The process of designing traditional fixed supply power amplifiers (PAs) has been well established for many years. Well defined metrics for performance assessment exist, and the PA designer’s job is ‘simply’ to design a PA with the best set of performance metrics. In reality of course this is far from a ‘simple’ task, but at least the assessment criteria are well established and well understood. For envelope-tracking (ET) PAs, the situation is more complex and requires the use of more sophisticated characterization techniques.
The objective of envelope tracking is to improve the efficiency of PAs carrying high peak to average power (PAPR) signals. The drive to achieve high data throughput within limited spectrum resources requires the use of linear modulation with high PAPR. Unfortunately, the efficiency of traditional fixed supply PAs when operated under these conditions is very poor. The efficiency of an ET PA is improved by varying the PA supply voltage in synchronism with the envelope of the RF signal. The PA’s fundamental output characteristics (power, efficiency, gain, phase...) now depends on two ‘control’ inputs (RF input power and supply voltage) and may be represented as 3D surfaces.
In a typical envelope tracking system, the supply voltage is dynamically adjusted to track the RF envelope at high instantaneous power. Here, the PA operates with high efficiency in compression. Its output characteristics are primarily determined by the instantaneous supply voltage. Conversely, when the instantaneous RF power is low, the supply voltage is held substantially constant and the PA output characteristics are primarily determined by the instantaneous input power (linear region). A transition region in which both supply voltage and input power influence the output characteristics exists between these two extremes (see Figure 1)
Figure 1 Instantaneous efficiency vs supply voltage – Isogain shaping