A Theory of a Unilateral Parametric Amplifier Using Two Diodes

T h e performance of a single-diode reflection-type parametric amplifier is often limited by the availability of a good circulator, which is essential to a practical amplifier. This becomes a more serious problem when the amplifier is to be refrigerated down to liquid helium temperature, since satisfactory circulator performance is then difficult to obtain. A unilateral parametric amplifier with two diodes, originally proposed by Baldwin, 1 does not require any circulator or isolator in front of the amplifier if the signal source impedance is reasonably well matched. Therefore, this amplifier might avoid the difficult circulator problems. This paper is prepared to show the theoretical characteristics of a unilateral parametric amplifier with two diodes separated by a quarter wavelength at the signal frequency. In Section IT, an exact expression for t h e scattering matrix of a simplified model of the amplifier is obtained. In Sections I I I and IV, expressions for power gain, noise figure a n d bandwidth are calculated. In Sections V and VI, the optimum noise figure and some design considerations of the amplifier are described. 1123