Multiport Noise Characterization and Differential Amplifiers

I address the issue of the definition and measurement of noise figure and parameters to characterize multiport devices, particularly differential amplifiers. A parameterization in terms of the noise matrix appears to be the most practical. The noise figure for a given output port is defined and related to the noise matrix and scattering parameters of the device. As an example, the case of a differential amplifier with reflectionless terminations is considered in detail. The noise figure, effective input temperature, and gains are related to the results of a series of hot-cold measurements, as in the familiar two-port case.