High-efficiency linear RF power amplification : a class-E based EER study case

Abstract

has put some of the old and still unresolved problems in the field of radio-frequency (RF) power amplification again in focus. Battery-operated handheld devices demand highly efficient RF power amplifiers (PA) in their transceivers in order to maximize the operating time between charging cycles. Since the PA is usually the most power-hungry building block of the transceiver, and even can dominate the power consumption of the whole handset, improvement of its efficiency is of major importance. On the other hand, high data rates in modern wireless standards mandate efficient use of the available spectrum. Spectrally efficient modulation schemes produce variable envelope RF signals that require linear and thus power-inefficient amplification. The work described in this thesis summarizes the research into possibilities of achieving high-efficiency linear RF power amplification, particularly for portable applications. Three possible PA/transmitter architectures have been considered: envelope elimination and restoration (EER), linear amplification with nonlinear components (LINC), and pulsemodulated PA systems. The benefits and limitations of these concepts are compared, in the context of the characteristics of the signals used in modern wireless systems and characteristics of the state-of-the art semiconductor technologies. The conclusion has been drawn that the most promising architecture is EER, and the feasibility of this concept has been further investigated in detail. Switched-mode Class-E power amplifiers prove to be prime candidates for the realization of EER systems and as such have been thoroughly investigated in the thesis. Extensive time-domain circuit simulations have been carried out that indicate that Class-E PAs, if properly employed in an appropriate EER scheme, can satisfy the stringent linearity requirements of the UMTS system. This was the first such quantitative demonstration in the literature. The design of Class-E PAs for use in the EER architecture has been discussed and inherent causes of intermodulation distortion of such an EER system have been identified. The influence of linear as well as nonlinear distortion of both the ampli- tude and phase path in an EER system has been clarified, and guidelines derived on the design of the system for a given linearity specification. Furthermore, novel explicit design equations have been derived for designing Class-E PAs with small DC-feed inductance. Two different approaches for supply modulation in the EER architecture have been considered: the linear (resistive) voltage regulation, and the pulse-width-modulation (PWM) based scheme. While the resistive regulation has excellent properties in terms of bandwidth and robustness, it suffers from a poor efficiency and thus proves unattractive for EER applications. The PWM-based supply regulation scheme offers much better overall efficiency, but is associated with PWM-inherent nonlinear distortion. A detailed analysis of this PWM-related distortion has been carried out and guidelines on how to design the system for a given linearity specification have been derived. The theoretical work has been followed by two design examples: a GaAs HBT-based Class-E PA for 2 GHz, and a two-stage GaAs PHEMT-based PA, also for operation at 2 GHz. Simulations indicate that the PAs exhibit both AM-AM and AM-PM effects. Excellent efficiency performance has been measured in the case of the GaAs HBT PA, whereas the other design exhibited discrepancies with regard to simulations. The possible cause for this discrepancy are insufficiently accurate transistor models obtained from the foundry. The work presented in the thesis shows that the EER concept is feasible as a means for high-efficiency linear power amplification over a wide dynamic range of the signal, but that the PA-inherent AM-AM and AM-PM effects must be resolved at the system level, in the form of DSP-based predistortion of the signal. Furthermore, a careful dynamic bias control of a multistage PA is needed to ensure optimal efficiency at all output power levels.