Two-stage integrated SQUID amplifier with series array output

A two-stage integrated DC superconducting quantum interference device (SQUID) amplifier has been fabricated which uses a compact series array of 100 DC SQUIDs as the readout device for a low-noise single SQUID. The output noise is dominated by the amplified noise of the input SQUID and substantially exceeds the input noise of a good room-temperature preamp. The input stage is a low-inductance double-loop SQUID with energy sensitivity of approximately 30-h (equivalent flux noise phi /sub n/ approximately=0.3 mu phi /sub 0// square root Hz), with an input transformer having input inductance L/sub in/ approximately=0.25 mu H and net coupling to the SQUID of k/sup 2/ approximately=0.1. The bandwidth extends from DC to about 390 kHz. The series array has an output voltage swing of 3-4 mV, providing a dynamic range of over 50 dB at full bandwidth. The results suggest the general utility of series SQUID arrays as readout devices for SQUIDs. >