As part of Onsala Space Observatory instrumentation activities, a 3.4-4.6 GHz and a 4-8 GHz cryogenic low-noise amplifiers were developed. These amplifiers will be used as cold IF amplifier for mm and sub-mm wave receivers with SIS and HEB mixer (Onsala 7 channel 3mm array and APEX projects). The 2-stage 3.4-4.6 GHz amplifier was fabricated in 7 copies with consistently very similar performance at 12 K ambient temperature as follows: gain 28 dB with 2.8 K noise temperature using Mitsubishi MGFC4419G GaAs transistor, and 2.2 K noise when tested with Chalmers InP transistor at the first stage. The 2-stage 4-8 GHz LNA demonstrates 25 dB gain with noise temperature of 5.0 K with GaAs transistors, 4.0 K with Chalmers InP transistor at the input stage. These performances are in a very good agreement with simulations and are believed to be among the best-reported using GaAs transistors. The amplifiers design was carried using Agilent ADS, HFSS and Momentum CAD software. The amplifier input circuitry was measured separately and optimised for the best noise performance, while special care was taken about accurate modelling of passive components, bond wires and having accurate S parameters at cryogenic temperature for the transistor. In this paper we present details on the amplifier design, performances (modelled and measured) and also gain-stability comparison between GaAs and InP transistor based amplifiers.
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