4 INTRODUCTION [Ch. 1 



pair, though in view of the answer obtained on that basis, the criterion 

 should have been viewed with a little suspicion. Thus if F12 and F21 

 are the respective noise figures of the cascade when amplifier No. 1 

 and ampHfier No. 2 are placed first, we have 



F. - 1 



(1.1a) 



(1.2) 



That is, amplifier No. 1 should come first if Eq. 1.2 is satisfied. 



Equation 1.2 implies that in a cascaded system of amplifiers, where the 

 earliest stages are obviously the most critical in regard to noise perform- 

 ance, the "best" amplifier is the one having the lowest value not of F 

 but of the quantity 



F - 1 

 M = ^--j- (1.3) 



'-G 



It is with M that we shall be most concerned, and we shall call it the Noise 

 Measure of an amplifier. 



In terms of M, and the fact that the available gain of a cascaded pair 

 of amplifiers is G = G1G2, Eq. 1.1a becomes 



^n = ^. + f^(— 4M=M. + AM(|fi) (1.4) 



