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 ∼ 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  Ω 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  Ω and a 4.4 MHz bandwidth starting from DC. At 1 MHz, the voltage and current noise amount to 1.3  nV / Hz and 12  fA / Hz , respectively, which yields an optimal source impedance of ∼ 100  k Ω .