Fluctuation Noise in Vacuum Tubes * 



By G. L. PEARSON 



The fluctuation noises originating in vacuum tubes are treated theoret- 

 ically under the following headings: (1) thermal agitation in the internal 

 plate resistance of the tube, (2) shot effect and flicker effect from space current 

 in the presence of space charge, (3) shot effect from electrons produced by 

 collision ionization and secondary emission, and (4) space charge fluctuations 

 due to positive ions. It is shown that thermal agitation in the plate 

 circuit is the most important factor and should fix the noise level in low 

 noise vacuum tubes; shot noise and flicker noise are very small in tubes 

 where complete temperature saturation is approached; shot noise from 

 secondary electrons is negligible under ordinary conditions; and noise from 

 space charge fluctuation due to positive ions is usually responsible for the 

 difference between thermal noise in the plate circuit and total tube noise. 



A method is deduced for the accurate rating of the noise level of tubes in 

 terms of the input resistance which produces the equivalent thermal noise. 

 Quantitative noise measurements by this method are reported on four difi'erent 

 types of vacuum tubes which are suitable for use in the initial stage of high 

 gain amplifiers. Under proper operating conditions the noise of these 

 tubes approaches that of thermal agitation in their plate circuits at the 

 higher frequencies and is 0.54 to 2.18 X 10"'^ mean square volts per cycle 

 band width in the frequency range from 200 to 15,000 cycles per second. 

 Below 200 cycles per second the noise is somewhat larger. 



The minimum noise in different types of vacuum tube circuits is discussed. 

 These include input circuits for high gain amplifiers, ionization chamber 

 and linear amplifier for detecting corpuscular or electromagnetic radiation, 

 and photoelectric cell and linear amplifier for measuring light signals. 



With the aid of these results it is possible to design circuits having the 

 maximum signal-to-noise ratio obtainable with the best vacuum tubes now 

 available. 



Introduction 



TT is well known that the noise inherent in the first stage of a high 

 -^ gain amplifier is a barrier to the amplification of indefinitely small 

 signals. Even when fluctuations in battery voltages, induction, 

 microphonic efifects, poor insulation, and other obvious causes are 

 entirely eliminated, there are two sources of noise which remain, 

 namely, thermal agitation of electricity in the circuits and voltage 

 fluctuations arising from conditions within the vacuum tubes of the 

 amplifier. The effect of thermal agitation in circuits outside the 

 vacuum tube is well understood, but in the case of tube noise there is 

 considerable confusion. In order to clarify the whole subject, the 

 present paper analyzes the various sources of noise in vacuum tubes 

 and their attached circuits, points out a new method for the measure- 

 ment of tube noise, reports the results of such measurements on four 

 different types of vacuum tubes, and discusses the minimum noise in 

 different types of vacuum tube circuits. 



* Published in Physics, September, 1934. 



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