To realize 10-Gbit/s optical transmission systems, we designed and fabricated a limiting amplifier with extremely high operation frequencies over 10-GHz using AlGaAs/GaAs heterojunction bipolar transistors (HBTs), and investigated their performances. Circuit design and simulation were performed using SPICE and LABRA. A discrete AlGaAs/GaAs HBT with the emitter area of 1.5${ imes}$10$mu extrm{m}$$^2$, used for the circuit fabrication, exhibited the cutoff frequency of 63GHz and maximum oscillation frequency of 50GHz. After fabrication of MMICs, we observed the very wide bandwidth of DC∼15GHz for a limiting amplifier from the on-wafer measurement. Ceramic-packaged limiting amplifier showed the excellent eye opening, the output voltage swing of 750mVulcorner, and the rise/fall time of 40ps, measured at the data rates of 10-Gbit/s.