Embodiments of the subject invention are drawn to power supply units and systems for supplying power to loads. Specific embodiments relate to systems incorporating the loads. The power supply units and systems can include a feedback mechanism for monitoring the system and maintaining a parameter of interest at or near a desired value (e.g., for maintaining the frequency of operation at or near resonance). The feedback mechanism is configured such that, if the at least one parameter indicates that the frequency of operation is away from a resonant frequency of the power amplifier, the feedback mechanism adjusts the frequency of operation closer to the resonant frequency of the power amplifier. The at least one load can have a variable impedance, though embodiments are not limited thereto.