The properties of envelope tracking DPS transmitters from Chapters 5 and 7 and those of direct polar DPS transmitters from Chapters 6 and 8 are often in opposition to each other. A concise summary of the defining properties for the envelope tracking and DP subsets of DPS transmitters is presented in Figure 11-1. The transition between these two DPST operating modes is initially explored in Chapter 10, which shows that the transition between ET and DP is strongly dependent on the transistor technology being used. Indeed, some transistor technologies never actually reach the ET operation but rather stay in this transition region.
When one or more operating modes of any circuit exhibit widely separated properties, a natural thought is to find methods to combine the operations so that the most desirable properties are achieved, or that undesirable properties are avoided. This directly leads to hybrid designs, and is the topic of this chapter.
Hybrid operation overview
Chapter 6 showed that optimizing an ET design for higher energy efficiency necessarily results in polar operation. To get the RF PA at its highest energy efficiency, it must shift from linear to compressed operation. Through this transition, the power supply noise suppression is eliminated from the increasingly nonlinear power amplifier and the precise value of the power supply becomes critical as it comes to directly control the envelope modulation. Signal drive must also increase as the gain drops due to circuit compression to maintain the output power. Such is the price of high efficiency operation.
Transitions from linear through ET to DP and back are summarized at a high level in Table 11-1. Activity requirements on the DPS and the associated PA linearity are well discussed in the prior chapters. The important additions in Table 11-1 are the associated modulation transfer functions for each of the DPST operating modes. The differences among these transfer functions points out the major challenge in getting any hybrid DPST implementation to work.