# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.4 Rectifier
In the second design, the maximum efficiency is 71% for 4.7 dBm input power. A common practice to improve the efficiency in RF energy harvesting systems is to add a boost converter to the output of the rectifier [53]. In [54], the authors manufactured a low-complexity double diode rectifier that was connected to a commercial boost converter. Implementing this practice, they achieved an efficiency of 21% for input power of \(-\)15 dBm at 97.5 MHz. In [55], a design of a compact rectifier for ambient wireless energy harvesting is presented that operates in two frequency bands, namely at 2.4 GHz and 5.8 GHz. The proposed rectifier consists of a dual-band matching network and a voltage quadrupler. It has an efficiency over 70% and can boost up the voltage level up to 9 Volts. In [56] we can find a Latour rectifier. This circuit is practically a modified voltage doubler. The authors achieved 38% efficiency for input \(-\)10 dBm at 850 MHz. In [57, 58, 59, 60], rectifiers that are voltage doublers can be found. These circuits operate at different frequencies and differ in the choice of materials (capacitors, diodes) and the substrate. The choice of substrate and diode are two factors that are very important in the design of a rectifier. The authors in [61] designed a Karthaus-Fischer voltage multiplier that which operates at 2.45 GHz. In [62] we can find a 7-stages voltage multiplier that resonances at 900 MHz. A system that works at 575 MHz, 900 MHz, and 2.45 GHz, can be found in [63]. The efficiencies for this system are 55%, 45%, and 30% for each frequency, respectively. In [64] there are four design approaches about differential connection. In [65] there is a 3-stage multiplier that operates at 915 MHz and at 2.4 GHz. The authors in [66] design a rectifier at 2.34 GHz with an input power of \(-\)25 dBm. A voltage doubler at 900 MHz can be found in [67] with an efficiency of 78.70% for input power of less than 10 dBm. In [68] a voltage multiplier with multistage Wilkinson was designed. The authors in [69] designed their dual-band rectifier on a polyimide-based substrate, a nonrigid substrate. They achieve at 900 MHz a 20% RF-DC efficiency for \(-\)15 dBm input and a 30% for \(-\)5 dBm input. At 2.4 GHz they achieve 10% efficiency. The authors of [70] design their rectifier on paper. This is a single diode rectifier because the input power is very low. The authors in [71] designed a single-stage voltage multiplier that operates at 900 MHz. They achieved 50.2% and 49.1% efficiencies for 14 dBm and 16 dBm, respectively. In [72] we can find an RF combiner. This circuit concatenates the RF signals of many rectifiers. Each rectifier is a triple band rectifier (GSM1800, UMTS2100, Wi-Fi). The whole system achieves 40% efficiency. In [73] we can find step by step the process of designing a rectenna on a thin flexible substrate. The whole system can achieve efficiency up to 60%. The designs of rectifiers with diodes suffer from the high-dropout voltage across the diodes. So, in order to overcome this limitation, the diodes must be replaced with diode-connected MOSFETs (metal-oxide-semiconductor field-effect transistor). Figure 10 below displays a 4-stage Dickson multiplier (Figure 10a) and a conventional differential rectifier (Figure 10b). Over the years and the evolution in circuit design, CMOS technology is preferred, because it assists in custom designing in electronics but also because CMOS transistors, at low voltages, are more sensitive than Schottky diodes [44]. On the other hand, MOS transistors suffer at high RF power due to a reverse current that drains some of the scavenged energy back to the input [74]. For instance, the authors in [37] designed a CMOS Villard multiplier that operates at 400 MHz and 2.4 GHz. The specific circuit achieves a 160% increase in output power over traditional circuits at 0 dBm of input power. To overcome some of the limitations such as the low-efficiency and limited working range of the RF energy scavenging systems, many designers manufacture rectennas with reconfigurable rectifiers.