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. Two examples are considered in detail: a 3-port differential amplifier and a 4-port mixed-mode amplifier, both with reflectionless terminations. The noise figures, effective input temperatures, and gains are related to the results of a series of hot-cold measurements, as in the familiar two-port case.
IEEE Transactions on Microwave Theory and Techniques
Noise Characterization of Multiport Amplifiers, IEEE Transactions on Microwave Theory and Techniques, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=5219
(Accessed May 30, 2023)