Bipolar Transistor DC LNA Characterization at 77 K

In the paper DC low-noise amplifier based on commercial silicon SSM2212 transistors and operating at 77 K are described. The amplifier circuit is a differential stage with one of the arms grounded. Its characteristics, depending on the amplifier operating current, are measured. The schemes for measuring the main parameters of the amplifier are described and the following characteristics are obtained: transfer function, input voltage and current noise densities (less than 0.4 nV/Hz1/2 and about 5 pA/Hz1/2 at 10 kHz, respectively). For the voltage noise, the flicker noise threshold frequency is increased to 20 Hz at 77 K. Based on the results obtained, the amplifier input resistance and some parameters of the bipolar transistor hybrid-π noise model are estimated. In addition, the optimal resistance of the signal source is estimated to obtain the minimum noise figure. The obtained results have shown the suitability of commercially available bipolar transistors for cryogenic applications and could be used to optimize a custom readout system using a cryogenic amplifier based on this type of bipolar transistor.