We present the design and the fabrication of an adder circuit in the extended phase-mode logic family. The phase-mode logic is a single flux quantum(SFQ) logic which utilizes an SFQ as an information bit carrier. The single-bit adder circuit is made up of an INHIBIT gate which is the basic device of the phase-mode logic. The circuit has been designed and fabricated using Nb/AlOx/Nb Josephson junctions with Josephson critical current density of 1. 0kA/cm2. In order to confirm the circuit operation the fabricated adder circuit has been tested at low speed. For investigating the possibility of a highfrequency operation dc voltages generated by fluxon pulse trains have been measured. From the Josephson voltage-frequency relation the result shows that the circuit has potential to complete the carry operation within 20psec.
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