A New Method of Characterizing Amplifier Noise Performance

Although the use of noise figure or noise temperature to characterize amplifier performance is a well-established practice, it is also recognized that this parameter provides only a partial description of the amplifier noise properties. In general, the noise figure (or temperature) depends upon the generator impedance and is thus a function of the signal-source and amplifier combination. Typically, the noise figure is measured by the Y-factor method using hot and cold noise sources that are nominally matched (reflectionless). The result of this measurement is of value as a figure of merit; however, if optimum performance is to be realized, the applications engineer must know whether to adjust the signal source impedance for maximum power transfer, minimum noise figure, or some other criterion, and he must know the deterioration in performance that results if this is not done. It is the purpose of this paper to present an alternative method of characterizing amplifier noise performance in terms of parameters that provide ready answers to these questions. In addition, the measurement of these parameters via a simple extension of the Y-factor method will be described.