Abstract
The optimization of radiofrequency amplifiers involving dc SQUIDs is discussed for both tuned and untuned input circuits. For a given frequency and input coil coupled to the SQUID, expressions are obtained for the optimum source resistance, gain, and noise temperature. The performance of two amplifiers designed according to these predictions is described. The gain of an untuned amplifier operated at 100 MHz and 4.2 K was 16.5±0.5 dB with a noise temperature of 3.8±0.9K; at 1.5 K the gain increased to 19.5±0.5 dB, while the noise temperature decreased to 0.9±0.4 K. A tuned amplifier operated at 93 MHz and 4.2 K had a gain of 18.6±0.5 dB and a noise temperature of 1.7±0.5 K. These results were in good agreement with predicted values.
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