The design and performance of a 38 GHz MMIC biphase amplitude modulator employing cold HBTs as switching elements are presented. Direct carrier modulation together with a balancing topology are employed to lower the circuit's complexity and cost, and to remove the amplitude and phase errors that occur at higher frequencies. Furthermore, by using a miniature microstrip coupler, the total area of the chip is reduced to 1 mm2. It is found that this technique is very robust, producing excellent performances at millimeter-wave frequencies. ©2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 215–218, 2000.