. Circuit theory of linear noisy networks. Electronic circuits; Amplifiers (Electronics); Noise. 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 â t'12 - -I'l -r ^ ^1 F F ^^'-^ ^2 The condition that F12 be less than F21 is ^2 i^2-l Gl or {F, - 1) - {F2 - 1)< ^^ - F2- 1 Gl or F,-\ ^F2-l Gi G2 () () That is, amplifier No. 1 should come first if Eq.
. Circuit theory of linear noisy networks. Electronic circuits; Amplifiers (Electronics); Noise. 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 â t'12 - -I'l -r ^ ^1 F F ^^'-^ ^2 The condition that F12 be less than F21 is ^2 i^2-l Gl or {F, - 1) - {F2 - 1)< ^^ - F2- 1 Gl or F,-\ ^F2-l Gi G2 () () That is, amplifier No. 1 should come first if Eq. is satisfied. Equation 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- () '-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. becomes ^n = ^. + f^(â4M=M. + AM(|fi) (). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Haus, Hermann A; Adler, Richard B. [Cambridge] Technology Press of Massachusetts Institute of Technology
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