A 0.4-5.8 GHz SiGe-MMIC quadrature modulator (Q-MOD) employing a self current controlled mixer for cognitive radio is described. The self current controlled mixer consists of a Gilbert cell mixer and a self current control circuit which is composed of both a current feedback circuit and an output buffer amplifier. The self current control circuit automatically controls the mixer current according to the output power level, and improves the linearity over wide radio frequency (RF) range. Simulation results show that the proposed Q-MOD realizes 1 dB compression point (P1 dB) improvement of more than 3.0 dB compared to the conventional Q-MOD at the frequencies of 0.4, 0.8, 1.95, 5.2 and 5.8 GHz. The fabricated Q-MOD achieves P1 dB improvement of more than 2.8 dB under the same condition. It also improves the output power with error vector magnitude (EVM) of 3.0% (Pout@EVM=3.0%), and achieves the Pout improvement of more than 2.7 dB under the modulation conditions of UHF wireless system (OFDM/16QAM, 0.4 GHz), W-CDMA (HPSK/QPSK, 0.8 GHz/1.95 GHz) and wireless-LAN (OFDM/64QAM, 5.2 GHz/5.8 GHz).