The paper presents a novel architecture of RF power generator, which serves as the transmitter in microwave power transmission (MPT) system. Existing architectures for MPT transmitter are briefly reviewed, problems of which are analyzed. A novel structure based on quadrature modulation and demodulation techniques is proposed, which features low cost, easy implementation and comparable high performance. Full amplitude and phase control of the RF power is realized, providing the chance to achieve versatile functions in MPT system. Operation principles and design considerations are elaborated in detail. Finally, a 16-channel 90W/915MHz prototype is constructed in the laboratory. The proposed architecture and related control strategy are fully verified on the prototype.