The importance of high-performance,low-cost and millimeter-wave transmitters for digital communications and radar applications is increasing.The design and performance of a Ka-band balanced in-phase and quadrature-phase(I-Q)type vector modulator,using GaAs heterojunction bipolar transistors(HBTs)as switching elements,are presented.The balanced technique is used to remove the parasitics of the HBTs to result in near perfect constellations.Measurements of the monolithic microwave integrated circuit(MMIC)chip with a size of 1.89×2.26 mm~2 demonstrate an amplitude error below 1.5 dB and the phase error within 3°between 26 and 40 GHz except for a singular point at 35.6 GHz.The results show that the technique is suitable for millimeter-wave digital communications. The importance of high-performance, low-cost and millimeter-wave transmitters for digital communications and radar applications is increasing. The design and performance of a Ka-band balanced in-phase and quadrature-phase (IQ) heterojunction bipolar transistors (HBTs) as switching elements, are presented. The balanced technique is used to remove the parasites of the HBTs to result in near perfect constellations.Measurements of the monolithic microwave integrated circuit (MMIC) chip with a size of 1.89 × 2.26 mm ~ 2 demonstrate an amplitude error below 1.5 dB and the phase error within 3 ° between 26 and 40 GHz except for a singular point at 35.6 GHz. The results show that the technique is suitable for millimeter-wave digital communications.