Fluctuation voltage in diodes and in multi-electrode valves

Abstract

This paper is concerned with the spontaneous fluctuation voltages generated in the anode circuit of thermionic valves. The general behaviour of commercial valves is examined and is shown experimentally to be consistent with the existence of shot and flicker effects: an expression, equation (7), is put forward that is found to be satisfactory experimentally, provided a correction factor A is introduced. The general form of this correction factor is examined; when the operating conditions are such that flicker effect is unimportant, A is found to be independent of the type of valve used, and of the nature of the cathode surface. Under such circumstances the value of A is unity when the current is very small, and is again unity when temperature limitation is reached; it has a minimum value of about 0.1 between these limits. This minimum is independent of frequency with thoriated tungsten filaments: with oxide-coated filaments the minimum increases, and occurs at a lower current value, as the frequency of operation is decreased; this is due to flicker effect. With oxide cathodes, except at very high frequencies, A exceeds unity as temperature limitation is approached; with thoriated tungsten, however, this does not occur. At low currents the value of A is always unity, and provided the operating conditions are chosen such that A decreases as I increases, flicker effect can be ignored. The optimum operating conditions as regards signal/noise ratio usually lie in this region, and indirectly- heated oxide-coated cathodes are therefore found to be the most satisfactory on account of the high mutual conductance that can be obtained.Thus in general use the shot effect is the important limit to amplification set by the anode-circuit fluctuations. Thermal agitation in the anode stream is shown to be non-existent or negligible; interpretation of the experimental results on such a hypothesis is not self-consistent, nor is it quantitatively accurate. Fluctuation voltages due to collision ionization are not apparent, and the whole fluctuation observed experimentally can be reconciled with the existence of shot and flicker effects alone.Multi-electrode valves are also considered, and it is shown that the value of A relevant to the current leaving the cathode is sensibly independent of the electrode arrangement, and of the subsequent distribution of current between the electrodes. When the cathode stream is shared between several electrodes the current arriving at any one electrode does not necessarily yield the same value of A as that arriving at other electrodes, but in general the values are of the same order: this is explained by the existence of a fluctuating current travelling round the circuit connecting any pair of current-sharing electrodes. When such sharing occurs it is important to make the current to the actual anode a large fraction of the total current in order to preserve a high value of g2/I, and give a good signal/noise ratio. The connection of similar valves in parallel improves the signal/noise ratio.