Microstrip superconducting quantum interference device radio-frequency amplifier:: Tuning and cascading

The resonant frequency of an amplifier based on a low-temperature dc superconducting quantum interference device (SQUID) is tuned by varying the capacitance across the end of the input microstrip by means of a varactor diode. The resonant frequency can be reduced substantially, for example, from 190 to 110 MHz, while maintaining an on-resonance gain of 27-28 dB. Two microstrip SQUID amplifiers with the same resonant frequency are cascaded to achieve a gain of 33.5 +/- 1 dB at 386 MHz; with the SQUIDs tuned to different frequencies, the gain is 22 dB or higher from about 260 to 420 MHz. (C) 1999 American Institute of Physics. [S0003-6951(99)02648-0].