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IEEE Transactions on Applied Superconductivity

Suzuki Stack Circuit With Differential Output

Yerzhan Mustafa, Selçuk Köse

Published Dec 21, 2022• 15 citations
Publisher

Abstract

Suzuki stack circuits, a type of latching high-voltage driver circuit, are widely used in Josephson–CMOS hybrid memories. The existing Suzuki stack designs utilize only single-ended signaling when connected to a CMOS amplifier. In this article, a Suzuki stack circuit with a differential output is proposed by producing both the positive and negative output voltages. The negative output voltage is produced by using a negative biasing network and a negative single-flux quantum (SFQ) input pulse, which is generated by the proposed positive-to-negative SFQ converter. As compared to a conventional single-ended Suzuki stack circuit, the proposed differential design can provide two times higher output voltage swing and better immunity to noise without degrading the operating margins. Potential advantages and drawbacks of the proposed circuit are explored in this article.

DOI
10.1109/tasc.2022.3230761

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Suzuki stack circuits, a type of latching high-voltage driver circuit, are widely used in Josephson–CMOS hybrid memories. The existing Suzuki stack designs utilize only single-ended signaling when connected to a CMOS amplifier. In this article, a Suzuki stack circuit with a differential output is proposed by producing both the positive and negative output voltages. The negative output voltage is produced by using a negative biasing network and a negative single-flux quantum (SFQ) input pulse, which is generated by the proposed positive-to-negative SFQ converter. As compared to a conventional single-ended Suzuki stack circuit, the proposed differential design can provide two times higher output voltage swing and better immunity to noise without degrading the operating margins. Potential advantages and drawbacks of the proposed circuit are explored in this article.
Abstract Josephson-CMOS hybrid memory leverages the high speed and low power operation of single-flux quantum logic and the high integration densities of CMOS technology. One of the commonly used type of interface circuits in Josephson-CMOS memory is a Suzuki stack, which is a latching high-voltage driver circuit. Suzuki stack circuits are typically powered by an AC bias voltage that has several limitations such as synchronization and coupling effects. To address these issues, a novel DC-biased Suzuki stack circuit is proposed in this paper. As compared to a conventional AC-biased Suzuki stack circuit, the proposed DC-biased design can provide similar output voltage levels and parameter margins, approximately two times higher operating frequency, and three orders of magnitude lower heat load of bias cables.
Superconductor–semiconductor hybrid circuits can combine the benefits of the high-speed and low-power operation of single-flux quantum circuits and high integration densities of CMOS technology such as memory. The Suzuki stack, a type of Josephson latching driver/amplifier, is a widely used interface circuit in Josephson–CMOS hybrid memories. Due to the limited cooling power at cryogenic temperatures, the power dissipation is becoming an important concern, especially in large-scale systems. An optimization technique to significantly reduce the power dissipation of Suzuki stack circuits is proposed in this article. The proposed design can reduce the power dissipation by 30–70% while causing a voltage drop of 2–9% in the output voltage depending on the circuit parameter configuration. The tradeoffs between the power dissipation and output voltage characteristics are discussed. The proposed design can operate correctly within at least$\pm$20% of process parameter variations as demonstrated with extensive simulations.
The application of superconducting circuits for high-performance computing and high-speed analog-to-digital converters suffers from the difficulty of amplifying the weak signals of superconducting circuits to the volt level of semiconductor electronics. We refer to the most accepted Josephson circuit used to amplify weak signals of about a millivolt to several tens of millivolts as a Suzuki stack. It can be directly triggered by a single flux quantum pulse and has a very high switching speed of about 25 ps. We report here an in-depth study of design issues for Suzuki stacks with extensive systematic simulations as well as experimental results. We explain the origin of previously reported phenomena such as multiple-level switching, high bit-error-rates, and interactions between stacks, and give design guidance for avoidance of such problems. We show experimental results for a set of Suzuki stacks operating correctly and independently with a single power supply. The presented design guidelines address important characteristics that need to be optimized: output voltage, bias margins, operating frequency, bit-error-rate, delay, power dissipation, and physical size.
Superconductor electronics for high-performance computing and high-speed analog-to-digital converters requires multi-channel digital data links. Most difficult is the amplification of the weak signals from the superconductor circuit to the volt level of semiconductor electronics. We developed a hybrid digital interface based on a Josephson latching driver, the so-called Suzuki stack, and a clocked CMOS comparator. The input can be directly triggered by a single flux quantum pulse and the output provides a 1 V CMOS-level signal. In contrast to existing systems, our interface is optimized for ultra-low-power consumption to enable its application for parallel multi-bit data interfaces. We provide evaluation of various types of Suzuki stacks and various CMOS comparators. We present experimental data on the delay of the overall hybrid interface and the total power consumption. The bit-error rate has been measured to be below 10-12. We will discuss the trade-off among circuit robustness, speed, and power consumption.
Data transmission from single flux quantum (SFQ) circuits to room temperature electronics requires the use of superconductor output drivers and semiconductor amplifiers. The state-of-the-art superconductor output drivers such as SFQ-to-DC converter, superconducting quantum interference device (SQUID) stack, Suzuki stack (Josephson latching driver), and nanocryotron (nTron) have a binary output that produces either a logical 0’ or 1’. Due to the limited cooling power at 4 K, a higher data rate of superconductor output driver is desired to reduce the number of cryogenic cables (i.e., heat load) from 4 K to a higher temperature stage. In this paper, a novel ternary digital output data link is proposed by transmitting the data in three logical states such as 0’, 1’, and 2’. The proposed ternary data link consists of an encoder and a ternary superconductor output driver, which is based on a re-designed Suzuki stack circuit. As compared to a conventional (binary) output data link, the ternary data link can either increase the data rate by 50% or reduce the number of cryogenic cables by33%.
In this study, we designed, fabricated, and experimentally tested a Josephson latching driver (JLD), a superconductor-to-semiconductor interface circuit for use in Josephson-CMOS hybrid memory, utilizing a 10-kA/cm2Nb process. The JLD is composed of a voltage driver known as a Suzuki stack and a Josephson four-junction logic (4JL) gate that serves as a preamplifier. The basic structure is based on our previous design using the 2.5-kA/cm2Nb process. In this study, we optimized the critical current and the load resistance of the 4JL gate, taking into account the parameters of the Josephson junctions in the 10-kA/cm2process. In experiments with the fabricated test circuit, we confirmed the correct operation of the JLD (bit error rate−11) at a target frequency of 5 GHz. We also confirmed the correct function of multichannel JLDs, which were used as interface circuits in Josephson-CMOS hybrid memory, in a low-frequency test.