A theoretical study has been made of a resistively shunted dc SQUID coupled to an input coil. The effect of a circulating current is taken into account. An analytical expression for the circulating current is obtained by replacing the nonlinear Josephson current with a linear ac current generator. It is shown that in the conventional SQUID, the circulating current changes considerably the input impedance seen from the input coil, and that the input impedance becomes nonlinear. As a result, previous optimization conditions of amplifiers and magnetometers, which were obtained by neglecting the circulating current, lead to errors amounting to 40%. In the case of a resistively shunted SQUID, where a damping resistance is attached in parallel with an inductance of the SQUID, it is shown that the circulating current can be suppressed significantly with the damping resistance. Therefore, the change of the input impedance caused by the circulating current can be eliminated in this case.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)