3-coil system Class-E power amplifier capacitive wireless power transfer circuit modeling coupling coefficient design guidelines design optimization electric field finite element analysis gallium nitride gradient methods impedance matrix inductive coupling inductive power transmission multiple coils multiple-input single-output multiports mutual inductance numerical analysis optimal load output characteristics power gain power measurement power transfer efficiency (PTE) power-transfer efficiency resonance resonance frequency resonance-based wireless power transfer (R-WPT) resonant scattering matrix