The noise properties of GaAs FET devices have been measured in the frequency range 0.2–20 MHz at temperatures 300, 77, and 4.2 K. Significant reduction in all contributions to noise parameters was found on cooling the devices. Cooled GaAs FETs were found to be suitable for use in the preamplifier of a pulsed NMR spectrometer. An amplifier noise temperature of 0.9 K was found at 1.16 MHz and the observed signal-to-noise ratio using a 3He gas sample was found to agree with calculation. Precise expressions for signal size are obtained using a spherical receiver coil, and employment of these facilitates the calculations and provides a basis for comparing sensitivities of different NMR spectrometers.
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Richards et al. (1986) studied this question.
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