Design and operation of voltage quadrupler cell for rapid-single-flux-quantum digital-to-analog converters

We propose a voltage quadrupler cell for the voltage multiplier in a RSFQ-DAC. It is composed of an input JTL and an output two-SQUID-stack. By operating the two-SQUID-stack in the second mode, fourfold voltage is obtained. A voltage quadrupler cell is fabricated using the ISTEC standard Nb process. The fourfold operation is confirmed in experiment. A conventional voltage multiplier cell in a rapid-single-flux-quantum (RSFQ) digital-to-analog converter is composed of an output dc-superconducting quantum interference device (dc-SQUID) magnetically coupled to an input Josephson transmission line. The authors previously demonstrated the operation of a voltage doubler cell that doubled the input voltage by introducing the second-mode operation. In this paper, we propose another type of a voltage multiplier cell, a voltage quadrupler, where the output dc-SQUID is replaced with two dc-SQUIDs sharing one coupling inductor. The proposed cell quadruples the input voltage when each SQUID is operated in the second-mode. A single voltage quadrupler has been designed and fabricated using the ISTEC standard process (STP2). Although operation margins for the bias current and the input SFQ repetition frequency are smaller than the values expected from simulation, its fundamental operation has been confirmed in experiment.