HELIUM COOLED RADIO FREQUENCY PREAMPLIFIER FOR USE IN NMR

A low temperature (4.2 K) radio frequency amplifier has been constructed using metal oxide semiconductor field effect transistors (MOSFET). Noise characteristics of the amplifier are compared to a model that includes two noise sources: shot noise and induced gate noise. The model is similar to models used to represent noise in vacuum tubes. Amplifier use for observation of low temperature nuclear magnetic resonance (NMR) signals is considered. Conditions for optimum signal-to-noise (S/N) are expressed in terms of the noise figure and are compared to those for room temperature vacuum tube amplifiers. Improvement of S/N by factors of 3 to 10 are readily obtainable. The S/N improvement with the low temperature MOSFET amplifier is demonstrated by observation of the NMR signal of 205Tl at 4.2 K at a frequency of 20 MHz. © 1968 The American Institute of Physics.