Control of return current flows in the single-flux-quantum circuits using positive/negative bias supply lines

Abstract

We propose a technique to use both positive and negative dc bias supply lines in single-flux-quantum (SFQ) circuits aiming to suppress stray return currents flowing on a ground plane. Negative bias lines are placed in parallel with positive ones, and are used to extract return currents immediately from a ground contact near each bias feeding point. We tested a large-scale shift register fabricated using the SRL 4-layer niobium standard process in three different ways to supply bias currents: did not apply any negative bias currents, and applied negative bias currents to ground contacts (conventional) and to the negative bias lines (proposed). The experimental results showed that the shift register never operated without negative bias currents, and the operating margin was improved in the proposed way of bias current supply. It suggests that unpredictable stray return currents can flow in the complicated layouts even when we apply negative bias currents in the conventional way, and careful control of the return current flows is required in large-scale SFQ circuits.