class E amplifier
Class E employs a single transistor operated as a switch shunted by a capacitor. In addition to this shunt capacitor, a series passive load network (LCR) is also connected across the transistor. As with the Class D amplifier case, the Q-factor of the resonator is assumed to be high enough to force a pure sinewave current (due to the flywheel effect) into the LCR branch. The DC voltage is supplied through a high-reactance choke. Two currents combine to flow into the switch-capacitor combination; the dc supply current and the sinusoidal resonant current in the LCR branch. When the switch (transistor) is ON, the collector is shunted to ground, current through the capacitor is also zero. When the transistor is OFF, the collector voltage waveform is produced by charging of the shunt capacitor. When switch changes from OFF to ON, the charge in the capacitor is rapidly discharged. Figure 3.8 shows the voltage and current waveforms for a Class E amplifier. The key point of operation is the near instantaneous transfer of current from the transistor to capacitor when the switch opens. The current and the voltage never coexist and hence there is theoretical 100% efficient conversion of DC to RF. MOSFET Class E amplifiers have been used as high-efficiency, high frequency switching amplifiers with power levels up to 1kW .
The Class F amplifier is one of the oldest techniques available to improve efficiency. This approach uses harmonic resonators in the output network to shape the output waveforms. A class F amplifier is nothing but a class B amplifier designed using an odd harmonic trap1 connected between the supply and the active device. This configuration gives better performance than with an inductive choke which is limited by its Q-factor. The transistor output is connected to a load via a high Q resonator, so as to provide open-circuit terminations at higher odd harmonics. Figure shows the voltage and current waveform a class F configuration. Since the devices are biased in Class B operation, the current waveform will be a half-sinusoidal waveform. The voltage waveform appears to have a square-wave like appearance due to the quarterwave even-harmonic short, coupled with the clamping effect occuring near the turn-on region of the transistor. Alternatively in an inverse class F configuration, the voltage is approximately a half sine wave and current is approximately a square wave. Squaring up of the sinewave can be improved by adding higher order odd harmonic traps which lead to an ideal class D amplifier. At any instant of time the voltage and current are not present simultaneously and hence the theoretical maximum efficiency is again 100 %.