Sec. 6.4] ALLOWED RANGES OF NOISE MEASURE 49 



by Mel 



^ _ Fe-l _ yWy 1__ 



'~ ,±~ jHP - TPTt)y {2kTo A/) ^^-^^^ 



Ge 



According to Eq. 6.9, the column vector y is determined, within a constant 

 multiplier, by the source impedance Zs at which the noise measure Me 

 of Eq. 6.25 is achieved. 



In the discussion of the noise performance of ampKfiers, it turns out to 

 be important to bear in mind the algebraic sign that Me assumes under 

 various physical conditions. These are summarized in Table 6.1. 



Table 6.1. Algebraic Signs or Exchangeable Gain 

 AND Derived Quantities 



RS RO Ge Fe-1 \Ge\ Me 



With reference to Table 6.1, it should be pointed out that for Rs > 0, 

 (Fe — 1) > 0. When Rs > 0, conventional available gain greater than 

 1 occurs in only two ways : 



(a) Ge > 1 



(b) Ge<0 



Case a holds whenever the amplifier has an output impedance with 

 positive real part and an available (or exchangeable) gain greater than 1. 

 Case b corresponds to an amplifier with an output impedance having a 

 negative real part. Such an amplifier has an infinite available gain in 

 the conventional sense. In both cases Me is found to be greater than zero. 

 In the succeeding portions of our work, we shall restrict our considera- 

 tion of amplifier performance to cases in which the source has an internal 

 impedance with a positive real part. This is the only case of practical 

 interest. Indeed, any amplifying system, however complicated, is 

 essentially a two-terminal-pair network driven by a signal transducer 

 with a positive real part to its output impedance (for example, antenna, 

 microphone, and so forth). 



6.4. Allowed Ranges of Values of the Noise Measure 



Let us consider a noisy two-terminal-pair network with the noise 

 column matrix 8 and the matrix of general circuit constants T. We 



