The sensitivity is a main problem for NMR experiments and it is laid down by the thermal noise of the receiver coil and its geometry. To improve the sensitivity of an NMR receiver coil, considering the postulate of Friis, the signal must be amplified as close as possible to the coil. Here we present a method to achieve optimum in situ low noise amplification for the sensitivity improvement of an NMR receiver coil or microcoil. In this paper we propose a realistic transducer model of an NMR coil and a reliable method to model an LNA. We introduce the requirements to achieve a low noise matching network for NMR receivers allowing an improvement of the sensitivity of the coil by a factor of 6. This sensitivity improvement has its interest in MRI by increasing the field of view with a higher SNR but also in MRS by reducing the limit of detection
