Minimum Noise Figure of the Variable-Capacitance Amplifier

The variable-capacitance diode is one of the most promising nonlinear elements for low-noise parametric amplifiers. In practice, however, these diodes have a small series resistance, and this limits the minimum obtainable noise figure; for the better diodes, the effect of the shunt conductance can be neglected. Taking the contribution of this series resistance into account, this paper discusses the minimum noise figure of parametric amplifiers under various conditions. It is shown that the minimum noise figures are basically determined by a dynamic quality factor of the diode, which will be defined in this paper, under the assumed model of a series resistance as the only parasitic element. Identical minimum noise figures are obtained for lower sideband amplifiers operated with optimum idler frequency, for those with the idler load at 0°K, and for the upper sideband up-converter. In terms of the over-all systems noise figure, however, the lower sideband amplifier is superior to the upper sideband up-converter, for here the gain is limited by the ratio of output to input frequency. Experimental values are given for the figure of merit of various diodes. Universal curves are also given which demonstrate noise behavior of the various systems as a function of the network parameters and component temperatures.