Spontaneous emission and the noise figure of maser amplifiers

Abstract

The Nyquist-Johnson formulation of the noise in circuits arising from thermal agitation is applied to circuit elements equivalent to and representing simple resonant absorption, such as occurs in paramagnetic materials or others having r-f line spectra. Such a model for a circuit resistance directly demonstrates the origin of the noise as spontaneous emission by excited particles, in complete analogy to black body radiation in free space. Extension of the conventional theorem to the domain of negative resistances at negative temperatures, as encountered in Maser amplifiers, is valid under circumstances of practical significance. Amplifiers of small noise figure can be made for frequencies ν such that hν kT 0 , where T 0 is the effective temperature of the signal source, is not large compared to unity. For example, an amplifier of high gain and noise figure less than 2 can be built if use is made of a medium describable by a negative effective temperature smaller, in absolute value, than T 0.