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. ET Basics 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
Almost all ET PAs are used in Class AB mode. And while the AM:PM distortion can't directly be controlled by the supply voltage, our experience with the vast majority of handset PAs is that correcting the AM:AM using IsoGain ET also brings the phase response into line. And simple memoryless correction of AM:PM ("DPD-Lite") is an increasingly common capability of the latest handset chipsets.
I agree that an envelope-tracking amplifier does not have to be a "class C" amplifier, although that would provide the greatest efficiency, it could easily cause some distortion. The other comment is certainly valid, which is that it takes a great deal more effort to design an amplifier that will deliver a satisfactory level of distortion at various operating voltages. Phase modulation certainly does happen and it must be compensated for in order for the distortion level to be acceptable, and designing the correct compensation will certainly add a lot to the design effort.
The surface response of an ET PA is more complex than a simple 3D RF-input vs battery-supply. Phase pushing and pulling will occur in such a 3D ET control solution.
One comment on the transistion region to be noted; it is the same break-point as system modulation pre-distortion uses. There are two other break-points needed to arrive at the most effective ET modulation efficiency which are not mentioned in the text.
The objection of the envelope tracking transmitters is to improve efficiency of the output signal.
It is collecting the input signal data and replaces it with something that we do not know at the time.
Envelope tracking reminds me of the two modulated stages in an AM transmitter with class "C" driver and output stages. Much more efficient than anything linear. But more demanding on the output filtering section.