## 7 RF-EH Circuit
### The Influence of Multiband Frequencies on the RF Harvester Architecture  
Radio-frequency energy harvesting has received a great deal of attention in the research community over the last few decades. Although a single narrow-band design promotes high efficiency, the amount of DC output power is limited. A multiband, broadband, or rectenna array can accumulate more power from weak ambient sources and produce more output power than a narrow-band rectenna, but the trade-offs may include decreased overall efficiency and increased dimension. The main challenge of wireless energy harvesting at the moment is how to improve power conversion efficiency at low-input power levels across a wide frequency band [167]. Due to the wide frequency range of the ambient wireless signals (ranging from 200 MHz to 4 GHz), multiband and broadband rectennas may be a promising solution to these problems. Rectennas of this type can receive more energy from a lot of different channels and sources at the same time, which leads to more DC output power [168]. To harvest energy from multiple RF bands, several RF harvester topologies can be investigated. The main distinction is in the RF bandpass filter design. The filter's function is to match the antenna impedance and the conjugate impedance of the rectifier input. Furthermore, the impedance of the rectifier's input varies with frequency and incident power. The impedance of the antenna can also change depending on the frequency. As a result, adjusting impedances at a single frequency is easier than adjusting them across an entire RF band. Adaptation to multiple RF bands, according to this, results in two types of losses: those caused by impedance mismatch and those caused by filter complexity [169]. In situations where there are multiple RF energy sources that are easily accessible, the quantity of energy harvested can be increased, which is especially useful if the system is intended to operate over a wide frequency spectrum. Figure 5 shows the schematic diagram of the architecture of the multiple RF circuits [169]. The author in ref. [170] described the multiband, differentially fed rectenna as being very useful for RF energy harvesting applications because it would have all the potential benefits of a differential structure, such as poor common mode noise, proper harmonic suppression, and so on. It would also have other advantages, such as steadily increasing RF to DC efficiency at even low RF power levels, because it would be extracting from multiple sources at the same time. It is mostly because of this that a differential rectenna can work in three different frequency ranges. It will be more difficult to maintain the next generation of sensors and circuits because they will be built right into the products themselves. If you cannot obtain a replacement battery, then batteryless devices are your only option. The environmental impact of these distributed and ubiquitous electronics must be minimized in order to avoid increasing pollution risks and lowering human life quality, as well as conforming to the host object's shape and cost [171]. Author in ref. [172] introduces a triple-band, high-gain multibeam ambient RF energy harvesting system for downlink channels of GSM-1800 (1.805-1.88 GHz), UMTS-2100 (2.11-2.17 GHz), and Wi-Fi (2.4-2.48 GHz) systems to overcome the challenge of battery charging and replacement in the IoT. Compared to previous work, this energy harvesting system provides a higher output DC power with a better RF-to-DC efficiency. As a result of its high antenna gain, this energy harvesting system is able to capture more energy from its surroundings while simultaneously taking advantage of frequency, spatial diversity, and polarization diversity. It is possible to change the methodology depending on what the researchers want to accomplish, such as using a single methodology (using only one method, software, and equation) or a hybrid methodology (combining several methods, software, and equations) (using mulitmethods, software, and equations).