A simple NMR marginal oscillator with a single MOSFET has been improved by incorporating an amplifier with additional MOSFET; the main part (cryogenic box) is immersed in liquid N2 or He and is put between the pole pieces of an electromagnet. The influence of static magnetic field upon the characteristics of the commercial MOSFET element was studied and the element was found to work well at low temperatures under magnetic fields up to 20.1kG. The stationary NMR measurements of Al27 nucleus in ruby crystal were carried out over the temperature range 1.5-300K by using the built-in NMR apparatus; in particular, the temperature dependence of the NMR linewidth was measured in this work.
