Cryogenic Differential Amplifier for NMR Applications

We have designed and characterized a cryogenic amplifier for use in \(^3\)He NMR spectrometry. The amplifier, with a power consumption of \(\sim 2.5\) mW, works at temperatures down to 4 K. It has a high-impedance input for measuring a signal from NMR resonant circuit, and a 50 \({\Omega }\) differential input which can be used for pick-up compensation and gain calibration. At 4.2 K, the amplifier has a voltage gain of 45, output resistance 146 \({\Omega }\) and a 4.4 MHz bandwidth starting from DC. At 1 MHz, the voltage and current noise amount to 1.3 \(\text{ nV }/\sqrt{\text{ Hz }}\) and 12 \(\text{ fA }/\sqrt{\text{ Hz }}\), respectively, which yields an optimal source impedance of \(\sim 100\) k\({\Omega }\).