison based on a set of chosen criteria is made. For a specified elliptic transfer function, filters with only grounded capacitors and those containing also floating capacitors emerge as alternative realizations, as are filters with a single input and those with distributed inputs. For third-order functions, a detailed comparison is performed of leapfrog (LF) and inverse follow-the-leader-feedback (IFLF), the most popular special cases, and of topologies that have also floating capacitors (LFf, IFLFf), as well as of a novel configuration that uses also distributed inputs (DIf) and leads to a reduced element count. Design guidelines and restrictions are given, which follow from the derived results with focus on the circuits' sensitivity performance and other properties important for IC implementation. Copyright © 2004 John Wiley & Sons, Ltd.

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