Sec. 6.1] EXTENDED DEFINITIONS OF GAIN AND NOISE FIGURE 43 



that pT in this representation must be related to the noise performance 

 of the network regarded as a transfer device. The noise figure has been 

 for many years the most widely used parameter describing the noise 

 performance of transfer devices. Consequently, we might well investi- 

 gate first whether or not pr has any relation with the noise figure. 



6.1. Extended Definitions of Gain and Noise Figure 



The noise figure is normally defined in terms of available power. We 

 have seen, however, that the available-power concept leads to diffi- 

 culties in cases involving negative resistances. Since such cases must 

 arise in any general theory of linear amplifiers, the available-power 

 concept should be replaced everywhere by the exchangeable power. 

 Accordingly, the same replacement should be made in the available-gain 

 definition: The exchangeable power Pes of the input source and the 

 exchangeable power Peo at the network output replace the corresponding 

 available powers. Specifically, 



where 



and 



We find that 



(6,1a) 

 (6.16) 



(6.k) 



(6.2a) 



(6.26) 



Observe that when Rs > and Ro > 0, Gg becomes the conventional 

 available gain of the two-terminal-pair network. 



The foregoing ideas lead to an extended definition, Fg, of the noise 

 figure of a two-terminal-pair network^ 



GekTo Af 



where Net is the exchangeable noise power at the network output when 

 the source has a given (noiseless) impedance Zs- Thus, Nei contains 



^ H. A. Haus and R. B. Adler, "An Extension of the Noise Figure Definition," 

 Letter to the Editor, Proc. I.R.E., 45, 690 (1957). 



