Development of A dc-SQUID Amplifier with Intra-coil Resistors

We have developed a dc-SQUID amplifiers with intra-coil resistors based on second-order parallel gradiometers. The measured results show that intra-coil resistors can significantly increase the flux-voltage transfer coefficient V Φ as well as smooth out resonances and produce a smooth flux-voltage (V-Φ) characteristic curve. It has been observed through a comparison of various intra-coil damping resistors that the flux-voltage transfer coefficient V Φ as a function of the intra-coil resistors rises as the corresponding electric conductance increase. The measured results of the flux noise at 4.2 K demonstrate explicitly that the flux-voltage transfer coefficient can greatly increase detection sensitivity and successfully reduce the noise contribution from the electronics operating at room temperature.