This paper is focused on resonant Piezoelectric Vibration Energy Harvesters (PVEHs). In all energy harvesting applications, and in particular in PVEH applications, the maximization of the extracted power is mandatory. It is well known that, in order to maximize the harvested power, it is necessary to get the optimal matching between the harvesting device and the load. In particular, in PVEH applications under purely sinusoidal input vibrations, in order to maximize the harvested power, the impedance of the load must be coincident with a well-defined optimal impedance ZOPT. However, in practical applications, the load of PVEHs is not a linear impedance, but it is nearly always represented by a non-linear AC/DC converter. In this paper, the f...