Highly efficient class E amplifiers

Modern portable telephones are required to operate for long periods from a small battery pack. This paper reviews previous work and describes a high efficiency (72%) 0.5 watt distributed Class E power amplifier operating at 1 GHz. Class E amplifiers are a type of switching amplifier where the losses normally associated with the `shorting out´ of the device shunt capacitance are negated by designing the load network to define the voltage across the switching device when it is off. These amplifiers unlike class C amplifiers offer the potential of 100% efficiency with a specified output power. To obtain minimum losses the voltage across the switch, when it is off, is arranged to ring and fall back to zero just before the switch turns on again. The design criteria for the voltage are: the voltage rises slowly at switch off; the voltage falls to zero by the end of the half-cycle; and the voltage has a zero rate of change at the end of the half cycle. A typical circuit is shown and it consists of a bias choke, a (transistor) switch, a shunt capacitor which incorporates the parasitic capacitance of the device, a series tuned LCR resonant circuit with an extra series reactance (inductance) which sets the load angle to obtain correct collector voltage waveform