# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.4 Rectifier  
The rectifier is the fundamental circuit of the rectenna, and its role is to convert the AC output voltage of the receiving antenna to DC voltage. It consists of passive elements (capacitors, coils, resistors) and diodes or diode-connected transistors. As Figure 8 depicts, there are three basic types of rectifier designs: single diode, voltage multiplier, and bridge of diodes. Single diode and bridge rectifiers can provide a DC voltage to the load, but the amplitude of the output signal is lower than the amplitude of the received signal. More specifically, a half-wave rectifier is the simplest rectifier circuit, but it is rarely used because it diminishes half of the input power [40]. The bridge rectifier circuit rectifies both cycles of the AC input. This circuit has 4 diodes operating simultaneously, hence there is a higher drop voltage. On the other side, a voltage multiplier is a rectifier circuit that not only converts the AC input to DC output but boosts it. At RF energy harvesting systems with a demand of high efficiency and high output power, at low input voltages, the full-wave rectifier is the best choice [48]. When the rectified power is \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Diode** & **Circuit** \\ \hline
[41] & HSMS2820 & Dual-Stage Voltage Doubler \\
[42] & SMS7630-079 & 3 Rectifiers in Series \\
[43] & SMS7630 & Voltage Doubler \\
[45] & SMS7630 & Single Diode Rectifier \\
[46] & HSMS-285C & 1-Stage Dickson Rectifier \\
[47] & HSMS 286C & Rectifier \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Types of diodes that are utilized in various rectifiers’ designs. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Diode** & \begin{tabular}{c} **Minimum Breakdown** \\ **Voltage V\({}_{\text{sn}}\)** \\ **(V)** \\ \end{tabular} & \begin{tabular}{c} **Maximum Forward** \\ **Voltage V\({}_{\text{f}}\) (mV)** \\ \end{tabular} & \begin{tabular}{c} **Maximum Forward** \\ **Voltage** \\ **V\({}_{\text{f}}\) (mV)** \\ \end{tabular} & \begin{tabular}{c} **Maximum Reverse** \\ **Voltage** \\ **(V) \(\oplus\) **I\({}_{\text{f}}\) (mA)** \\ \end{tabular} & \begin{tabular}{c} **Maximum Reverse** \\ **(nA) \(\oplus\) V\({}_{\text{f}}\) (V)** \\ \end{tabular} & \
\begin{tabular}{c} **Maximum** \\ **Capactance C\({}_{\text{f}}\)** \\ \end{tabular} &
\begin{tabular}{c} **Special Dynamic** \\ **Resistance R\({}_{\text{p}}\)** \\ **(\Omega)** \\ \end{tabular} \\ \hline HSMS282\(\times\) & 15 & 340 & 0.5-10 & 100–1 & 1.0 & 12 \\ SMS7630-079 & 2 & 60–120 & 50 & - & 0.3 & 5000 \\ HSMS-285C & 3.8 & 150–250 & 1 & 0.175-2 & 0.3 & - \\ HSMS286C & 3 & 250–350 & 0.35–1 & - & 0.3 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Schottky diodes. insufficient and there is a demand for further increasing voltage, a better approach is the use of stacking single rectifiers into series (Figure 9), making a voltage multiplier [6,44]. Figure 8: Types of rectifier configurations (**a**) half-wave rectifier (**b**) full-wave rectifier (**c**) bridge rectifier [1]. Figure 9: Topologies of voltage multipliers: (**a**) Villard Topology (**b**) Dickson Topology. In [49] the authors manufactured a rectifier prototype with a single HSMS-2860 Schottky diode at 2.45 GHz. The authors in [50] have designed a rectifier based on the approach of a multiplier. Also, to improve the efficiency of their circuit, they placed a coil at the input of the circuit. As a result, they achieved an efficiency of 81.65% for 0 dBm input at 868 MHz. A different approach of designing a rectifier in a rectenna is presented in [51, 52]. In both works, the authors manufactured a modified Greinacher rectifier that receives input from a rat-race coupler, that was used to divide the input RF signal into two signals with equal power and a 180\({}^{\circ}\) phase shift. The conventional Greinacher circuit was placed inside the rat-race coupler structure. In the first design, the efficiency enhanced over 5% for input ranges from \(-\)20 dBm to \(-\)10 dBm.