1. A superconducting digital logic amplifier for interfacing between a Josephson junction logic circuit having output current and a higher voltage semiconductor circuit input, said amplifier comprising: a. an input terminal for connection to a b. an output terminal for connection to a semiconductor circuit input; c. an input, lower critical current, Josephson junction having first and second terminals; d. a first series string of at least three lower critical current Josephson junctions, said first series string being connected to said first terminal of said input Josephson junction such that the first series string is in series with said input Josephson junction to provide a series combination, and said input terminal being connected to said first terminal of said input Josephson junction, and with said critical current of said lower critical current Josephson junctions of said input Josephson junction and said first series stirng Josephson junctions being less than the output current of said low voltage Josephson junction circuit; e. a second series string of at least four higher critical current Josephson junctions, said second string being connected in parallel with said series combination to provide parallel strings having an upper common connection and a lower common connection, said lower common connection being connected to said second terminal of said input Josephson junction and said upper common connection being connected to said output terminal; and f. a pulsed DC current source connected to said parallel strings at said upper common connection, said DC current source having a current at least equal to the critical current of said higher critical current Josephson junctions.