Brush up on the theory before designing a high power Class‐E amplifier

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.

1. Abstract

This article covers the Class-E theoretical basis initially introduced in the 1970’s by Dr. N.O. Sokal and A.D. Sokal (1) and the idealized operation mathematically explained by Dr. Raab (2). Then we’ll discuss the effects caused by the various non-ideal effects present in the circuit (3), and finally, we’ll present a 500-W class-E amplifier designed with two SD4933 from STMicroelectronics in parallel.

For Class-E amplifiers, the theory offers useful equations for starting a new design, all obtained by making mathematical assumptions for the voltage at the device's drain and considering lossless components. Nevertheless when all these hypotheses are gradually removed, it's difficult to obtain closed-form equations, and the circuit analysis can be done using Electronic Design Automation software (EDA).

 

2. Idealized Class‐E operation

The basic form of a Class-E power amplifier is shown in Figure 1. The topology consists of a transistor, a shunt capacitor C, RF choke L1, and a series L2-C2 resonator followed by the load resistance R.


Figure 1: Basic Class-E amplifier.

The Class-E amplifier works in switched mode — the transistor is driven hard enough to saturate in order to act as a switch with two discrete states, ON and OFF. The circuit operation is determined by the transistor when it is ON and by the transient response of the load network when the transistor is OFF. The previous circuit is equivalent to the one shown in Figure 2.


Figure 2: Equivalent schematic of the Class-E circuit.

In Figure 2, the transistor has been replaced by a switch—the capacitor C1 comprises the external capacitance, C, and the transistor’s output capacitance. The series combination L2-C2 is tuned at the frequency of operation ωc. Moreover, the series reactance jX (at the switching frequency fsw) can model a circuit mistuning or a change of the operating frequency.

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