A single Josephson junction device is arranged in a single branch which comprises an external source resistor connected in series with an output resistor and a Josephson junction device. An input node and an input resistor are in series and connected to the node between the output resistor and the Josephson junction device. Voltage signals applied to the input voltage node are amplified by connecting a high gain voltage output in parallel with the output resistor and providing sensing means for sensing the voltage output across the output resistor only when the Josephson junction device is switching from its low impedance state to its high impedance state.