A 300 GHz amplifier is fabricated using indium-phosphide (InP) double-heterojunction bipolar transistor (DHBT) technology. The cascade chain in the amplifier contains six unit cells each using a differential-pair of common-base DHBTs. A total of six signal lines provide connection to the unit cell to obtain the differential-mode amplifier gain while providing proper dc bias. Measured results show the peak gain of 17.3 dB at 290 GHz with 10-dB gain-bandwidth of 20 GHz. This design technique could be extremely powerful in generating high terahertz amplifier gain.