Dynamic characteristics of S-band DC SQUID amplifier

A low-noise RF amplifier based on a dc SQUID (SQA) has been tested in the frequency range 3.0-4.6 GHz in the openloop configuration. The following parameters have been measured for the single-stage balanced type SQA at 4.0 GHz: gain (12+1) dB, 3 dB bandwidth of 500 MHz and noise temperature (1.0+0.25) K. For the nonbalanced type SQA at 4.0 GHz gain was (15+1) dB, 3 dB bandwidth 200 MHz and noise temperature (0.5+0.25) K. The improved performance is obtained due to the increased characteristic voltage (~420 (mu)V) of the small-area (down to 0.7-0.9 (mu)m/sup 2/) highquality Nb-AlO/sub x/-Nb SIS junctions. The saturation power (normalized to 1 GHz) referred to the input at 1 dB gain compression is estimated as ~55 K*GHz at a bias voltage of 60 (mu)V. The reasons for saturation of the SQA are discussed.