Inductive Isolation in Stacked SQUID Amplifiers

Amplification of SFQ pulses to higher signal levels has been achieved using the general technique of inductive isolation. A multiple-flux-quantum generator, which produces quantum-accurate waveforms suitable for ADC applications, and an output data link were demonstrated. Both circuits were implemented using critically damped, dc-powered SQUIDs stacked in series. In the past, the voltage output achievable with this approach has been severely limited by parasitic capacitance to ground and to the control lines. Parasitic capacitance inhibits SQUID operation both at turn-on and turn-off. Using inductive isolation between SQUIDs, parasitic capacitance was completely tuned out. A 100-flux-quantum waveform generator, based on a series stack of 100 SQUIDs, operated at 6 Gb/s, fabricated in a 4.5 kA-per-square-cm Josephson junction process. The output amplifier produced a 1 mV swing at 12 Gb/s non-return-to-zero. This circuit is based on a stack of only two SQUIDs, and uses both inductive isolation and asymmetric resistive shunting to enhance the out-of-phase oscillation mode.