Current recycling: New results

Long ago, current recycling or serial biasing technique was recognized as a promising solution for the reduction of bias current in VLSI dc biased digital superconductor circuits. Despite a significant number of declarations, not so many successful experiments have been reported so far. In the paper we discuss our recent experiments with serially biased 16-bit counter-flow RSFQ shift registers, connected by special interfaces that are able to transfer SFQ pulses between the galvanically isolated driver and receiver. The required functionality is achieved using a superconductor transformer with isolated input and output coils. The known drawback of such interfaces is their parasitic sensitivity to magnetic fields induced by remote currents. To depress this sensitivity we have implemented a special superconductor guard that uses six superconductor layers. We have achieved stable operations of such registers with 16 current recycling stacks. The circuits have been fabricated at MIT Lincoln Laboratory.