Optimum Noise Performance of Linear Amplifiers

A single quantitative measure of amplifier spot noise performance is established. It removes difficulties heretofore associated with the effect of feedback upon spot noise performance. This measure, (Me)opt, is a function of the amplifier noise and circuit parameters alone. It determines the lowest spot noise figure achievable at high gain with a given amplifier, which is used either alone or in a passive interconnection with other amplifiers of the same (Me)opt. Moreover, passive interconnection of amplifiers with different (Me)opt cannot lead to a spot noise figure at high gain better than that obtainable by using only amplifiers of the smallest (Me)opt. (Me)opt is, therefore, a valid measure of the absolute quality of amplifier noise performance. In many important cases the best noise performance attainable with a particular type of amplifier is actually achieved by a simple cascade in which the input of each stage is properly mismatched. The mismatch conditions for each stage do not in general coincide with those normally used to "minimize" its noise figure. In the case of a two-terminal-pair negative resistance amplifier, a limiting form of which is the maser, optimization may always be obtained using an ideal circulator.