Design and Evaluation of Multi-Flux-Quantum Drivers Using Under-Damped Josephson Junctions

The lack of driving ability is one weakness of single-flux-quantum (SFQ) devices. At present, pulse splitters, which generate double or triple flux quanta from an SFQ input, are used when multiple-flux quanta (MFQ) must be obtained from an SFQ input. However, the splitter-based MFQ driver requires a very large circuit area and high latency. In order to overcome these problems, we investigated an MFQ driver using under-damped Josephson junctions. The advantage of the proposed MFQ driver is its current driving ability. The proposed MFQ driver can be used as a bit-line driver in an SFQ memory or as an output driver in a current-recycling system, which requires a large gain. The proposed MFQ driver was implemented using the ISTEC 2.5 kA/cm² Nb standard process (STP2), and its driving ability was investigated. We obtained a histogram of the output pulse number of the MFQ driver as a function of the bias current. In addition, we examined the propagation delay of the MFQ driver through circuit simulations. The results indicate that the MFQ driver can generate MFQ from an SFQ input more rapidly than splitter-based MFQ drivers.