Improved dc SQUID read-out electronics with low 1/f noise preamplifier

An improved read-out electronics without flux modulation for de superconducting quantum interference devices (de SQUIDs) is presented where the preamplifier is directly coupled to the SQUID. Compared to our previous designs, the 1/f voltage and current noise levels of the preamplifier were reduced by about a factor of 4 (rms) by using bipolar matched transistors instead of operational amplifiers at the input. Three types of flux-locked-loop electronics were developed and successfully tested: a low-power (120 mW) version for large multichannel arrays, a high-speed version with a 15 MHz bandwidth and a very fast (300 ns) reset mode, and a low-noise (0.44 nV/root<(Hz)over bar>) version with a fast and precise bias reversal circuit. The flux-locked-loop electronics are compatible with each other, so that only one type of low-power (180 mW) control unit is required to operate them. All essential specifications of practical SQUID systems (noise, dynamic behavior, and temperature stability) were determined by using, Liquid-helium and liquid-nitrogen cooled SQUIDs, respectively. (C) 1997 American Institute of Physics. [S0034-6748(97)02511-2].