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The noise voltages depend mainly on the collector direct voltage as 

 shown in Fig. 29. While they do vary with the other operating parameter 

 at constant collector voltage, such variations rarely exceed 10 db, which 

 is much less than the variations with collector voltage. 



More important than the actual level of the noise is its relation to thermal 

 resistance noise, which is the ultimate limit to amplification. This relation- 

 ship is conveniently expressed by means of the noise figure, or number of 

 times noisier than amplified thermal noise in the output of the amplifier. 



Fig. 29 — Transistor noise versus operating point. 



A representative noise ligure for the Type A transistor at 100() cycles is 

 60 db, with individual units ranging from 50 to 70 db. 



Noise figure formulas for the three single-stage connections are given 

 in Fig. 30. The noise performance of the three connections would usually 

 not be ver} different if it were not for stability considerations, which may 

 render unusable the generator impedance which would give optimum per- 

 formance. Mainly, on account of stability, the grounded base connection 

 may be said to give the best noise performance, with the grounded emitter 

 running a close second. 



The noise figure of any device depends upon the generator impedance 

 out of which it works but does not depend upon the load. Accordingly, 

 there exists an optimum generator impedance which gives the best noise 



