Design and Operation of a Double-Flux-Quantum Amplifier Excluding Flux Bias Lines

In addition to their fast switching with low power dissipation, precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits. This feature is attractive for realizing ac voltage standards consisting of SFQ-based digital-to-analog converters (DACs). Among several key components in SFQ-based DACs, a double-flux-quantum amplifier (DFQA) is a potential candidate to amplify the output voltage. In this paper, we present a redesigned DFQA without flux bias lines. We found a parameter set that provided wide bias margins and zero-flux-bias operation. Improved bias regions were numerically demonstrated, whereas a ±20-fold DFQA excluding flux biases was fabricated using Nb integration technology and tested in a liquid helium bath.