Theory of Multi-Electrode Vacuum Tubes

N T R O D U C T I O N of the three-electrode vacuum tube into the field of communications and in other applications represented such a tremendous advance over the possibilities of any other known device that, despite some of its rather obvious limitations, it proved entirely adequate for the service required until comparatively recent years. However, with increasing demands made by service requirements for larger power output at higher efficiency, reduced distortion, higher * Published in November 1934 issue of Electrical Engineering. Scheduled for presentation at W i n t e r Convention of A. I. E. E . , New Y o r k City, J a n u a r y 22-25, 1935. 44 THEORY OF MULT I-ELECT RODE VACUUM TUBES 45 gain, amplification at higher frequencies and greater frequency discrimination, it eventually became necessary to investigate the possibilities of making changes in vacuum tubes enabling them to meet these requirements more satisfactorily. Measures taken to meet this situation have included improvements in the three-electrode tube that reduce the effects of some of its limitations, and the development of vacuum tubes having more than three electrodes. The purpose of this paper is to present, in simple form, the physical principles underlying the characteristics and performance of multi-electrode vacuum tubes. For present purposes, such tubes may be defined as those having more than the three electrodes of the conventional triode. The procedure will be to show that the definitions of electrical tube parameters applicable to triodes are, with certain modifications in their interpretation, also applicable to tubes having more than three electrodes; and, utilizing the theory of the triode, to analyze the characteristics of a few typical multi-electrode structures that illustrate the types of characteristics found in many such tubes.