Performances of a flux-flow-type Josephson amplifier

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 _s to a voltage V_dc across the Josephson line, and the I_s-V_dc characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I_s -V_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 Ω for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 μm. It is also shown that the linear operating range of the amplifier is as large as 1 mV