Manufacturers of modern digital cellular telephones compete principally on the basis of price, size and battery lifetime. All of these factors are predictably affected by the selection of components within the phone, not least the transmitter amplifier. The efficiency with which dc energy from the battery is converted to radio frequency energy for voice transmission is crucial to the battery drain and the amount of heat generated by the amplifier which must be dissipated. The greater the power amplifier efficiency, the longer the battery lifetime and the smaller the heat sink required.
Typically, an existing amplifier in a digital cellular telephone is 35% efficient, with 65% of the power wasted as heat. This gives a typical talk time between battery charges of 2hr. By increasing the efficiency to 70%, only 30% needs to be dissipated as heat and the `talk’ time can be effectively doubled.
Efficiency, however, is not the only criteria in modern digital cellular amplifiers. Linearity of amplification is also a very critical parameter, in some cases requiring distortion levels much better than those achieved with the best `hi-fi’ equipment. Achieving linearity and efficiency at the same time is a very difficult technical challenge and the current range of products are a long way from achieving the theoretical maximum performance.
A number of new techniques has recently emerged, with the most popular being a technique known as Cartesian Loop linearisation. Wireless Systems International (WSI) has been a pioneer in the commercialisation of this technology, and, in conjunction with the US based IC house Maxim, is developing a Cartesian Loop IC for the handset market. This development will go some way to improving the efficiency of amplifiers (typically achieving 50%) and will certainly allow a cost saving to manufacturers of mobile phone equipment.
Recently, however, a new approach to power amplification has been developed by researchers at Bristol University. Called Callum (Combined Analog Locked Loop Linear Modulator), it has the potential for much higher linearity and efficiency than any previous technique. Indeed, a development programme at WSI aims to prove the potential for such a technique by developing hardware based around the University’s concept.
The technique itself uniquely synthesises the output signal from two or more constant amplitude RF phase-modulated sources, much like the synthesis employed in modern keyboards. It can be viewed as a pair of cross-coupled phase locked loops, excited by the modulating waveform represented with Cartesian components. WSI believes that a further improvement can be obtained by adding a high-level modulation stage, as shown in Figure 1.
The overriding advantage to this approach is that the phase modulated components can be amplified prior to summation using highly efficient non-linear power stages, eg switching amplifiers.
To date, computer simulation of the technique has shown that the approach has the potential to achieve linearity in excess of that required for digital cellular, with projected efficiencies of 60% to 80%. This, it is claimed, is substantially better than that obtained by other linear power amplifier solutions in the market.