An apparatus includes a film bulk acoustic resonator and a field effect transistor. The film bulk acoustic resonator includes first and second electrodes separated by a piezoelectric material. The piezoelectric material is configured such that the application of a potential difference between the first and second electrodes enables the generation of an acoustic wave and associated surface charge in the piezoelectric material. The field effect transistor includes a channel, and source and drain electrodes configured to enable a flow of electrical current through the channel when a potential difference is applied between the source and drain electrodes. The surface charge generated in the piezoelectric material induces a corresponding charge in the first electrode causing a variation in the electrical current flowing through the channel via a portion of the first electrode, the variation in electrical current producing an output signal having a frequency corresponding to that of the acoustic wave.