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The nonlinear response of Superconducting Quantum Interface Devices (SQUIDs) has limited their usefulness. We propose the SQUID Operational Amplifier, which consists of several stages of SQUIDs with high open-loop current gain. When connected in a negative-feedback configuration by passing some of the output current through a feedback coil connected to the first stage, the response is linearized. An analog of the semiconductor op-amp, the SQUID op-amp can be used in superconducting equivalents of op-amp circuits such as current amplifiers, current-to-voltage converters, differentiators, and integrators. We present experimental results with a x 10 current amplifier as well as a 750 ohm} current-to-voltage amplifier which can couple directly to a room-temperature amplifier without a transformer or a feedback line.
Irwin, K.
and Huber, M.
(2001),
SQUID Operational Amplifier, IEEE Transactions on Applied Superconductivity, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=6767
(Accessed October 9, 2025)