Abstract :
Upon introduction of the notions of gain and noise figure, it appears that preamplifier stages using velocity-modulation tubes are unsuitable at u.h.f. and s.h.f. under operational conditions considered hitherto. A special arrangement, connected with such a tube and consisting of three electrode pairs spaced along the electron stream, is considered in this paper. The first pair is connected to a resonance cavity or line, constituting a pre-circuit. The second pair is connected to the input, and the third pair to the output circuit. It can be shown that the transfer of initial spontaneous velocity fluctuations to density fluctuations along the electron stream may be neglected. The reverse effect is considered in an appendix, and it also is negligible under practical conditions. It is shown that, by the use of a properly detuned pre-circuit, the noise figure may be reduced from a few thousand to, say, 10 under optimal conditions, retaining gain figures of, say, 100. The usefulness of the arrangement under consideration is discussed and estimated to be favorable under actual conditions of operation. With traveling-wave tubes noise figures below 10 are thought to be attainable by application of the present device.
