It is shown that DC-SQUID read-out electronics can be realized utilizing positive feedback without deteriorating the SQUID noise performance. The required gain rise is achieved by interconnecting the SQUID output and a flux modulation coil via a cooled FET acting as a voltage-controlled resistor; different SQUIDs with different types of FETs have been studied experimentally. Possible ways of adaptively controlling the feedback gain produced by the positive feedback are briefly discussed.
Seppä, H., Ahonen, A., Knuuttila, J., Simola, J., & Vilkman, V. (1991). DC-squid electronics based on adaptive positive feedback: Experiments. IEEE Transactions on Magnetics, 27(2), 2488 - 2490. https://doi.org/10.1109/20.133723