Piezoelectric Energy Scavenging Systems (PESS) are used to convert the energy of mechanical vibrations into electrical energy exploiting the piezoelectric effect. Their output is a voltage which strongly varies in time; to obtain a suitable supply source an AC-DC conversion of the output voltage of these transducers is needed. Since the output power level of the energy transducer can be very low, the conversion should be as efficient as possible. The paper describes an active AC-DC converter, based on the voltage doubler topology, in which two different driving circuitries have been implemented. The proposed solutions are fully autonomous, i.e. they are supplied by the energy that they harvest. To reduce and to optimize their power consumption a bias circuit has been designed to make the total bias current supply independent. A test chip has been diffused using STMicroelectronics 5V CMOS technology. The performances of the two solutions were compared with the ones of a passive Schottky based voltage doubler. The figures of merit were the average power supplied by the piezoelectric transducer and the average power delivered on a load resistance. Furthermore, the significance of such parameters is also discussed, and a more general figure of merit is defined with the advantage of also weighting the ability of the converter to harvest all the power available at the transducer terminals.