NOISE 



noticing that the nature of the material itself does not enter into the calculation. 

 Notice also that we do not have here any kind of perpetual motion device; 

 the load supplies an equal power back to the source and there is no net 

 transfer of energy. 



If R is the resistance of the noise source and Vj^ is the R.M.S. voltage 

 delivered to the load, we have 



Noise power = —jt- 



Kjr^ = kTBR 

 Vl = {kTBRfl^ 

 and the R.M.S. open circuit noise voltage will be twice this 



Foe = 2 {kTBRfl'' 



A; = 1-37 X 10~^^ joules per degree absolute, so the open circuit R.M.S. noise 

 voltage generated by a 1 megohm wire-wound resistance at 24°C as measured 

 via an amplifier of band- width 0-10 kc/s is 



2 (1-37 X 10-23 X 3 X 102 X 104 >. io«)i/2 



= 2(411 X 10-11)^2 



= 12-8 microvolts 



CARBON RESISTOR NOISE 



The most commonly used resistors in electronics are not composed of a 

 homogeneous material; they are an aggregate of carbon particles bound 

 together, and through molecular agitation these particles continually vary in 

 the intimacy of their contact one with another, as a result of which the 

 overall resistance of the component is itself subject to continual minor 

 fluctuation. Thus a steady current through the resistor causes a fluctuating 

 potential difference to appear across it. Noise caused in this manner is over 

 and above ordinary Johnson noise, and is dealt with again in Chapter 20. 

 For the present it is sufficient to bear in mind that carbon resistor noise 

 is a function of the current the component is carrying. 



VALVE NOISE 



Shot effect 



This is caused by the random manner in which electrons are emitted from the 

 cathode. It is most pronounced in the saturated thermionic diode, so much 

 so that saturated diodes are commonly used in certain pieces of test apparatus 

 whose function it is to generate noise. It can be shown^ that the R.M.S. noise 

 voltage appearing at the anode of such a diode is given by 



V=:^(lIeBfl^xR 



where / is the diode current, e is the charge on an electron = 1-6 X 10~^^ 

 coulombs, B is the band-width in c/s and R is the diode load. Thus if the 

 diode anode current = 1 mA, the band-width is 10 kc/s and the anode load is 

 100 kQ, the R.M.S. noise voltage produced is 



F= (2 X 10-3 X 1-6 X 10-19 X 10^/^ X 105 



= 179 microvolts 



254 



