11
High-Efficiency Power Amplifiers
High efficiency of the power amplifier can be obtained by using Class D, Class F, or Class E operation modes and their subclasses, depending on the technical requirements. In all cases, an efficiency improvement in practical implementation is achieved by providing the nonlinear operation conditions when an active device can simultaneously operate in pinch-off, active, and saturation regions resulting in nonsinusoidal collector current and voltage waveforms, symmetrical for Class D and Class F and asymmetrical for Class E (DE, FE) operation modes. In Class F amplifiers analyzed in frequency domain, the fundamental-frequency and harmonic load impedances are optimized by short-circuit termination and open-circuit peaking to control the voltage and current waveforms at the device output to obtain maximum efficiency. In Class E amplifiers analyzed in time domain, an efficiency improvement is achieved by realizing the on/off active device switching operation (the pinch-off and saturation modes) with special current and voltage waveforms so that high voltage and high current do not concur at the same time.
11.1 CLASS D
The efficiency of a power amplifier can be maximized if the active device is ideally operated as a switch. When the transistor is turned on, the voltage is nearly zero and high current is flowing through the device; that is, the transistor acts as a low resistance (closed switch) during this part of a period. When the transistor is turned ...
