Superconducting quantum interference devices with different damped junctions operated in directly coupled current- and voltage-bias modes

We investigate niobium thin film superconducting quantum interference devices (SQUIDs) with different Steward-McCumber parameters β_c operated in both current- and voltage-bias modes. We experimentally prove that there is no difference between the two bias modes with respect to the SQUID intrinsic noise and the noise contribution from the preamplifier. Furthermore, the relationships of the SQUID dynamic parameters, (R_d)_{current bias} ≈ (R_d)_{voltage bias} and (∂V/∂Φ)_{current bias} ≈ (∂i/∂Φ)R_{dvoltage bias}, are always satisfied. For a strongly damped SQUID with β_c ≈ 0.25, additional positive feedback (APF) and noise cancellation (NC) were employed to enhance ∂V/∂Φ, the former showing a degradation in the linear flux range but otherwise the same with NC. For a weakly damped SQUID with β_c ≈ 3, it is directly connected to the preamplifier without APF or NC, and a low SQUID system noise of about 4 μΦ ₀/√Hz is measured, which is close to its intrinsic noise.