Etude et developpement d'architectures d'amplification a deux branches pour les systemes de communication sans fil

The design of RF power amplifier is becoming a crucial task in order to meet higher trade-offs between linearity and power efficiency. New spectral efficient modulations with high envelop dynamic lead in to poor performances when conventional single branch amplifiers are used. This work concerns the study of two branches amplification systems, particularly the symmetrical LINC technique and a second one with asymmetrical branches. The LINC method consists on converting variable envelop signals in two constant envelop components, having opposite phase, and which can be efficiently amplified by two identical amplifiers. Based on recent literature, we first investigated the performances and the behavior of the LINC system when both adapted and Chireix combiner are used. A study of this technique with modulated signals has been done. Then, we have presented a new technique to synthesize the Chireix combiner using digital predistorsion when excited with ideal sources. A mathematical formulation of this method has been developed to demonstrate the equivalence between physical and digital implementations of the Chireix combiner. Validations through simulations and experimental measurements of this new concept have been done. Next, a study of the LINC system with real amplifiers through realistic simulations and experimental measurements was done to describe its behavior and to build an efficient and linear LINC amplifier. Simulations have shown that phase predistorsion of the LINC system with Chireix combiner offers good linearity with a power efficiency outperforming the LINC system with stubless combiner. Experimental measurements partially confirmed the results obtained in the simulations because of the lack of imbalance of the implemented set-up. Besides, we have studied a new LINC based technique, called MILC method. We have presented the principles of this technique and studied its performances in the adaptive modulation context. Simulations and experimental results have shown that this technique offer better linearity vs. efficiency trade-off compared to conventional LINC technique.;Key words: Power Amplifier, Power efficiency, linearity, LINC.;In the second part of this thesis, we have investigated the performances of a novel amplification technique using two asymmetrical branches. The principle of this method is to split the input signal in two components. The first one has a constant envelop, or a reduced dynamic, containing most of the information and can be efficiently amplified with low distortion. The second component is the residual signal having an average power much below the main component and is amplified by a linear amplifier. The idea with such decomposition is to force the overall efficiency to be dominated by the main branch one. We have developed an analytical expression of the power efficiency of this system. Poor performances have been obtained when used the system is excited with MQAM signals. However, when CPM signals were considered, particularly MSK signal, we have obtained excellent power efficiency level with very good linearity expressed in terms of BER and/or ACPR. A comparison with single-ended class B amplifier shows that the proposed architecture offers better performances.