We have been developing an analog amplifier utilizing a flux flow phenomenon in a Josephson line. i.e., Flux-Flow-Type Josephson Amplifier (FFA). The FFA converts a signal current Isto a voltage VDCacross the Josephson line, and the Is-Vdccharacteristic determines performances of FFA. such as a gain and a saturation level. Studies have been made to improve the Is-Vdcrelation, such as a transresistance r.=dVdc/dI and a linear operation range. It is shown that the transresistance can be improved significantly by reducing a width W of the Josephson line, and by utilizing a kinetic inductance of the line. The kinetic inductance can be increased by thinning the thickness t of an electrode of the line, and by using the electrode with large London penetration depth such as NbN. The obtained transresistance is as large as r.=6.8Ω for NbN/Pb-alloy Josephson lines with t=5nm and W=4μm. It is also shown that the linear operation range of the amplifier is as large as lmV. On the basis of experimental results, evaluation is made of the performances of FFA.
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