Stacked double-flux-quantum output amplifier

A suitable data link from RSFQ to semiconductor electronics remains challenging. We report an output amplifier that uses a new pulse multiplier circuit. The pulse multiplier consists of several stages arranged in series; a double-flux-quantum gate at each stage promotes the SFQ pulse to the next stage. Characteristics of the circuit design are: 1) DC power. As with the DFQ gate, every unshunted Josephson junction in the amplifier is loaded by critically-damped junctions that prevent voltage-state modes. 2) Equal rise and fall times that scale with output amplitude. In our case, 50 ps rise and fall time and 1-2 mV output amplitude can be realized, which is ideal for a data rate of 10 Gb/s. 3) Quantum accurate voltage multiplication independent of bias current. 4) Non-return-to-zero (NRZ) operation. The circuit was designed and successfully tested in our 8 kA/cm ² foundry process. A 60 GHz pulse train, generated on-chip, was gated with RSFQ logic to produce a data pattern that was then fed into a pulse multiplier of either ten or twenty stages. Voltage multiplication was observed with operating margins of ±21% on bias current. Fast rise time was directly observed; unfortunately, fall time was spoiled by ringing on a 1 ns time scale. It is plausible that small changes to the physical layout of the circuit would produce desired operation.