Conclusions 



The developments that we have undertaken have been rather lengthy. 

 Therefore, it is worth while to reassess and summarize our principal re- 

 sults as well as our omissions. 



As pointed out in the introduction, the original motivation for the 

 present work was the desire to describe in a systematic manner the single- 

 frequency noise performance of two-terminal-pair linear amplifiers. It 

 was necessary at the outset to elect a criterion of noise performance, which 

 we chose to be the signal-to-noise ratio achievable at high gain. This 

 criterion is not clear for systems without gain, nor for multiterminal-pair 

 networks. For multiterminal-pair networks, the noise parameter pT 

 expressed in terms of the general circuit constants has been set down as 

 an extension of the two-terminal-pair noise-measure definition but has 

 not been given any physical interpretation in this work. One reason for 

 this omission is the fact that a general-circuit-constant (or wave-matrix) 

 description of multiterminal-pair systems has been of little use in the past. 

 There have not been any systems incorporating gain whose noise per- 

 formance on a multiterminal-pair basis was of interest. It is true that 

 in the past some special problems involving frequency conversion have 

 called for proper interpretations, and that two-terminal-pair networks 

 processing sidebands may be analyzed theoretically as multiterminal-pair 

 networks. But a sophisticated theoretical approach to noise problems of 

 this nature was never necessary. Problems of this type were easily dis- 

 posed of by inspection. 



Recently, parametric amplifiers (nonlinear-, or time-varying-, reactance 

 amplifiers) have received a great deal of attention because of their low- 



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