RF power system efficiency is becoming increasingly important in low-power systems when it comes to wringing out the last minute of operation from a battery, and in high-power base stations where electric bills can be substantial. One obvious way to improve the situation is to emit only the needed RF power. This is relatively easy to do, as the amplifiers operate as AB amplifiers, and the drive signals can be backed off when lower output power is desired.
This can be taken a step further by reducing the supply voltage to the amplifiers when lower output power is desired. Figure 1 illustrates this; it presents amplifier efficiency as a function of output power for two different supply voltages. Lowering the supply voltage improves efficiency but limits how much power the amplifier can deliver.
Lower supply voltage improves efficiency but limits output power
Speed of response can be an issue in these types of systems, as the bandwidth of switching power supplies is usually limited to the tens of kilohertz, and the modulation requirements can be in the multiple megahertz range. There have been combinations of linear regulators and switching power supplies developed, such as the LM3290 and LM3291. These devices feature linear regulators with 50+ MHz control loop bandwidths coupled with a high-performance switching regulator.
The switching regulator provides enough headroom to the low dropout regulator (LDO) for envelope tracking while keeping the power loss low. There also have been efforts to improve control loop speeds by pushing the power supply switching well past 1 MHz with advanced switching devices such as GaN.
In addition to a fast control loop, an envelope-tracking power supply needs to source and sink current. That is, the power supply needs to take charge off the output capacitor to quickly reduce the output voltage, rather than letting the load discharge it. Otherwise, there can be a significant energy loss due to the discharge.