wireless industry
components and configurations needed to cover the required transmit bands and modulations (see Figure 5). Many alternatives are inferior, because they solve similar problems with techniques that potentially add to the cost and complexity of each component.
We can illustrate this point with a block diagram of an ET design that we have put together. (see Figure 5). Further improvements in high-power efficiency may result from adding pre-distortion – this could increase high-power efficiency by allowing the PA to operate closer to compression (see Figure 3 for an example of how this can increase system efficiency).
However, effective, high-bandwidth envelope tracking demands a DC-DC converter that is capable of delivering the rapid, accurate changes in voltage required to follow the envelope of high PAR modulations. Unfortunately, gains in PA efficiency resulting from this approach are offset by declines in the conversion efficiency of the ET DC-DC converter and the circuitry needed to rapidly follow the modulation envelope (operating at a wider bandwidth reduces the converter efficiency). The good news, however, is that the system ‘up-lift’ is favourable. In other words, the PA efficiency improvement, minus the ‘cost’ of the ET implementation circuitry, is beneficial rather than detrimental.
As transmit power is lowered, a point can be reached where the PA benefit is effectively cancelled by the conversion efficiency of the ET circuitry. When this happens, the efficiency enhancement method must be changed for optimum performance. Several options are available: Switching the system to an APT mode with greater DC-DC conversion efficiencies; adjusting PA bias, or using similar low-power efficiency enhancement techniques; or adopting optimum combinations of these techniques, which come together to ensure high- efficiency transmit operation over the full range of conditions.
Bringing it all together We believe that these new generations of RF platforms will require closer co-ordination of transceiver, power management and power-amplifier blocks. These latest designs must also be capable of handling complex communication, as well as timing signals to ensure proper operation with the transceiver – there needs to be a seamless, well executed transition between the various efficiency enhancement techniques providing transmit operation over a wide range of modulations, frequencies and power levels. Desirable qualities from the power management include rapid, accurate changes in voltage capable of following a high PAR envelope if ET is used, plus the prevention of excessive switching noise, preservation of spectral purity and maintenance of sufficiently high power conversion efficiencies to reap the rewards of this approach.
Additionally, the PA must be designed with the optimum load line for operation across the dynamic range. This implies proper device scaling, and careful trade-offs, such as judicious selection of the supply capacitance so that the smart device fulfils the complex requirements of the wireless network.
It is clear that for the foreseeable future, consumer demand for mobility and high data rate connectivity in smart devices will continue to drive complexity and challenges in the RF front-end market. In response, engineers will continue to improve and optimise the energy usage, thermal performance and battery life of modern data devices through innovation in the individual components, as well as integration of these components into a cohesive RF platform.
© 2012 Angel Business Communications. Permission required.
Figure 4. A graphic
representation of the PAE improvements for several enhancement methods, neglecting conversion efficiency
associated with implementation
Figure 5.
A representation of an RF front end showing PA power management (DC-DC
converter) to multiple PA components
July 2012
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