A transmit apparatus having a supply modulator is provided. The apparatus includes a detector and the supply modulator. In the method, the detector detects an output signal of the supply modulator. Also, the supply modulator receives the detected output signal of the supply modulator from the detector and calibrates a modulation characteristic of the supply modulator. The transmit apparatus having a supply modulator includes a modulator/demodulator (modem) and the supply modulator. The modem provides a calibration signal for calibrating a modulation characteristic of the supply modulator, to the supply modulator. The supply modulator outputs a modulated signal in accordance with the calibrated modulation characteristic of the supply modulator based on the calibration signal from the modem.