C Band Cryogenic Low Noise Amplifier

This article presents a cryogenic low noise ampli?er for a C band receiver ofthe VLBI division, Shanghai Astronomical Observatory. The ampli?ers, which arewith three-stage single-ended architecture, have the room-temperature performance of4650 ～ 5150MHz frequency, ～ 32dB gain and < 0.6dB in-band gain ripple, > 16dBreturn loss, less than 45K noise temperature and unconditional stable; and when theampli?er is cooled down to about 15K ambient temperature, with proper biasing, thefrequency response keeps almost the same while the noise temperature can be reducedto 6 ～ 8K. Three this type of ampli?ers has been assembled as the pre-ampli?ers ofthe C band receiver, and their performances are the same. Based on the theories for linear two-port networks, general problems in the single-ended ampli?ers developing procedure have been discussed, which include the mea-surement techniques to obtain the HEMT's S parameters and noise parameters, thedesign skill for the matching networks and the improvement of the stability etc. TheNoise Master system, which is used to test the noise of the cryogenic LNAs, is intro-duced, further more, an error due to the load temperature uncertainty has been cured.