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Revue de Physique Appliquée

Negative resistance effects with Josephson junctions

Helmut Kanter

Published Jan 1, 1974• 0 citations
Publisher

Abstract

Conventional negative resistance effects can be generated with a Josephson junction quite similar to that obtained with Varactor diodes, with the additional advantage that the internal, self-generated oscillations serve as a pump, i. e., only a dc source is needed instead of a high frequency pump source. However, another negative resistance effect typical for a Josephson junction is possible as has been predicted recently for the current controlled, highly damped junction model. The effect has recently been demonstrated experimentally at 9 GHz. Characteristically, the pump (Josephson) frequency is smaller than the frequency of the amplified signal and the pump mechanism is effective over a broad range of frequencies in contrast to the conventional parametric negative resistance effect where the pump frequency exceeds that of the signal and idler and where the frequency of operation is limited to a narrow range. The broad-band feature renders the amplifier operation uncritical to tuning and bias voltage fluctuations. The fact that the junction is the element of lowest impedance in the circuit alleviates to a degree the matching problem generally encountered with Josephson contacts. An assessment of the competitiveness of this effect in microwave technology is given.

DOI
10.1051/rphysap:0197400901022700

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Conventional negative resistance effects can be generated with a Josephson junction quite similar to that obtained with Varactor diodes, with the additional advantage that the internal, self-generated oscillations serve as a pump, i. e., only a dc source is needed instead of a high frequency pump source. However, another negative resistance effect typical for a Josephson junction is possible as has been predicted recently for the current controlled, highly damped junction model. The effect has recently been demonstrated experimentally at 9 GHz. Characteristically, the pump (Josephson) frequency is smaller than the frequency of the amplified signal and the pump mechanism is effective over a broad range of frequencies in contrast to the conventional parametric negative resistance effect where the pump frequency exceeds that of the signal and idler and where the frequency of operation is limited to a narrow range. The broad-band feature renders the amplifier operation uncritical to tuning and bias voltage fluctuations. The fact that the junction is the element of lowest impedance in the circuit alleviates to a degree the matching problem generally encountered with Josephson contacts. An assessment of the competitiveness of this effect in microwave technology is given.
The negative-resistance effect recently predicted for current-controlled highly damped Josephson junctions is experimentally demonstrated at a frequency of 9 GHz. Characteristically, the pump (or Josephson) frequency is smaller than the amplified one, and the pump mechanism is effective over a broad range of frequencies in contrast to the conventional parametric negative-resistance effect where the pump frequency exceeds that of signal and idler and where the frequency of operation is limited to a narrow range. The broad-band feature renders operation uncritical to tuning and bias voltage fluctuations. The fact that the junction is the element of lowest impedance in the circuit alleviates to a degree the matching problem generally encountered with Josephson contacts.
The parametric nature of supercurrent in Josephson junctions may be exploited for amplification of high frequency signals in several modes of operation, where in each case the reactance variation is provided by the internal oscillations due to the average contact potential, a)One mode is negative resistance amplification with a single idler. This mode is entirely equivalent to that generally employed in conventional parametric amplifiers with varactor diodes. b)Negative resistance amplification with several idlers at further combination frequencies. This mode of operation is typical for the resistively shunted junction model, c)Amplification by upconversion, which is conceptually employed in the rf SQUID used in magnetometry. Experimental verifications of these various modes are described and their suitability for low noise amplification discussed. The upconverter experiment permitted the measurement of device noise by variation of the temperature of the input termination. An upper noise limit of 15°K is established. This experiment demonstrates that in high frequency application of active self-oscillating Josephson junctions the fundamental noise achievable is not greatly in excess of the thermal limit.
A self-pumped Josephson junction can be operated in a mode which is described in first order by a two-idler parametric amplifier analysis for a sinusoidal elastance variation. Thus the analysis and operation follows the two-idler negative resistance behavior predicted by Penfield and Rafuse for varactor diodes. Experimental observations on parametric negative conductance excitations at 9 GHz by a Josephson point contact are readily explained by the two-idler model. A total signal power gain of 12 dB was achieved with an experimental device consisting of a niobium titanium point contact in a shallow rectangular cavity coupled through a room-temperature circulator.
The high-frequency amplification properties of transformer coupled, resistively shunted Josephson tunnel junctions have been investigated. The importance of the shunt loop inductance is stressed. It allows a high cutoff frequency, of significance for good high-frequency performance. The self-pumped parametric amplifier showed none of the excessive noise rise, which has hitherto plagued the development of externally pumped Josephson junction amplifiers. Around 10 GHz, we estimated a noise temperature less than 30 K for an amplifier pumped by a Josephson oscillation with a frequency well above twice the signal frequency. The corresponding gain of 5 dB may be increased in a better impedance matched circuit. The gain was very stable against variations in the bias conditions. A gain-bandwidth product as high as 0.3 was registered. The experimental results agreed well with the established theory for self-pumped parametric Josephson amplifiers. It should be possible to extend the low noise amplification by this device to mm wave frequencies. A relaxation oscillation occurred at a subharmonic of the Josephson frequency when the shunt loop inductance became large. The amplification in this mode followed closely the predictions of a simple model, where the signal modulated the switching of the sawtooth-like (relatively low frequency) relaxation current. Gains of about 15 dB were measured around 10 GHz, but the amplification was sensitive to bias conditions and noisy in this case where the relaxation frequency fell well below the signal frequency. Much improved properties were registered when the inductance was decreased so that the relaxation frequency approached the Josephson frequency and exceeded twice the signal frequency. The behavior then resembled that of a Josephson mode parametric amplifier, but the high content of harmonics of a relaxation oscillation meant that the amplifier became noisier due to converted noise from the many idler frequencies.
Gain and noise properties of a non-degenerate one-frequency parametric amplifier using a self-pumped Josephson junction are investigated both theoretically and experimentally. The amplifier uses the phenomenon of microwave negative resistance of the junction. Theoretical analysis made in usual resistively shunted junction (RSJ) model shows that the minimum noise temperature is equal to approximately 42T. It takes place at a signal frequency f close to the junction characteristic frequency f0. Maximum effectivity (the product of the square root of power gain and relative bandwidth) can reach 0.4 at f≃f0. Gain up to 20 dB was obtained at X-band using amplifier with Nb-Ta point-contact junction. Minimum noise temperature at T = 4.2K was about 200K.
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.