396 



HEI.I. SYSTEM TECllXICM. JOIRXAL 



in the transistor and contribute (onclalcd noise oulpul to both tlie genera- 

 tors of the circuit rej^rescntation. 



The transistor noise is of two types. One is a rushing sound somewhat 

 similar qualitatively to thermal resistance noise; the other is a frying or 

 rough sound which occurs erratically, usually in the noisier units. The noise 



40 60 80 100 200 400 600 1000 2000 4000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 28 — Transistor noise versus frequency. 



10,000 20,000 



power per unit bandwidth varies almost exactly inversely with frequency 

 as shown in Fig. 2S, being in this respect reminiscent of contact noise. 



Since the noise dependence on frequency is known, its level may be given 

 as noise voltage per unit bandwidth at a reference frequency (1000 cycles). 

 The collector noise usually dominates as far as practical effects on the output 

 are concerned. Representative values are about 100 microvolts per cycle 

 at 1000 cvcles for the collector, and one or two microvolts for the emitter. 



