This paper presents an active rectifier, which aims to extract maximum power from piezoelectric generators for vibration energy harvesting. A nonlinear impedance matching scheme called Synchronized Switch Harvesting on Inductor (SSHI) is highly effective [1]. However, control of the internal capacitor flip timing is complicated to result in a complex controller or suboptimal flip timing. We proposed an active rectifier based on the series SSHI configuration [2]. It ensures an optimal capacitor flip timing, yet the controller is simple to dissipate low power and hence achieve high efficiency. One potential problem of the series SSHI scheme is a non-robust operation due to parasitics and a variety of characteristics of the piezoelectric generators. We present an improved rectifier, which address the problem. The improved rectifier is designed in 0.25 μm BiCMOS process. Post-layout simulation results indicate robust operation and high efficiency of 93%, which is comparable to the original design in [2] and far higher than other existing SSHI rectifiers reported.