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To implement a flux quanta counting (FQC) scheme in the wide range magnetic field measurement using a superconducting quantum interference device (SQUID), we present a SQUID proportional feedback amplifier (PFA) circuit. It is a simplified version of the direct readout flux-locked loop (FLL) electronics without an integrator. It achieved a periodically repeated (with a period of 1Φ0), quasi-linear flux-to-voltage characteristics with an input range extended to 2Φ0. Its working point is reset spontaneously with 1Φ0 of flux jump caused by the over range flux input. The resultant wide range measurement is achieved by the synthesis of the PFA voltage output and the flux jumps recorded using the FQC scheme. We simulated and experimentally verified the working principles of SQUID PFA. The FQC scheme using SQUID PFA was demonstrated with measurements in both shielded and unshielded environment. SQUID PFA is simple and easy-to-operate suited for applications in the Earth's magnetic field. [ABSTRACT FROM AUTHOR]

• We presented a simple two-terminal SQUID amplifier circuit for wide range magnetic field measurement. SQUID merits of wide bandwidth and wide range are fully exploited in this scheme. • This scheme uses only two terminals led out with coaxial cable to achieve both high speed and low heat dissipation. • This SQUID amplifier scheme can be easily fabricated on one chip as a novel SQUID integrated circuit specific for unshielded application systems in Earth's magnetic field. To implement wide range measurement using direct current Superconducting Quantum Interference Device (dc SQUID), we presented a simple two-terminal SQUID amplifier consists of a shunt resistor and a conventional SQUID in series with feedback coil. Only two terminals are led out with coaxial cable for high speed and low heat loss. Periodically repeated quasi linear flux-to-voltage transfer characteristics are achieved by internal flux feedback. Wide range flux input is modulated by the SQUID amplifier into voltage signal with rapid transient edges. The reconstructed output is the synthesis of measured voltage and counts of flux-quanta counting. Working principles of linearized SQUID amplifier were simulated and experimentally verified. Wide range measurement with error within ±0.1Φ 0 was demonstrated in the experiment. [ABSTRACT FROM AUTHOR]