3.2. Parametric Amplifiers

Abstract

This chapter provides an overview of parametric amplifiers, which are being used in most sensitive continuum-receivers and are gaining importance as preamplifiers in spectral-line receivers because of their broad bandwidth and low noise temperature. Most parametric amplifiers or converters contain at least three frequency-selective circuits, which are coupled to the nonlinear reactance: the signal, the pump, and the idler circuits. In most practical parametric amplifier and converter applications, three frequency-selective circuits with midband frequencies are employed. The noise of modern parametric amplifiers for the microwave region originates primarily from the loss resistance of the varactor and is, therefore, of thermal nature. Before designing a parametric amplifier it must be cleared whether the system under consideration requires a very large bandwidth, extremely low noise, a high degree of stability, or a very good linearity. In contrast to communications and radar applications, radio-astronomy receivers usually require a high degree of stability and are rarely affected by nonlinearities because of the low signal-power levels involved.