Introduction 



The principal example of a linear noisy network, and the one of greatest 

 practical importance in electrical engineering, is a linear noisy amplifier. 

 The noise performance of such amplifiers involves many questions of 

 interest. One very significant question is the extent to which the amplifier 

 influences signal-to-noise ratio over a narrow band (essentially at one 

 frequency) in the system of which it is a part. We shall address ourselves 

 exclusively to this feature, without intending to suggest that other fea- 

 tures of the much larger noise-and-inf ormation problem are less important. 

 The term "spot-noise performance" or merely "noise performance" will 

 be used to refer to the effect of the amphfier upon the single-frequency 

 signal-to-noise ratio. It is essential to emphasize right at the beginning 

 the very restricted meaning these terms will have in our discussions. 



We undertook the study reported here in the hope of formulating a 

 rational approach to the characterization of amplifier spot-noise per- 

 formance, and to its optimization by external circuit operations upon the 

 terminals. Fortunately, a characterization has resulted which is based 

 on a single hypothesis about the essential function of an amplifier and 

 which turns out to avoid pitfalls previously associated with the effect of 

 feedback upon noise performance. In developing the aforementioned 

 noise characterization of amplifiers and in pursuing the relevant optimi- 

 zation problem, we encountered a number of illuminating features relating 

 power and noise in linear multi-terminal-pair networks. Indeed, it 

 eventually became clear that the major issues could be presented most 

 simply by postponing until last the questions we had originally asked 

 first. The result is a work of broader scope than was originally envisaged, 



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