VACUUM TUBES FOR RADAR LP. AMPLIFIERS 



399 



in the tube itself is due to random fluctuations in the emission of electrons 

 from the cathode. For a parallel plane diode in a circuit such as that shown 

 in Fig. 10(a), the mean square noise current is^ 



il = ir^hAf (18) 



where e is the electronic charge, h is the d-c cathode current, A/ is the 

 incremental bandwidth, and F is a factor which takes into account the 



BY-PASS 



CONDENSER 



BY-PASS 

 CONDENSER 



BY- PASS 

 CONDENSER 



BY- PASS 

 CONDENSER 



BY-PASS 

 CONDENSER 



Fig. 10a — Diode, negligible circuit impedance. 

 Fig. 10b — Pentode, negligible circuit impedance. 

 Fig. 10c — Pentode, impedance in grid circuit. 



"cushioning effect" of space charge. For anode-cathode spacings and 

 operating conditions such that the transit time is not too large a faction of 

 the period of the frequency involved, F is of the order of 0,20 when the 

 zero-field emission is several times larger than h ■ Under temperature- 

 limited conditions F is unity. That is, under favorable space-charge- 

 limited conditions, the fluctuation noise component in h is only about 20% 

 of the value for the same cathode current under temperature-limited condi- 

 tions. Although the introduction of grids between the cathode and anode 



' "Fluctuations in Space-Charge-Limited Currents at Moderately High Frequencies," 

 B. J. Thompson, D. O. North, W. A. Harris, R. C. A. Review, October 1940, Vol. 5, p. 244. 



