Flux Feedback Amplifier based on Superconducting Quantum Interference Device

Highlights: • With Flux Feedback Amplifier (FFA) model, the SQUID readout circuit is simplified as a low noise differential amplifier with no more than two operational amplifiers. Two FFA schemes were conducted to build very simple FLLs. • The dynamic performances of two simple FFAs were characterized and compared in experimental results. The one-pole FFA scheme is suited for wideband FLL, the two-pole FFA scheme is suited for high slew rate FLL. • The SQUID in-loop characterization and working point reset in FFA is enabled with only one switch. Operations of FLL are simplified for practical applications. -- Abstract: We introduced the Flux Feedback Amplifier (FFA) to design simple and easy-to-use Flux-Locked Loop (FLL) used to operate Superconducting Quantum Interference Device (SQUID) for practical applications. The SQUID FFA is a hybrid operational amplifier (op amp) with SQUID as flux input end and feedback coil as flux output end. The room temperature circuit is simplified as a low noise differential amplifier with no more than two op amps, and operated with only one switch. Two SQUID FFA schemes were demonstrated and the dynamic performances were characterized and compared in the experimental results. The scheme with a monolithic op amp achieves one-pole open loop gain with slope of 20 dB/decade and is suited for the wideband FLL. The scheme with two op amps achieves two-pole open loop gain with slope of 40 dB/decade and is suited for the FLL with high slew rate (up to 1Φ₀/μs).