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Japanese Journal of Applied Physics

Superconducting Quantum Interference Device Voltage Swing Related to Additional Positive Feedback Parameters

Youichi Takada, Gen Uehara, Naoki Matsuda Kado

Published Nov 1, 1994• 3 citations
Publisher

Abstract

The relationship between superconducting quantum interference device (SQUID) voltage swing and additional positive feedback (APF) parameters has been investigated. Voltage swing is depressed as APF gain increases. In a symmetric-bias-injection SQUID, the voltage depression can be explained by the voltage divider action of APF resistance and the SQUID dynamic resistance. On the other hand, the voltage depression in an asymmetric-bias-injection SQUID is expressed as the effect of the voltage divider action multiplied by the effect of thermal noise of APF resistance. These two effects should be considered when designing SQUIDs with an APF.

DOI
10.1143/jjap.33.l1595

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We investigated the flux noise characteristics of a superconducting quantum interference device (SQUID) with additional positive feedback (APF) by comparing calculated results with experimental results. Equations for the minimum flux noise of a SQUID with APF and the optimum condition of APF gain (βa) are deduced. We examined the relationship of flux noise versus βaand experimentally confirmed that it has a minimum point. The deduced equations agree well with experimental results. From the viewpoint of total flux noise of SQUID, it is useful to employ the APF technique for a low-intrinsic-noise SQUID, but not for a high-intrinsic-noise SQUID.
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A simple approach to understanding and analyzing direct readout schemes for superconducting quantum interference devices (SQUIDs) is presented. It is shown that the existing methods for suppression of room temperature amplifier noise are based on voltage feedback and current feedback in the SQUID, that were introduced in the original schemes of additional positive feedback and bias current feedback. It is also shown that the way the SQUID is biased (at constant current or voltage) does not affect the noise suppression.