A cryogenic SiGe low noise amplifier developed for radio astronomy

A two stages cryogenic Silicon-Germanium heterojunction-bipolar-transistor (SiGe HBT) low-nose-amplifier (LNA) for 0.2-4GHz frequency range has been designed, fabricated and measured with the temperature range from 300K to 4K. Both of the stages of the amplifier utilize the commercially available plastic packaged SiGe transistors NXP BFU725F. The temperature dependent DC characteristics, namely β and gm of the SiGe transistor have been investigated. This amplifier achieves an averaged noise temperature as low as about 10K with a gain over 30 dB for the frequencies in the range of 0.2-4GHz at physical temperature of 4K. To verify the feasibility of the cryogenic SiGe LNA for astronomy observation, we embedded the amplifier into a Superconductor-Insulator-Superconductor (SIS) receiver as the first stage IF amplifier and measured the performance of the receiver. We found that the receiver exhibited low noise without bringing any unexpected effect to the SIS mixer. In addition, the measurement result of receiver stability in term of Allan variance verified stable operation at cryogenic temperature. Above results indicated that the SiGe LNAs based on the commercially available transistors are applicable to radio astronomy observation.