Abstract
The paper is concerned with a general analysis of noise in dc-pumped amplifiers based on transverse waves. It is shown that any amplifier involving either the cyclotron waves or the synchronous waves has the same minimum noise temperature determined by the conditions at the cathode. The noise temperature can be reduced to an arbitrarily small value by increasing the magnetic field at the cathode or by reducing the cathode diameter. It is also shown that the minimum noise temperature is reduced by the smoothing effect of the potential minimum near the cathode. The minimum noise temperature can be realized by inserting a suitable noise transformer between the cathode and the amplifier. In general, structures with electric or magnetic fields having rotational symmetry such as used in normal electron guns can perform the required transformation. Some specific examples of noise transformers are discussed.
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