NOISE IN RESISTANCES 



167 



Now, by applying (25) and (26) we obtain for tlie noise m the plate 

 current 



i| = 2eI,Bil - (1 - r')(V7.)). 



m 



It is to be noted that if T^ = 1, that is, if the cathode current is random, 

 the noise in the plate current is purely shot noise. The screen cannot make 

 the plate current noisier than shot noise since it does not act to produce 

 bunches of electrons. 



The noise m the screen current can be obtained by substituting /« for Ip 



^^ 



1 



ceo 



5| 



UJ, 



HII 



^111 



Fig. 4 — Electrons randomly hitting or missing the screen grid make a tetrode noisier 

 than a triode. 



in {S3). There is a correlation between the screen and plate noise currents; 

 the total noise in the screen current plus the plate current must, of course, be 



is + H = il = T'2eIoB 



(34) 



and not the sum of i% and i1 • 



Partition noise has been discussed by Thompson, North and Harris.* 



V. Space Charge Reduction of Noise 



In this section an approximate derivation of noise in a space charge lim- 

 ited diode will be presented. The derivation leads to an expression valid 

 for many practical tubes and illustrates the nature of the noise in space 

 charge limited flow. 



Consider a parallel plane diode of unit area and spacing x, with an applied 

 voltage Vq, as shown in Fig. 5. When the voltage is applied, the electron 

 convection current in the diode rises to value h - Neglecting thermal 

 velocities of electron emission, this current is such that the electronic 



