SQUID operational amplifier

The nonlinear response of Superconducting Quantum Interference Devices (SQU IDs) has limited their usefulness. We describe the SQUID Operational Amplif ier, 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. rin analog of the semiconductor op-amp, the SQ UID op-amp can be used in superconducting equivalents of op-amp circuits su ch as current amplifiers, current-to-voltage converters, and differentiator s. We present experimental results with a x10 current amplifier as well as a 750 Omega current-to-voltage amplifier which can couple directly to a roo m-temperature amplifier without a transformer or a feedback line.