The Center-Frequency Properties of Negative-Conductance Amplifiers

Masers, tunnel diodes and parametric devices, in their simplest forms, can each be described at a center frequency as a linear noisy negative-conductance one-port. Various coupling schemes may be used to convert the one-port to a two-port amplifier. This two-port, in turn, is useful as a lownoise preamplifier feeding a high-gain postamplifier or receiver. The properties of the "over-all amplifier" can be calculated from the noise properties of the negative-conductance one-port, the coupling scheme and the properties of the receiver. Four of the most popular coupling schemes are considered in detail and the minimum achievable noise figure for each case, regardless of the receiver, is , where is the noise temperature ratio of the negative-conductance one-port. In each case the conditions for minimum noise figure require also that the over-all amplifier be infinitesimally close to the oscillating condition. This suggests the introduction of a stability factor which approaches zero as the over-all amplifier nears the oscillating condition. Then, when a condition of finite stability (finite positive ) is imposed, clearcut differences between the different couplng schemes appear and the noise properties of the receiver come into the picture. It is shown that coupling which is restricted to reciprocal networks is definitely inferior to circulator coupling. The stability factor can be related to allowable tolerances in the negative conductance and in other circuit variables as, for example, the source conductance. Experimental and theoretical methods of determining are discussed.