A signal modulation device and a signal amplifier cooperative therewith. The signal modulation device includes a local oscillation signal source, a baseband signal source, a first NMOS transistor, and a second NMOS transistor, wherein the first and second NMOS transistors are coupled with the baseband signal source and form a circuit architecture of a Gilbert-cell based differential pair to be directly switched by a differential baseband signal, and a high-frequency signal from the local oscillation signal source is controlled by the baseband signal so as to generate an amplitude-modulation high-frequency signal at an output end. The single-stage signal power amplifier amplifies the amplitude-modulation signal from the preceding circuit so as to increase the magnitude of signals transmitted and simplify the preceding digital/analog signal conversion circuit in a conventional amplitude-modulation circuit.