Ch. 1] INTRODUCTION 5 



where AM = M2 — Mi is the difference between the noise measures of 

 the second and first amplifiers of the cascade. Equation 1.4 shows that as 

 long as Gi and G2 are greater than 1 the noise measure of a cascade of 

 two amplifiers Kes between the noise measures of its component amplifiers. 

 In the particular case when the noise measures of the two amphfiers are 

 equal, the resulting noise measure of the cascade is that of either amplifier, 

 even if the available gains of the individual amplifiers are different. 



Furthermore, since the available gain G = G1G2 is supposed to remain 

 the same for either order of cascading, the result (Eq. 1.2) and the defini- 

 tion (Eq. 1.3) show that the lowest noise measure for a cascaded pair of 

 amphfiers results from placing at the input the amplifier with the lowest 

 individual noise measure. 



Compared with the noise figure alone, which always deteriorates in a 

 cascade (Eqs. 1.1) and which does not suffice to determine which amplifier 

 should come first, the noise measure alone is evidently a more satisfactory 

 and self -consistent single criterion of amplifier noise performance. 

 Moreover, since noise measure and noise figure become essentially the 

 same for amphfiers with sufficiently high gain, the final performance 

 evaluation of a practical multistage amphfier always rests numerically 

 (if not in principle) upon the familiar noise-figure criterion. 



From such reasoning, we evolved a criterion for amplifier noise per- 

 formance. The criterion is based on the plausible premise that, basically, 

 amphfiers are supposed to provide "gain building blocks" without adding 

 excessively to system noise. In its final stage of evolution, the criterion 

 can be described as follows. 



Suppose that n different types of amplifiers are compared. An un- 

 hmited number of amphfiers of each type is assumed to be available. 

 A general lossless (possibly nonreciprocal) interconnection of an arbitrary 

 number of amphfiers of each type is then visuahzed, with terminals so 

 arranged that in each case an over-all two-terminal-pair network is achieved. 

 For each amplifier type, both the lossless interconnecting network and the 

 number of amplifiers are varied in all possible ways to produce two 

 conditions simultaneously: 



1. A very high available gain (approaching infinity) for the over-all 

 two-terminal-pair system when driven from a source having a positive 

 real internal impedance. 



2. An absolute minimum noise figure Fmin for the resulting high- 

 gain system. 



The value of (Fmin — 1) for the resulting high-gain two-terminal-pair net- 

 work is taken specifically as the ^^ measure of quality of the amplifier type 

 in each case. The "besf amplifier type will be the one yielding the smallest 

 value of (Fmin — I) at very high gain. 



