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International Journal of Electronics

Review of microwave distributed superconducting vortex-flow transistor amplifiers

R.C. Yalamanchili, ZHENG AN QIU, Yen-Chu Wang

Published Sep 1, 1992• 3 citations
Publisher

Abstract

The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance rm of a VFT is very small. Some methods for maximizing rm and minimizing the output impedance r0 also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-Tc superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.

DOI
10.1080/00207219208925693

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The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance rm of a VFT is very small. Some methods for maximizing rm and minimizing the output impedance r0 also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-Tc superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.
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Vector S-parameter measurements of the superconducting vortex flow transistor (VFT) are presented. The measurements were obtained for frequencies up to 100 MHz on VFTs that had a calculated transmit-time cutoff frequency of 5 GHz. An equivalent circuit model that includes calculations of the VFT transresistance, input inductance, and feedthrough capacitance is derived from these measurements. The measurements are limited to an upper frequency of 100 MHz due to crosstalk in the low-impedance system.>
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.>
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Microwave two-port S-parameter measurements and modeling of superconducting flux flow transistors are presented. The transistors, based on the magnetic control of flux flow in any array of high temperature superconducting weak links, exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive while the output impedance is both resistive and inductive. It is shown that the characteristics of these devices are useful in numerous applications including matched amplifiers, phase shifters and active impedance convertors.>