RF Power Amplifiers for Wireless Communications (RF vermogenversterkers voor draadloze communicaties)

Latest standards for wireless communications have resorted to the Orthogonal Frequency Division Multiplexing (OFDM) technique to reduce channelimpairments. Multipath fading and intersymbol interference (ISI) can belimited with the use of multiple orthogonal sub-carriers. Nevertheless this OFDM signal exhibits large instantaneous signal amplitudes comparedto its average values. This compels the use of linear amplifiers working with several dB in backoff from their 1 dB compression point (P1dB). In this way, the entire OFDM signal is amplified linearly; however at thecost of a very poor efficiency. This increases the energy consumption and reduces the battery life of mobile terminals. Hence, providing linearamplification with high efficiency has become one of the major challenges for current RF amplifier design for wireless communications. Several techniques are under research to tackle this problem. These techniques can be classified as efficiency and linearity oriented techniques. Predistortion, feedforward and feedback techniques are examples of the first sort of techniques, which mainly correct the inherent distortionof the amplifier by providing an inversion function of the amplifier orby comparison with the original input signal. Examples of the second type of technique are dynamic biasing, dynamic load and Envelope Elimination and Restoration (EER), which basically vary the bias or load of the amplifier according to the input signal. This thesis has focused on Doherty amplifiers (dynamic load technique) and high efficiency class-E amplifiers (main amplifier in EER) applied to WiMAX application at 3.5 GHz. In the design process of these amplifiers, the concept of equivalent capacitance is introduced and applied successfully. Measurements of the designed Doherty amplifier confirm the potential benefits of this technique in terms of overall efficiency and linearity in comparison with legacy class-AB amplifier. Nonetheless, this design task is not possible without a reliable and sturdy device model. Hence, the task of constructing a non-linear model for a GaAs HEMT device was undertaken in this thesis, which gave satisfactory outcomes. The conjunction of this reliable model with the equivalent capacitance concept has made possible the contemporization of aged techniques to face the stringent requirements of modern wireless communications.status: publishe