Devices that harvest electrical energy from mechanical vibrations have the problem that the frequency of the source vibration is often not matched to the resonant frequency of the energy harvesting device. The source vibration may vary with time, and it may have a broad spectrum. Previous work has recognized that maximum output power can be obtained over a fairly broad spectrum using a tunable, complex reactive impedance at the output of the device. The present paper explores the bias-flip (BF) technique, along with other microelectronic techniques, to implement this tunable, complex reactive impedance.
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