DC SQUID small signal analysis

A small signal analysis for the DC SQUID (superconducting quantum interference device) is presented. This analysis involves the linearization of the SQUID equations for a given flux and current bias. Considering the SQUID to be a two-port device, an impedance matrix can be determined for a SQUID coupled to a general input circuit. The two intrinsic noise sources referred to the input are computed. One advantage of this analysis over other models of the DC SQUID is that an analytic expression can be determined for the cross-correlation power spectral density of the noise sources. These results should be useful in determining the signal-to-noise ratio of low-noise SQUIDs for a given application. Other differences and similarities of the small signal analysis to other DC SQUID models are discussed.