A HIGH-T-C SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE WITH AN ALTERNATING-CURRENT BIAS DOIT AND ADDITIONAL POSITIVE FEEDBACK

This study shows that the alternating current (AC) bias direct offset integrated technique (DOIT) reduces low-frequency noise without increa sing the level of white noise in a high-T-c DC superconducting quantum interference device (SQUID). Additional positive feedback (APF), whic h can improve the effective flux-to-voltage transfer function, it used to reduce the equivalent flux noise due to the voltage noise of the p reamplifier of the magnetometer. The use of APF combined with the AC b ias DOIT reduced the noise of the magnetometer by factors of 1.5 at 10 0 Hz, 3.5 at 10 Hz, and 5.3 at 1 Hz when compared with noise levels ob tained with the static current bias DOIT.