industry wireless
compression point (see Figure 3 for the compression curve and efficiency of a typical power amplifier). However, when PAR increases, the PA spends more time operating further from compression. In other words, efficiency is compromised to maintain sufficient linearity and ultimately preserve peak amplitudes.
Transmission: The big battery drainer When smart devices are transmitting, especially at higher power levels, the power amplifier draws a great deal of energy from the battery. The time that the smart device can go between charging, which is also known as the battery life, is governed by the efficiency of the amplifier portion of the RF front-end. To extend battery life, engineers are developing amplifier front-end designs that excel in efficiency and linearity, so that the PA can operate closer to compression and thus hit higher efficiency.
There are several approaches for meeting these goals. One is pre-distortion – making the system compensate for PA non-linearities by applying inverse non-linearities to the amplifier’s input signal, so that the PA operates closer to compression. Alternatively, one can use ‘feed- forward’, a technique based on correction of non- linearity by subtracting an estimate of the non-linearity at the PA output. Yet another option is ‘feedback (Cartesian or Polar)’: Detecting the output signal,
comparing it to the desired input signal and correcting the input. And last but by no means least, there is ‘envelope elimination and restoration’ (EER). This involves amplification of a constant envelope signal featuring phase information, onto which is superimposed amplitude information produced by modulating the amplifier’s power supply.
At RF Micro Devices, which is headquartered in Greensboro, NC, we believe that envelope tracking (ET) – a variation of EER with additional back-off to guarantee linearity – is rapidly rising to the top as the most effective power optimization technology for higher power RF transmission. One of its greatest attributes is that it works well across a wide frequency range, a wide bandwidth and across multiple modulations. This makes it well suited to satisfying the needs of the global smart device market. However, it’s worth noting that in some applications the best performance might result from a combination of technologies previously discussed (see Figure 4 for a graphic representation of relative efficiency improvements for several methods).
Throttling back
Figure 3. The
compression curve and power-added efficiency (PAE) for a typical power amplifier showing the effect of greater PAR
28
www.compoundsemiconductor.net July 2012
In modern digital networks, power control schemes reduce the total transmit power during the times when full power is not necessary. The benefits are not limited to saving energy, as this approach also minimizes unnecessary interference, leading to a higher system capacity. The key to working well in this type of network is to design the RF front-end for efficient transmission over a large dynamic range of output powers. There are many ways to enhance RF efficiency over a large dynamic range. Two of the most obvious are to turn off the final PA stages when the power is low enough to be delivered by just the driver stages, and to trim the bias current at lower powers to cut current consumption. Alternatively, engineers can adopt a Doherty configuration, tuning the main amplifier to favourable (high) impedance at low power and turning on an auxiliary amplifier at higher powers to maintain correct impedance. Another option is to employ load manipulation techniques, such as ‘chain matching’ or ‘load switching’, with favourable impedances maintained over a wide power range through the switching in and out of matching elements. And there is also average power tracking (APT) or multi-state power management. With this approach, the PA is kept close to saturation by turning down the amplifier’s power supply voltage when the average power falls.
In practice there are trade-offs in complexity, cost and benefit for all these techniques that can deliver efficiency enhancement over wide ranges of modulations, frequency bands, bandwidths, and output power requirements. However, we have seen a rise in the popularity of systems taking advantage of PA power management, such as DC-DC converters. Go down this route and a single converter technology can be cost- effectively applied across all the different PA
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