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IEEE Transactions on Magnetics

Wide margin voltage locked array of Josephson junctions and its use as amplifier

Meir Gershenson

Published Mar 1, 1987• 1 citation
Publisher

Abstract

The use of unshunted junctions to close the superconducting loop of two shunted junctions in series is shown to improve the voltage locking range compared to the more common use of passive elements such as resistors or capacitors. Stacking pairs of junctions in a ladder-like form with one junction common for each two steps makes it possible to phase lock more than two junctions. Simulations show that four junctions with 5% matching can be locked from 0 to 55μV. The device can also be used as a voltage amplifier by driving one of the junctions. Simulations show that an array of four series junctions operates as an amplifier between 12μV to 33μV input. Input impendance was 6 ohms and output impedance was 12 ohms.

DOI
10.1109/tmag.1987.1064917

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The use of unshunted junctions to close the superconducting loop of two shunted junctions in series is shown to improve the voltage locking range compared to the more common use of passive elements such as resistors or capacitors. Stacking pairs of junctions in a ladder-like form with one junction common for each two steps makes it possible to phase lock more than two junctions. Simulations show that four junctions with 5% matching can be locked from 0 to 55μV. The device can also be used as a voltage amplifier by driving one of the junctions. Simulations show that an array of four series junctions operates as an amplifier between 12μV to 33μV input. Input impendance was 6 ohms and output impedance was 12 ohms.
Previously published experimental results are used to deduce an equivalent circuit for this amplifier, based on the shunted junction model. It is found that the very small inductances of the junction shunt resistors play an important role in determining the shape of the current-voltage curves in the voltage-locked region. Once the circuit is determined using the shunted junction model a combination of an approximation method (the method of slowly varying amplitudes) and the shunted junction model is used to approximately maximize the power gain of the circuit. The maximum gain achieved in these simulations of the two-junction amplifier is 11.3. This gain occurred with a negative resistance imput impedance of −0.41R and a signal source impedance of 0.167R, where R is the junction shunt resistance. It is estimated that the bandwidth of this amplifier is about 1 GHz.
We have studied a new type of Josephson junction amplifier which is based on a two-junction array; the junctions are mutually phase locked at the Josephson self-oscillation frequency. With this frequency at 82 GHz, the voltages of the junctions remain equal (locked) for a bias current range as large as 60% of the critical current. Over a much smaller bias range, with an applied signal frequency of 1 kHz, a small-signal power gain of 19 dB was measured, accompanied by a negative resistance input impedance. This performance is consistent with a quasistatic theory of the amplifier.
Frequency locking effects in two interacting Josephson junctions are investigated. The junctions are described in the usual resistively-shunted-junction (RSJ) model and coupling between junctions is provided by a resonant circuit. The system can be used to simulate interactions between superconducting weak links in a resonant cavity or via a general impedance shunt. Various types of v-i curves in the voltage locking region are obtained in this calculation by using a first harmonic approximation in the high-frequency regime. Stability of order parameter phase oscillations for two identical junctions in the voltage-locked region is examined. We also show that dissimilar junctions can be synchronized in a resonant cavity using a common series bias current.
NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.
Mutual phase locking has been demonstrated in series arrays of two and four Josephson junctions at millimeter-wave frequencies. Experimental observations are in good agreement with theory reported earlier. This technique increases the output power available from a Josephson junction source. Available output power is expected to be proportional to the square of the number of junctions until the array impedance approaches the load impedance. The output frequency is voltage tunable over as much as an octave. Theory indicates that the technique can be extended to even larger arrays.