A low-noise NMR preamplifier with transformer feedback

摘要

A low noise preamplifier for NMR receivers comprises a common-source FET 3 with feedback from the drain circuit through a transformer 6. The transformer enables matching of the amplifier's input impedance to the impedance of the signal source connected to terminal 1. A second transformer or a series input capacitor may be added to enable the optimum impedance to be presented at the FET gate for lowest noise figure while also achieving input impedance matching (figures 2 and 3). The input impedance may be 50 or 75 ohms. HEMTs and GaAsFETs may be used. Alternatively, a bipolar transistor or a thermionic valve may be used. The preamplifier may be of narrowband or broadband type.