IMPROVED DC SQUID READ-OUT ELECTRONICS WITH LOW 1 F NOISE PREAMPLIFIER/

An improved read-out electronics without flux modulation for de superc onducting quantum interference devices (de SQUIDs) is presented where the preamplifier is directly coupled to the SQUID. Compared to our pre vious designs, the 1/f voltage and current noise levels of the preampl ifier were reduced by about a factor of 4 (rms) by using bipolar match ed transistors instead of operational amplifiers at the input. Three t ypes of flux-locked-loop electronics were developed and successfully t ested: a low-power (120 mW) version for large multichannel arrays, a h igh-speed version with a 15 MHz bandwidth and a very fast (300 ns) res et mode, and a low-noise (0.44 nV/root<(Hz)over bar>) version with a f ast and precise bias reversal circuit. The flux-locked-loop electronic s are compatible with each other, so that only one type of low-power ( 180 mW) control unit is required to operate them. All essential specif ications of practical SQUID systems (noise, dynamic behavior, and temp erature stability) were determined by using, Liquid-helium and liquid- nitrogen cooled SQUIDs, respectively. (C) 1997 American Institute of P hysics. [S0034-6748(97)02511-2].