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 hi-impedance input for measuring a signal from NMR resonant circuit, and a 50 $\mathrm{ \Omega}$ 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 $\mathrm{ \Omega}$ and a 4.4 MHz bandwidth starting from DC. At 1 MHz, the voltage and current noise amount to 1.3 $\mbox{ nV}/\sqrt{ \mbox{ Hz}}$ and 12 $\mbox{ fA}/\sqrt{ \mbox{ Hz}}$, respectively, which yields an optimal source impedance of $\sim 100$ k$\mathrm{ \Omega}$.
Categories: physics.ins-det physics.app-ph