Analysis and Design of Class-E_M Power Amplifier at Subnominal Operation

In this paper, an analytical design procedure for subnominal class $E_{M}$ power amplifier (PA) is proposed. The main circuit only satisfies zero voltage switching (ZVS) and zero current switching (ZCS) conditions, which provide two extra design freedoms compared to the traditional class $E_{M}$ PA. All the parameters in the circuit can be expressed as a function of the two design freedoms, and more selectable circuits with higher efficiency can be provided. Meanwhile, the design freedoms can also be applied to improve circuit performance by selecting suitable values. To validate the analytical process, two subnominal class $E_{M}$ circuits are fabricated and measured with specific design freedoms at the operating frequency 2 MHz. The obtained output power for the two designs are 5.92 W and 13.45 W, and the efficiency can reach 96.1% and 97.3%, respectively. Compared to the traditional class $E_{M}$ PA, the transistor peak voltages of the main and auxiliary circuits in the first design can be reduced by 10% and 27%, and the transistor peak voltage of the auxiliary circuit in the second design is reduced by 25%, which demonstrates that the subnominal operation can be well applied for decreasing the transistor voltage stress. The experimental results show the validity of the presented theoretical analysis.