## 3 Fabrication of Class-E Amplifier and Experiment Verification  
To verify the previous power transfer efficiency analysis and the effect of MMs, a high-frequency Class-E power drive was designed. The Class-E amplifier has a simple topology and operating frequency up to the MHz level. It is suitable to be used as a high frequency excitation source for the WPT system with the MM. The Class-E amplifier consists of the DC voltage source \(V_{I}\), RF choke \(L_{f}\), shunt capacitance, \(C_{1}\), Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), \(S\), resonant capacitance, \(C\), and inductor, \(L\). It can reach the soft switching state with the appropriate parameter of the electrical component [28, 29, 30]. Furthermore, the RF Class-E topology needs to be chosen to meet the four-coil system property. Figure 9 shows the circuit sketch of the WPT system with the Class-E amplifier. We set the equivalent output resistance of the Class-E to 10 \(\Omega\), namely, the \(Zin\) needed to be adjusted to 10 \(\Omega\) by varying the distance between coils. The operating frequency of system was 2.80 MHz. The quality factor was set as 8. The rest of the parameters were calculated by expression (10). Table 2 lists the parameters of the Class-E amplifier. \[\left\{ \begin{array}{l}L_{f}=2(\frac{\pi^{2}}{4}+1)\frac{R}{f}\approx\frac{7 R}{f}\\ C_{1}=\frac{8}{\pi(\pi^{2}+4)wR}\\ C=\frac{1}{wR[Q-\frac{\pi(\pi^{2}-4)}{16}]}\\ L=\frac{\text{Q}R}{w}\end{array} \right. \tag{10}\]  
Figure 8: The relative permeability as a function of the resonance frequency of the MM unit cell by varying: (**a**) The width of the copper strip; (**b**) the interval between the copper strips; (**c**) the turn number of the copper strip; and (**d**) the thickness of the substrate. A few megahertz switching frequency and hundreds of watts can be achieved by the using of silicon carbide devices (C2M008012D), and its conduction resistance and parasitic capacitance are very tiny. The driver chip (IXDD614) was employed in this circuit template, as shown in Figure 10a. Figure 10b shows that voltage waveform of the test load resistance, the waveform was a sinusoidal wave approximately and its working frequency was 2.80 MHz. The duty ratio of the signal was less than 50%, ensuring that the amplifier operated in a soft-switched state. To verify the proposed the method, a four-coil WPT system with the MMs was built, as shown in Figure 11. The radius of the \(Tx\) coil was set to 25 cm, a turn of 10, and a pitch of 4 cm, with the radius of the copper tube as 4 mm, the same as the Rx coil. Additionally, both the drive loop and the load loop were the single turn circular coil with a diameter of 40 cm. The additional capacitor was \begin{table}
\begin{tabular}{c c c} \hline \hline
**Meaning** & **Symbol** & **Value** \\ \hline Resonant frequency & f & 2.80 MHz \\ Shunt capacitance & C\({}_{0}\) & 1.04 nF \\ Resonant inductance & L & 4.54 uH \\ Resonant capacitance & C & 0.83 nF \\ Quality factor & Q & 8 \\ Choke inductance & L\({}_{0}\) & 31.56 uH \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the proposed Class-E amplifier. Figure 10: **(a)** The physical circuit board of each component of the Class-E amplifier. **(b)** The voltage waveform of the test load resistance. Figure 9: The equivalent circuit diagram with the Class-E amplifier. welded to the four coils to achieve a resonant frequency of 2.80 MHz. The parameters of the four-coil WPT system is shown in Table 3. For simplification, the skin effect and proximity effect were ignored. The unit cells were assembled periodically as a \(5\times 5\) MMs slab. The MMs slab was located between the \(Tx\) coil and the \(Rx\) coil. All the modules, including the drive loop, the \(Tx\) coil, the MMs slab, the \(Rx\) coil, and the load loop, were placed coaxially. The DC power source supplied energy for the Class-E amplifier, the signal generator emitted the high frequency PWM electrical level, and the oscilloscope measures the waveform of the output resistance.