Active envelope load pull system suitable for high modulation rate multi-tone applications

Power amplifiers used in modern wireless systems need to efficiently transmit signals, without distortion, that have complex envelopes with variable phase and amplitude. The design of these power amplifiers require that transistors are characterized under complex excitations into variable load impedances, so that determination of perfect matching conditions for optimum performance is achieved. This paper presents a new active load pull system design, based on the envelope load-pull concept, which can provide for the required load-pull capability under high modulation rate multi-tone excitations. This performance is realized by incorporating analogue controlled delay elements into the envelope feedback loop to compensate for the inherent group delay encountered in the feedback loop. Measurements on a ldquoThrurdquo using this active load-pull system have demonstrated that it can provide constant, drive independent control of load impedances for multi-tone characterization of transistors and power amplifiers used in wireless communication systems such as GSM-EDGE.