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Much of the current interest in negative impedance circuits is directed toward potential applications as components of transmission networks-filters, equalizers, and similar structures. To permit maximum flexibility in the analysis and design of such structures, a complete specification of the characteristics of the negative impedance circuit is required. Four parameters are required for such a specification. The present discussion in terms of the ABCD matrix is particularly useful when a tandem connection of negative impedance circuits and passive networks is to be investigated. The behavior of a practical circuit can be represented as the sum of a matrix stating the ideal behavior and an error matrix. In the cases studied, the elements of the error matrix are inversely proportional to the gain of the amplifier used. Stability of an over-all circuit and sensitivity to component variations are expressed in terms of the elements of the scattering matrix.