## 6 Active Load Modulation for Simultaneous Short/Long Range WPT  
To further enhance the ability of adaptive/reconfigurable rectifiers to operate over a wide input power range, an active load modulation has been applied to actively modulate and vary the output load as in Figure 8. Because the output DC power is mainly dependent on the voltage breakdown and the output load, the rectifier output load is indeed one of the crucial parameters that can affect the overall power efficiency [4; 9]. Accordingly, the total PCE can be studied as a function of the load impedance (Figure 9). Here, the concept of active load modulation can be explored using active components, such as transistors, that can adjust the circuit load to increase and enhance the circuitry efficiency of a predefined input power range.  
Figure 8: Illustration of the adaptive rectifier configuration with the active load technique.  
Figure 7: Adaptive rectifier technique compared to a conventional diode rectifier, (**a**) Stacking two diodes, (**b**) Stacking three diodes, (**c**) Adaptive system, (**d**) I-V curves, (**e**) Power Conversion Efficiency.  
The design keys of having active load modulation are (i) to achieve a high voltage at a low input power to maximize the efficiency and (ii) to reduce the output voltage at a high input power to prevent the rectifier from reaching its breakdown voltage. Therefore, at low input powers, a large output load is desirable to produce a high output voltage and thus to increase the overall PCE. Once the input power increases, it is appropriate to minimize the output load to avoid the rectifier reaching its voltage breakdown, thus maintaining high PCE over a wide range of input power levels [15].  
The proposed active load block, illustrated in Figure 9(a), contains three resistors (\(R_{1}\), \(R_{2}\), and \(R_{3}\)) and a switching transistor. The transistor is utilized to modify and adjust the load impedances amongst low and high input powers. For low power levels, the transistor is in the OFF-state. Consequently, the output load corresponding to impedance, as depicted in Figure 9(b), is  
\[R_{eq\ OFF\ state}=R_{1}+R_{2}. \tag{10}\]  
As the input power rises, the transistor changes to an ON-state situation as shown in Figure 9(c). Thus, the output load corresponding to impedance is then  
\[R_{eq\ ON\ state}=\frac{(R_{1}+R_{2})(R_{3}+R_{T})}{R_{1}+R_{2}+R_{3}+R_{T}}. \tag{11}\]  
Note that at high driving voltages, the transistor equivalent resistance (\(R_{T}\)) can be neglected compared to \(R_{3}\), leading to a simplified load equivalent impedance expression as in (12).  
\[R_{eq\ ON\ state}=\frac{(R_{1}R_{3})+(R_{2}R_{3})}{R_{1}+R_{2}+R_{3}}. \tag{12}\]  
Figure 10: Operation simplification of the active load equivalent circuit configuration: (**a**) active load circuit, (**b**) OFF-state, (**c**) ON-state.  
Figure 9: Sweeping the adaptive rectifier loads versus input power level.  
Therefore, by applying active load modulation, a high and low load impedance can be achieved to produce a high output voltage at low power and prevent the rectifier from reaching its voltage breakdown at a high input power, thus maintaining high PCE over a wide range of input power levels.