Noise Analysis for Amplifiers with Superconducting Input

A method is developed to adapt the conventional definition of the noise temperature to amplifiers with superconducting inputs. In particular this method is applied to a crossed-film cryotron amplifier with a superconducting input to calculate its noise temperature. We then compare it with that of other amplifiers and masers. In addition to the noise temperature, a new criterion is evolved here: to estimate the figure of merit of amplifiers and detectors. This involves a new parameter called herein the action factor, which is the product of the smallest energy measurable by a device, with its smallest possible response time. The action factor and the noise temperature are worked out for the cryotron and a few other amplifiers and detectors for comparison. The advantage of the cryotron amplifier is brought out.