A device that amplifies very low-level signals. A preamplifier is generally placed close to its signal source and has a very low noise figure as it is the principal determinant of electronic noise within the system. Preamplifiers used in NMR systems usually have a 50 ohm input impedance, and require a matching network to interface to the RF coil, although preamplifiers with high input impedance may be used with surface coils. Such devices typically use a field effect transistor (FET) as their input stage.
An arrangement of reactive elements (inductors and capacitors) used to transform an input impedance of a given value to an output impedance of a second value. Such circuits are used in interfacing high impedance RF coils to low impedance (usually 50 ohms) transmission lines that feed RF energy to the coil or send the MR signal to an MR preamplifier.