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In an usual RF-SET system, the SET is the first-stage amplifier and a
high electron mobility transistor (HEMT) works as a second stage amplifier.
The noise temperature of the HEMT amplifier should be in the subkelvin
range in order to reach the physical limit of the RF-SET, set by the
shot-noise of the SET.
We have designed a cryogenic, 700 MHz HEMT amplifier, based on Agilent
ATF35143. We used cryogenic measurement data on S-parameters in conjunction
with the Pospiezalski noise model to build an amplifier that shows a noise
temperature of ~ 3 K.
Technologically, noise temperatures of cryogenic HEMT amplifiers seem to
be limited to a few Kelvins. In order to decrease the noise temperature of
the GHz amplifiers by an order of magnitude, DC-SQUID based amplifiers
have
been constructed. However, there are open issues related to their input
match and stability to be solved before they can be used as general-purpose
amplifiers in the GHz range.
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Photograph
of the amplifier without cover. The coaxial rf-connections are made with SMA-connectors.
The scale units at the bottom are centimeters.
Noise temperature and gain of our LNA.
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