## 7 RF-EH Circuit
### A rectifier's Configuration  
Signal rectifying can be performed with diodes, transistors, and complementary metal-oxide-semiconductor (CMOS) technology. A rectifier is a device that converts alternating current (AC) created in an antenna by received radio frequency signals into direct current. Signal rectification topologies are classified as follows:  
1. Half-wave (HW) rectification is either the positive or negative half of an alternating current wave passing while the other half is blocked. Rectifiers produce unidirectional but pulsating direct current (DC).  
2. Full-wave (FW) rectification is a technique that converts the entire input waveform to an output waveform with constant polarity (either positive or negative) [138].  
Completely rectifying an input waveform converts both polarities of the waveform to pulsing direct current (DC) and results in a higher average value output voltage.  
Half-wave rectifiers produce far more ripples than full-wave rectifiers, necessitating the use of far more filtering to remove alternating current frequency harmonics from the output. The most common rectifying circuit consists of a single diode in a serial design that functions as a half-wave rectifier (HWR). Lower built-in voltage diodes, such as Schottky diodes [138], can generally achieve higher conversion efficiency than higher built-in voltage diodes. The series and shunt single diode topologies provide the option of reducing the loss of the diode, which is measured in terms of the diode junction resistance [139]. Figure 3 shows the half-wave rectifier in three different configurations: (a) shunt, (b) series [140], and (c) full-wave rectifier [16].  
Figure 3: Half-wave rectifier with (a) shunt configuration and (b) series configuration [140], and (c) full-wave rectifier configuration [16].  
Schottky diodes have been utilized, and the Dickson charge pump is the most frequent topology [141]. However, due to the high threshold voltage of diodes, obtaining an efficient output is difficult. Metal-oxide-semiconductor field-effect transistor (MOSFET) technology has overcome the limitations of diodes and has emerged as a viable replacement for rectifiers [142]. The voltage developed at the output is given as follows:  
\[Vout\ (dc)=2 Vin\ (RF)\ -\ Vth \tag{8}\]  
It is necessary to use four diodes in an FW bridge rectifier design to reduce the number of diodes and hence minimize the total losses. Given that the resultant output may be decreased by accounting for the losses associated with the diodes, a full-wave rectifier was discovered to be a suitable alternative for higher input power-density levels. When the power received is insufficient to power the circuit, the rectifier input must be magnified to provide enough power. Alternatively, a rectifier circuit with a multiplier may be employed in certain situations [143]. Circuit designs for rectifiers can be divided into three categories: the primary rectifier, which uses a capacitor and single diode; the voltage doubler, which uses capacitors and two diodes; and the voltage multiplier [138].  
The performance of the rectenna is further improved by the use of a Greinacher rectifier circuit (GRC) [144; 145]. GRCs are analogous to two-stage VDR circuits arranged in a bridge configuration, as illustrated in Figure 4. It consists of two branches, each with a pair of diodes, and it is connected to a power source. In addition, each diode's biasing voltage can be moderately generated by the output of the previous diode, reducing the need for external power. As a result, the proposed GCR design is suitable for a wide operating frequency range (1.8-2.5 GHz) [144], while the proposed GRC's 180\({}^{\circ}\) phase difference was achieved using a hybridized rat-race coupler [145].