UHF ultra low noise cryogenic FET preamplifier

A design is presented for a stable liquid helium cooled 430 MHz preamplifier, using a GaAs MESFET, with a 3 dB bandwidth of 35 MHz. Changes in device parameters with temperature are considered. Accurate matching is achieved by the use of GaAs varactor diodes and may be optimised in situ by controlling the bias voltages. The expected improvements in performance from both the increase in transconductance, and the decrease in the level of the Johnson noise sources is observed. By this technique the authors have achieved a noise temperature T N <or approximately=10K with a single stage gain of 19 dB. The output power for 1 dB gain compression is +7 dB m.