A 0.4-5.8GHz high linearity SiGe-MMIC quadrature-modulator (Q-MOD) having a novel self mixer current control circuit is described. This control circuit, which consists of an output power detector and an envelope current feedback circuit, is applied to unit mixers, and it automatically controls the mixer current according to the output power level. As a result, high linearity can be obtained. In simulation, the Q-MOD employing the proposed unit mixers shows the P1dB improvement of more than 2.6dB under the CW condition of the 0.4-5.8GHz frequency range compared with the conventional Q-MOD. In measurement, the Q-MOD achieves P1dB improvement of more than 3.0dB under the same condition. It also improves the maximum output power with EVM of 3.0% (Pout@EVM=3.0%) for both modulation conditions of UHF wireless systems (16QAM/OFDM, 0.4GHz) and IEEE 802.11a W-LAN (64QAM/OFDM, 5.2GHz).