Cryogenic amplifier for ∼1 MHz with a high input impedance using a commercial pseudomorphic high electron mobility transistor
ABSTRACT A cryogenic amplifier for ∼100 kHz to a few MHz is presented which uses a commercially available pseudomorphic GaAs / InGaAs high electron mobility transistor and dissipates less than 0.5 mW in the cryogenic stage. The input-referred voltage noise and current noise of the amplifier at approximately 2 MHz are measured to be approximately 0.7 nV Hz -1/2 and 25 fA Hz -1/2 , respectively. A superconducting resonant circuit can be used to provide a high input impedance over a narrow bandwidth, or a low input impedance can be used to yield a large bandwidth. Although suitable for many applications, the amplifier has been developed for measurements of current noise from a high-impedance source, and its long-term stability is such that by averaging over 30 min , it could be used to detect approximately 0.6 fA Hz -1/2 .