Small Signal Analysis of SQUID Direct Readout Schemes

TL;DR

This paper provides a comprehensive small-signal circuit analysis of SQUID direct readout schemes, revealing how feedback configurations influence transfer characteristics and noise performance, with experimental validation.

Contribution

It introduces a complete circuit model for SQUID readout schemes, analyzing the effects of internal feedback on transfer and noise, which was not previously detailed.

Findings

Amplifier noise suppression depends on flux-to-voltage transfer coefficient.

Internal feedback schemes improve transfer coefficient and regulate dynamic resistance.

Experimental results validate the circuit analysis and theoretical predictions.

Abstract

To better understand the working principles of Superconducting Quantum Interference Device (SQUID) direct readout schemes, which work in different bias and amplifier modes with different internal feedback schemes, we present the complete circuit analyses on SQUID small-signal model. SQUID bias and amplifier circuits are analyzed using SQUID Thevenin equivalent circuit, and the general equivalent circuit of SQUID with different internal feedback schemes is derived and analyzed with a trans-impedance amplifier model. Transfer characteristics and noise performances of different direct readout schemes are analyzed and experimentally characterized. It is shown that amplifier noise suppression is only dependent on SQUID flux-to-voltage transfer coefficient and is irrelevant to the configuration of bias and amplifier; SQUID with an internal feedback scheme improves the transfer coefficient with voltage feedback and regulates the dynamic resistance with current feedback.