Advanced version of two stage dc SQUID-based amplifier

An advanced, version of a two stage de SQUID-based low frequency amplifier and its measured signal and noise parameters are presented in this report. An effective input inductance of about 2.9 mu H, a SQUID's inductance of ab out 11 pH, an effective coupling k(2) of about 0.24 and a white noise level of about 0.87 mu Phi(o)/Hz(1/2) at 4.2 Kelvin were achieved. The gradiomet ric scheme, a modified distributed damping filter and a modified geometry o f coupling transformers have been used to suppress the influences of the ex ternal noise and resonances in the SQUID microstructure.