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parametric amplifier(pa-ră-met -rik) A type of negative-resistance amplifier that employs a nonlinear circuit element, such as a varactor diode, to act as a time-varying capacitance. These amplifiers have low noise figures at high frequencies and are useful in radio telescopes in the range 1–30 gigahertz.
a radio-electronic device in which the power level of a signal is increased by means of energy from an external source—a pump—that periodically varies the capacitance or inductance of a nonlinear reactive element in the electric circuit of the amplifier. Parametric amplifiers are used mainly in radio astronomy, in long-range space and satellite communication, and in radar low-noise amplifiers of weak signals at the input of radio-receiving equipment, principally in the
superhigh-frequency range. The reactive element commonly used in these amplifiers is a varactor diode. In addition, electron-beam parametric amplifiers are used in the superhigh-frequency range, and ferrite, or ferromagnetic, in the low-frequency (audiofrequency) region.
Two-frequency, or two-circuit, parametric amplifiers are the most common. In the centimeter range these are negative resistance amplifiers that maintain the signal frequency (Figure 1,a), and for the decimeter range these are amplifying-up converters (Figure 1,b). The receiving oscillatory circuit and the oscillatory circuit tuned to the auxiliary, or idler, frequency (which is usually equal to the difference or the sum of the signal and pump frequencies) are generally in the form of cavity resonators containing varactor diodes. The pumps use avalanche transit time diodes, Gunn diodes, varactor frequency multipliers, and, more rarely, reflex klystrons. In most cases, the pump and the idler frequencies are chosen close to the critical frequency fcr of the varactor diode—that is, close to the frequency at which the parametric amplifier no longer amplifies; in addition, the signal frequency must be substantially less than fcr. In order to achieve minimal noise temperatures (10°-20°K or less), parametric amplifiers are cooled to the temperature of liquid nitrogen (77°K), the temperature of liquid helium (4.2°K), or intermediate temperatures (usually 15°-20°K). For amplifiers that are not cooled, the noise temperature is 50°-100°K or more. The maximum obtainable gain and bandwidth of a parametric amplifier depend basically on the parameters of the reactive element. Power gains of 10 to 30 decibels and bandwidths of 10 to 20 percent of the signal carrier frequency have been achieved.
REFERENCESEtkin, V. S., and E. M. Gershenzon. Parametricheskie sistemy SVCh na poluprovodnikovykh diodakh. Moscow, 1964.
Lopukhin, V. M., and A. S. Roshal’. Elektronnoluchevye parametricheskie usiliteli. Moscow, 1968.
SVCh—poluprovodnikovye pribory i ikh primenenie. Moscow, 1972. (Translated from English.)
Kopylova K. F., and N. V. Terpugov. Parametricheskie emkostnye usiliteli nizkikh chastot. Moscow, 1973.
Penfield, P., and R. Rafuse. Varactor Applications Cambridge, Mass., 1962.
V. S. ETKIN