A simple ‘quasi static’ (i.e. memoryless) behavioral model of a PA may be constructed if its AM/AM and AM/PM characteristics are known. These characteristics, along with other key PA metrics such as power and efficiency are profoundly influenced by the mapping between instantaneous RF envelope and applied supply voltage. In an ET system this mapping is determined by the contents of a ‘shaping table’ in the envelope path (see Figure 2)
Figure 2 Envelope Tracking PA System
A mapping of particular interest is ‘ISOgain’ shaping, in which the instantaneous supply voltage is chosen to achieve a particular constant PA gain (see Figure 3).
Figure 3: RF O/P power - Supply voltage mapping - ISOGain shaping
With this mapping, the ET PA system achieves low AM-AM distortion despite operating in compression over much of the envelope cycle as shown in Figure 4. The equivalent trajectory for fixed supply operation is also shown in Figure 4 – from this it is apparent that ET can actually be used to linearize a PA, reducing ACPR and EVM.
Figure 4: ET PA gain characteristics - Isogain shaping
The system trade-off associated with using the shaping table to linearize the PA is a small loss of efficiency (compare Figure 1 and Figure 5) for a substantial improvement in linearity (compare Figure 4 and Figure 6). The choice of shaping function also has a strong influence on the bandwidth requirement of the envelope path. A smooth transition between the linear and compressed regions results in a lower bandwidth requirement for the envelope amplifier for modest (1-2%) loss in system efficiency.
Figure 5: ET PA Efficiency - Optimum efficiency shaping
Figure 6: ET PA gain characteristics - Optimum efficiency shaping
When designing a fixed supply linear PA, a great deal of attention must be paid to the achieving adequate linearity characteristics at maximum output power. Many factors influence the linearity (e.g. fundamental technology characteristics, biasing and RF matching) and it is up to the PA designer to achieve the best trade-off between efficiency and linearity. For an ET PA, things are different, as the linearity of an ET PA in the compressed region is no longer a self contained PA parameter. The PA still has to be linear in the low power, low voltage region, but at higher powers the AM linearity constraint is removed and the PA can be designed for optimum ET efficiency without regard to AM linearity. Unlike AM distortion, phase distortion is not directly controlled by the envelope shaping table. However, it is observed that many PAs actually show reduced PM distortion when operated in ET mode.
As a result of this ‘self linearization’, it is possible to push harder into compression at signal peaks than with a fixed supply amplifier, allowing increased output power for given linearity. Figure 7 shows measured ACLR and EVM performance for a PA operated in Fixed Supply and ET modes. In this example the PA output power for -40dBc ACLR is 2dB higher in ET than fixed supply mode.
Figure 7: ET PA / Fixed supply PA linearity comparison