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We proposed and successfully demonstrated a high-speed Josephson IC to semiconductor IC output interface circuit combining a high electron mobility transistor (HEMT) amplifier and Josephson high-voltage drivers successfully. We developed a 0.5-/spl mu/m gate 77-K wide-band analog monolithic HEMT amplifier for the interface. The HEMT device consisted of InGaP/InGaAs materials stable even at 77 K. The amplifier has a differential amplifier as a first stage to cancel out ground-level fluctuations in the Josephson IC and showed a voltage gain of 23 dB and /spl sim/3-dB frequency of 8 GHz. A 0.63-V/sub p-p/ output was obtained from a 5-GHz, 30-mV/sub p-p/ complementary input signal. We succeeded in transfer ring a voltage signal from 10-stack Josephson high-voltage drivers to a 50-/spl Omega/ system at room temperature with 0.7-V/sub p-p/ amplitude at 300-MHz clock using the HEMT amplifier.