Brush up on the theory before designing a high power Class‐E amplifier: Page 4 of 15

June 29, 2016 // By Alfio Scuto, Roberto Cammarata, ST Microelectronics
The most promising switched-mode PA for RF applications, among the several types available, is the Class-E for several reasons: high efficiency, simplicity of the load network, and a satisfying performance even with a non-optimal drive signal.

A feature of the Class-E operation that distinguishes this class from other switching-mode PA configurations is the so-called “soft switching.” In other words, the switch closes precisely at the instant where the shunt capacitor is completely discharged.

Therefore, to achieve the soft switching at the drain voltage, at the turn ON the output’s network must force the electrical boundary conditions as initially introduced by Sokal:

 → Equation 6

→ Equation 7


After some calculations, each component of the output network can be found by the following equations:

→ Equation 8


→ Equation 9


→ Equation 9


PDC and RDC are respectively the power absorbed from the power supply totally delivered to the load and the resistances the DC supply sees. The idealized model already presented in addition with the boundary conditions (Equation 6 and Equation 7) produces a theoretical power efficiency of 100%. In reality, the power efficiency will decrease because the ideal assumptions are not met and also because we must add the power losses of each component.

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