Fluctuation Noise in Vacuum Tubes

Abstract

The fluctuation noises originating in vacuum tubes are treated theoretically 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 different 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 2.18×10⁻¹⁶ 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.