A theory of fluctuation noise

Abstract

From a consideration of the physical phenomena involved in thermionic conduction and in thermal or Johnson noise, it is deduced that the noise in a space-charge-limited valve is best expressed as a thermal noise, and it is shown that a small correction must be applied to the valve slope resistance to give the value of resistance effective as a noise source. The theoretical temperature of this resistance is then shown to be approximately half the cathode temperature. In a temperature-limited valve, both the resistance and the temperature of the conducting path are indeterminate. The “pure shot noise” formulae are applicable to this case, and represent the maximum noise which can result from the passage of a given current through the system. It is incorrect, however, to state that in space-charge-limited conditions “pure shot noise” is smoothed out, while thermal noise in the internal resistance of the valve appears as an additional factor; shot noise and thermal noise in the valve's internal-resistance are essentially the same phenomenon, but are modified by the differing conditions of electron transit.Nyquist's expression for thermal-agitation noise is derived from the atomic mechanism in the case of a metallic conductor.