Radio-frequency amplifier with tenth-kelvin noise temperature based on a microstrip direct current superconducting quantum interference device

A dc superconducting quantum interference device (SQUID) with a resonant microstrip input and a cooled heterostructure field-effect transistor as a postamplifier is used as a radio-frequency amplifier in the frequency range 90-500 MHz. At liquid He-3 temperatures, gains of 24 and 20 dB and intrinsic noise temperatures of 0.06 +/- 0.02 and 0.12 +/- 0.10 K were achieved at 89.6 and 438 MHz, respectively. The system noise temperature at 438 MHz was also estimated from the Nyquist noise produced by a resonant circuit coupled to the input of the microstrip SQUID. (C) 1999 American Institute of Physics. [S0003-6951(99)02731-X].