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Envelope tracking (ET) is a strong contender architecture for enhancing the power efficiency performance of power amplifiers (PAs) in emerging communication systems. However, the design and characterisation of envelope tracking power amplifiers (ET-PAs) introduces a number of significant technical challenges related to the optimisation and interaction of the numerous subsystems involved, namely the PA itself, envelope detection/generation, the supply modulator and linearisation elements. This Ph.D. research extends the current state of the art in ET-PA measurement and characterisation and considers new measurement and characterisation capabilities that provide for the rapid development of ET-PA architectures. The research starts by fully implementing a new ET-PA measurement system and includes the characterisation and validation of the requirements for such a system. Following this, the realised system is used to investigate the important area of interaction between an PA and a supply modulator in the presence of voltage ripple representative of an actual switching modulator. By varying the ripple magnitude as a proportion of the modulated drain voltage, the effects on the linearity of the PA are observed and analysed, providing the system designer with insight into the amount of ripple that is tolerable, and at what cost in terms of other key parameters. Additionally, potential countermeasures including digital pre-distortion (DPD) and shaping function optimisation are explored and the influence of the ripple magnitude on an ET-PA is quantified. The second part of the thesis presents an integration of a modulated active load-pull system, allowing simultaneous broadband impedance environment emulation and DPD linearisation, in one integrated measurement system. This novel combination allows investigation of for example, how well a microwave power transistor, operating in an optimal RF impedance environment, responds to linearisation with DPD techniques. Following this demonstration, a fully emulated ET-PA environment is realised by adding a dynamic supply voltage capability, and excited using industry-standard modulated. As a result, a measurement setup has been demonstrated that enables the PA designer to characterise device operation within fully emulated PA modes of operation, under realistic modulated signal conditions, as well as allowing, in real time, the rapid investigation into how well these modes respond simultaneously to ET and DPD techniques.