Fluctuation Noise in Vacuum Tubes

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 effects, 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 v a c u u m tubes of the amplifier. The effect of thermal agitation in circuits outside the v a c u u m t u b e is well understood, b u t 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 v a c u u m tubes and their attached circuits* points o u t a new method for the measurem e n t of tube noise, reports the results of such measurements on four different types of vacuum tubes, and discusses the m i n i m u m noise in different types of vacuum tube circuits. · Published in Physics, September, 1934. 634