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A comprehensive review of the electrical performance of passive transformers fabricated in silicon IC technology is presented. Two types of transformer construction are considered in detail, and the characteristics of two-port (1:1 and 1:n turns ratio) and multiport transformers (i.e., baluns) are presented from both computer simulation and experimental measurements. The effects of parasitics and imperfect coupling between transformer windings are outlined from the circuit point of view. Resonant tuning is shown to reduce the losses between input and output at the expense of operating bandwidth. A procedure for estimating the size of a monolithic transformer to meet a given specification is outlined, and circuit examples are used to illustrate the applications of the monolithic transformer in RF ICs.