A front-end system includes a first terminal that receives a radio frequency signal from a transceiver and a second terminal coupled to an antenna. A front-end module including a power amplifier amplifies the radio frequency signal when powered by a supply signal. The front-end module includes a reference signal generator that generates a reference signal and modulates the reference signal frequency based on the supply signal. The frequency-modulated reference signal is transmitted to the transceiver through the first terminal. The front-end module can also include a voltage sensor that senses a voltage level at the first terminal when the frequency-modulated reference signal is being transmitted to the transceiver to determine at least one control signal for controlling the front-end module.