DC SQUID series array amplifiers with 120 MHz bandwidth

We report on the performance of de SQUID (Superconducting Quantum Interfere nce Device) series array amplifiers from de to 500 MHz. The arrays consist of up to 100 de SQUIDs, with varying degrees of intracoil damping; the flux -focusing washer of each SQUID is electrically isolated from the SQUID loop (L-sq = 18 pH). Using an rf network analyzer, we have observed high-freque ncy resonances in the response at bias points corresponding to distortions in the de transfer functions. Increasing distance between SQUIDs in the arr ay reduces the distortions. Distortions are also more pronounced, and bandw idth reduced, in devices incorporating the flux-focusing washer into the SQ UID body. With intracoil damping of 0.25 Omega per turn on the input coil, the voltage-flux transfer characteristics of the isolated-washer design and 300 pm center-to-center SQUID spacing are free of significant distortions, and the bandwidth is not degraded compared to undamped devices. The 100-SQ UID array has 150 nH input inductance, 500 V/A transimpedance, 2.5 pA/ Hz e quivalent input current noise at 4 K, and 120 MHz bandwidth.