Analysis and Design of Multi-Stage Doherty Power Amplifiers for Wireless Communications

Most modern wireless communication systems employ variable-envelope modulations, causing the Power Amplifiers to work at low efficiency levels. Several solutions to this have been proposed in the literature and the Doherty Power Amplifier (DPA) is one of the most promising ones. Despite being introduced in 1936, the efficiency enhancement scheme devised by Doherty can be applied to modern FET-based RF Power Amplifers with good results. This works explores the problem of the low efficiency levels in backoff and how the active load modulation can be used to counter this effect. Theory about the conventional two-device Doherty is developed and extended to the three-device case, considering two different implementations. The theoretical results are accompanied by two design examples in order to validate the design equations and show the improved efficiency brought by the DPA. The designed amplifiers manage to keep a high efficiency level, over 60%, over an Output BackOff (obo) power of 6dB and 10dB, for the two and three device DPA respectively.