**3.2 Rectifiers**

The power efficiency, seen as Power Conversion Efficiency (PCE) in the **Figure 9**, is the capability of a rectifier to transform radio frequency (RF) energy into DC current. The PCE depends on the diode conduction and reverse leakage losses. The input voltage varies according to the frequency, which means the diode impedance varies, leading to a difference in the performance loss. In low input power, the efficiency is low because the input voltage dynamic is lower or equal to the forward biasing voltage of the diode.

In general, the PCE varies with the input dynamic which in turn depends on *V <sup>j</sup>*, *Vbr*, and *RL*, representing the diode forward voltage drop (in the pn junction), the breakdown voltage and the dc load resistance of the rectenna, respectively. As shown in **Figure 10**, the efficiency sharply decreases as the voltage swings, when a diode exceeds *Vbr*, the breakdown voltage. The peak efficiency is an optimum between: the forward (junction) loss and the reverse (breakdown) leakage loss. Moreover, the PCE is also affected by the production of higher order harmonics. Diodes produce harmonics and inter modulation as a result of their nonlinear nature, which reduces power conversion efficiency. Due to increased parasitic losses caused by harmonic generation, the power levels are reduced, which in turn

**Figure 10.** *Diode efficiency function depends on the breakdown voltage and the load resistance.*

limits the performance. As a result, all of the above-mentioned parameters follow a tradeoff depending on the requirements. High threshold voltage diodes are favoured for low power applications, whereas high reverse break down voltage diodes are preferred for high power applications.
