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Abstract: SQUID bootstrap circuit (SBC) is a SQUID direct readout operating in the voltage bias mode. It consists of two branches connected in parallel: SQUID-inductance La branch and inductance Lb-resistance Rb branch. Its bias voltage is connected to the input terminal of preamplifier. In the SBC the voltage noise from the preamplifier creates the current noise which can be modified by the parameters. Based on the equivalent circuit for SBC and its network equations we studied its characteristics and the effect of parameters on current noise due to biased voltage noise by numerical simulation. Results suggest that SBC current noise can be effectively reduced by modifying parameters, in optimal condition the current noise can be totally suppressed. The effect of resistance Rb on the excess noise is discussed. [Copyright &y& Elsevier]

The SQUID Bootstrap Circuit (SBC) for direct-coupled readout of SQUID signals in voltage bias mode was recently demonstrated. In addition to the conventional dc SQUID, the SBC incorporates a shunt resistor Rs, and two coils coupled to the SQUID via mutual inductances M1 and M2. In this paper, basic equations of SBC are formulated based on its equivalent circuit model. The expression of equivalent flux noise from the preamplifier is also given. The effect of the three adjustable parameters (M1, M2 and Rs) on the characteristics of SBC and the preamplifier noise suppression are numerically simulated. The SBC combines current and voltage feedbacks in one circuit, allowing for an effective suppression of the preamplifier voltage noise through increased flux-current transfer coefficient and dynamic resistance. In contrast to other direct-coupled schemes, it offers not only a good noise performance, but also tolerance to a wide range of adjustable parameters. [ABSTRACT FROM AUTHOR]

• The superconducting quantum interference working principle of DC SQUID is intuitively interpreted with an interferometric equivalent circuit and the concise dynamic system model. • The interference phenomena inside DC SQUID are simulated and visualized with distributions of the AC current and voltage inside the SQUID to improve the understanding of interferometer mechanism of DC SQUID for both SQUID design and application. Flux modulated static current-voltage characteristics are the basis of DC SQUID being the flux-to-voltage convertor utilized in high-performance magnetic field measurement systems. Those static behaviors are the results of the internal quantum interference between two Josephson junctions. However, the interferometric working principle and the interference phenomena are difficult to be intuitively visualized, either through simulation with conventional models, or through the in-situ measurement inside DC SQUID. Therefore, we redraw the equivalent circuit of DC SQUID in form of the microwave interferometer, and transform it into a concise dynamic system model to study the interferometric mechanism. The double-slit like interference phenomena inside the DC SQUID are visualized through simulations of the AC oscillation current and voltage along the SQUID washer. The concept that DC SQUID is a microwave integrated circuit working as the microwave interferometer rather than a static nonlinear transducer is demonstrated and emphasized. Those concepts help users and designers improve their understandings of quantum interference mechanism of DC SQUID for both circuit design and the practical application system development. [ABSTRACT FROM AUTHOR]