2 INTRODUCTION [Ch. 1 



and one for which the title "Circuit Theory of Linear Noisy Networks" 

 seems appropriate. 



Since the introduction by Friis^ and Franz^ of the concept of spot- 

 noise figure F for the description of ampUfier noise performance, this 

 figure has played an essential role in communication practice. The 

 noise figure is, however, merely a man-made definition, rather than a 

 quantity deduced from clearly defined postulates or laws of nature. The 

 possible consequences of this fact were never questioned deeply, although 

 it has always been known that the (spot-) noise figure F does not con- 

 stitute a single absolute measure of amplifier noise performance. 



In particular, the noise figure is a function of the impedance of the 

 source connected to the amplifier input. Thus in giving an adequate 

 conventional description of amplifier noise performance, the source 

 impedance, as well as the noise figure, must be specified. 



Usually, when regarded as a function of source impedance alone, the 

 noise figure has a minimum value for some particular choice of this 

 impedance. If with this source impedance the gain of a given amplifier 

 remains sufficiently high, its noise figure will prescribe the noise figure 

 of any amphfier cascade in which it is used as the first stage. In this way, 

 it is possible to build an amplifier cascade with any desired high gain, 

 and with a noise figure set by the minimum (with respect to source 

 impedance) of the noise figure of the original amplifier. 



If a cascade is to be composed of several individual amplifiers, each of 

 which alone has a "high enough" gain when driven from the source 

 impedance that yields its minimum noise figure, the previous argument 

 shows that the amplifier with the lowest minimum noise figure should be 

 used as the first stage. Any other choice would result in a higher over-aU 

 noise figure for the cascade. 



The foregoing discussion seems to suggest that the minimum value 

 (with respect to source impedance) of the noise figure of an amplifier may 

 be used as an absolute measure of its noise performance and as a basis for 

 comparison with other amplifiers. The validity of the argument, how- 

 ever, is based upon the two previously mentioned restrictions : 



1. Each stage has ^^high enough"" gain when driven from the "optimum" 

 source that yields the minimum noise figure. 



2. Only the source impedance of each stage is varied in controUing the 

 noise performance. 



The inadequacy of this viewpoint becomes clear when stage variables 

 other than source impedance and stage interconnections other than the 



1 H. T. Friis, "Noise Figure of Radio Receivers," Proc. I.R.E., 32, 419 (1944). 

 ^ K. Franz, "Messung der Empfangerempfindlichkeit bei kurzen elektrischen 

 WeUen," Z. Elektr. Elektroak., 59, 105 (1942). 



