Rapid Single Flux Quantum (RSFQ) Circuits

In this chapter, the most popular type of superconductive digital logic—rapid single flux quantum (RSFQ) logic—is described. The basic building blocks of digital RSFQ circuits are reviewed in this chapter. Different types of transmission lines for this technology—JTL and PTL—are introduced, and the advantages and disadvantages of these interconnect structures are discussed. Certain basic structures of RSFQ circuits, flux storage loops, balanced comparators, and buffers, are described here as a framework to demonstrate the operation of more complex gates. Bias current distribution in RSFQ circuits is also discussed in this chapter. Resistive bias distribution, commonly used in RSFQ circuits, is described, and the disadvantages of this approach are reviewed. Energy-efficient modifications to RSFQ logic to reduce or eliminate static power dissipation are also introduced, along with the advantages and disadvantages of these different circuit topologies. In particular, the ERSFQ bias scheme, which utilizes inductive current distribution with JJs as current regulators and feeding JTLs as voltage references, is introduced.