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A broadband low-noise four-stage high-electron-mobility transistor amplifier was designed and characterized in a cryogen-free dilution refrigerator at the 3.8 K temperature stage. The obtained power dissipation of the amplifier is below 20 mW. In the frequency range from 6 to 12 GHz its gain exceeds 30 dB. The equivalent noise temperature of the amplifier is below 6 K for the presented frequency range. The amplifier is applicable for any type of cryogenic microwave measurements. As an example we demonstrate here the characterization of the superconducting X-mon qubit coupled to an on-chip coplanar waveguide resonator.

The problem of broadband matching of active elements in terms of noise figure is inevitably encountered in the design of broadband low-noise microwave amplifiers. Despite the fact that this problem differs from the classical problem of broadband matching of signal source and load, it can be reduced to a form suitable for applying methods for solving the classical problem. For this purpose, in this work, the own parameters of a reactive two-port network are derived that match active elements in terms of noise figure in the entire frequency band, where the data for calculating this coefficient are determined. The own parameters of such a two-port network, on the one hand, make it possible to construct methods for the synthesis of input matching circuits of low-noise amplifiers and other devices where low noise matching is required. On the other hand, the own parameters allow one to construct estimates of the maximum achievable bandwidth for a matching circuit of a given complexity.

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