Exact noise figure of a noisy two-port with feedback

Exact expressions for the noise figure of a two-port with shunt or series feedback are presented in terms of standard four-noise parameters and imittance parameters. The general case of an arbitrary feedback network, characterised in terms of a two-port imittance matrix, is analysed and the results are applied to the more practical case of a single lossless feedback element. The results are presented in the form of the standard two-port noise figure equation, with additional terms which illustrate clearly the effect of feedback. The equations presented make use of directly measurable device parameters, and are therefore design-oriented.