VALVE NOISE 



If the diode is operated under space-charge limiting conditions the noise 

 generated falls. This may be explained by a 'smoothing out' of anode current 

 by the space charge. Under these conditions the noise voltage is given by the 

 Johnson noise that would be caused by a resistance, equal to the diode 

 incremental resistance /-„, raised to a temperature 0-64 of that of the diode 

 cathode, i.e. 



V„, = 2(0-64 T,,,, kBrjy^ 



and the proportion of this which appears across an anode load R is 



R 



J^ + r, 



2(0-64 T,,,, kBrjy' 



Thus taking R = 100 k^, /•„ = 50 kQ, T,^i„ = 1,000 absolute, B = 10,000 c/s 

 we get 



Va = ,J^,^c.^ X 2 X (0-64 X 103 X 1-37 X 10-23 X 10^ X 5 X 104)V' 

 " 100 + 50 



= 2-8 microvolts 



If now a grid be introduced and the valve become a triode, the generated noise 

 is very slightly raised. It can be shown that the expression for space-charge 

 hmited diode noise voltage just given should be multiplied by a factor 



{1 + (1/^)}V2. 



Partition noise 



With tetrodes and pentodes additional noise is caused by the random fashion 

 in which the cathode current divides between the screen and the anode. 

 This is called the 'partition noise'. The space charge does not extend out to 

 the partition region, so the expression for partition noise is related to the 

 expression for a diode under saturated conditions. It follows from equations 

 derived by Starr that the R.M.S. partition noise voltage appearing at the 

 anode of a pentode or beam valve is 



F. = (2 eB ^'' ' ^" 



p 



h + l 



^sg 



where I^g is the screen grid current and /„ is the anode current, R is the anode 

 load and e and B have the usual meanings. Thus if I^ = 0-8 mA, 4 

 0-2 mA, B = 10 kc/s, R = 100 kQ, the R.M.S. partition noise is 



0-8 X 0-2\i/2 



F^ == I 2 X 1-6 X 10-19 X 104 X 0-8 X 103 X ^^ ^ ^ 1 X 10^ 



= 6-8 microvolts 



Flicker effect 



This is a very slow noise-like phenomenon of peculiar interest to electro- 

 physiologists, whose amplifier pass-bands commonly extend to lower fre- 

 quencies than those of most electronic engineers. Flicker noise, unlike 

 Johnson, shot and partition noise, is not distributed evenly throughout all 



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