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Japanese Journal of Applied Physics

Flux-Flow-Type Josephson Linear Amplifier with Large Gain and Wide Linear Range

Tadao Nagatsuma, Keiji Enpuku, Hiroaki Iwakura, Keiji Yoshida

Published Aug 1, 1985• 16 citations
Publisher

Abstract

A linear amplifier using a flux-flow phenomenon in a long Josephson junction has been designed and tested by connecting a load resistance. The present experiment has demonstrated that a current gain of 500/W s (1/µm) is obtained for the load resistance of 9 mΩ when an input signal is applied with a control line of width W s (µm). An equivalent circuit of the amplifier has also been presented, which shows that the device is most suitable for current amplification with large gain in low impedance circuits.

DOI
10.1143/jjap.24.l599

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A linear amplifier using a flux-flow phenomenon in a long Josephson junction has been designed and tested by connecting a load resistance. The present experiment has demonstrated that a current gain of 500/W s (1/µm) is obtained for the load resistance of 9 mΩ when an input signal is applied with a control line of width W s (µm). An equivalent circuit of the amplifier has also been presented, which shows that the device is most suitable for current amplification with large gain in low impedance circuits.
The authors have been developing an analog amplifier utilizing a flux flow phenomena in a Josephson line, i.e. the flux-flow-type Josephson amplifier (FFA). The FFA converts a signal current I/sub s/ to a voltage V/sub dc/ across the Josephson line, and the I/sub s/-V/sub dc/ characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I/sub s/-V/sub dc/ relation. It is shown that the transresistance can be improved significantly by reducing the width of the Josephson line and increasing the kinetic inductance of the line. The kinetic inductance can be increased by thinning the line electrode and by using an electrode with large London penetration depth, such as NbN. The transresistance obtained is as large as 6.8 Omega for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 mu m. It is also shown that the linear operating range of the amplifier is as large as 1 mV.>
A current amplifier design based on the principle of fluxon propagation in a multijunction exponential flux shuttle has been investigated. In this design, the critical current of the junctions is increased exponentially and the SQUID (superconducting quantum interference device) inductance is a Josephson-junction equivalent inductance. Current gain can be achieved by generating fluxons at the low end and dissipating them at the high end where the load is located. Advantages over other types of linear devices are discussed. Two parallel exponential flux shuttles can be used to duplicate flux from a high-inductance input coil to a low-inductance output. The device performance of the two circuits is evaluated by computer simulation, and noise performance is discussed.>
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.
The authors have modified the design of the superconducting current injection transistor (Super-CIT) for use in a distributed amplifier. The super-CIT consists of a resistively damped long Josephson junction in which the maximum Josephson current is modulated by the magnetic field produced by current in a control line. The device is modified by DC-coupling the control line from the Josephson junction so that the Super-CIT can be embedded in a distributed amplifier configuration. The distributed amplifier design is used to raise the input impedance and the power gain of the amplifier over a wide bandwidth. The small signal current gain of the device can be increased by biasing the Josephson junction in an asymmetric configuration. A simulation using practical values results in an amplifier with an input impedance of 6.5 Omega using only four devices, which produces 15 dB of gain into a 2.5- Omega load over a bandwidth of 90 GHz with very little reverse gain.>
A niobium-based superconducting amplifier consisting of a series array of 60 vortex flow transistors (VFT) was fabricated and tested. Each VFT consisted of a long Josephson junction biased in the flux flow state, magnetically coupled to a current based on a distributed amplifier configuration. The transresistance of the amplifier in a 50- Omega environment for frequencies up to 1 MHz was measured. The transresistance of the array was less than the expected value calculated from measurements obtained on a single VFT. The difference was probably due to nonuniformities in the fabrication process, which caused some of the junctions not to be biased in the flux flow state or receive the same amount of magnetic field from the control line. However, measurements of the array showed that a large number of VFTs could be connected in series and simultaneously be biased into the flux flow state.>
Josephson fluxonic bipolar junction transistor (JFBJT) has been proposed as a superconducting amplifying element. The results on a simple realization of this device are provided to demonstrate its working principle. These experiments demonstrate that JFBJT provides current gain without suffering from the problems associated with the regular long junction vortex flow transistors. In particular, no resonance structure is present in its I-V characteristics and its impedance can be matched to any load without degradation of its characteristics. Suggestion is made on how a practical device can be fabricated.