Jerry Miller from Anadigics discusses how high-efficiency power semiconductors extend battery life in wireless handsets

Wireless phones are fast moving from a simple voice communication device to the centre of the digital mobile lifestyle. In recent years, wireless users have moved beyond voice and basic text messaging to embrace Internet access, digital photography, MP3 audio and high-quality video.

Yet even as these potentially power-hungry capabilities grow in popularity, users still place a premium on longer battery life – they want the capabilities, but not necessarily at the expense of longer talk times and standby times.

Great progress has been made on the baseband side in achieving power-efficient operation. Intelligent power management dynamically controls power for maximum efficiency and lowest consumption. The strides made in power efficiency are clearly seen by simply comparing today’s phones to those of just a few years ago.

Researchers have now turned their attention to the radio section in general and the power amplifier in particular. By their nature, power amplifiers consume a considerable amount of power in a handset, which means saving power here can significantly affect battery life.

Power amplifier for dual-band W-CDMA

Various wireless standards specify a maximum output power from the amplifiers to drive the antenna. Amplifiers work fairly efficiently at high output levels. Phones seldom need to operate at full output power to maintain a strong connection and clear communications.

The actual output power levels are less than +10dBm for over 80 per cent of the time – rather than the +28dBm (for CDMA) maximum output capability of the amplifier. Users typically need full power less than 5 per cent of the time and most often during start‑up and connection while the base station determines the proper power level for the handset’s transmissions.

Base stations set the power levels for attached handsets to maintain all incoming signals at essentially the same received power level so that the station does not adjust its gain control up or down too far and thereby drop connections.

The challenge created by running a typical power amplifier at lower power levels is inefficiency. At +28dBm, the amplifier has an efficiency of around 40 per cent for CDMA and 42 per cent for WCDMA. Efficiency drops off precipitously at +16dBm to only 9 per cent. Quiescent current in the standby mode is 50mA. Thus power amplifiers operating at mid-range power levels offer a fruitful area for significant improvements in lowering power consumption.

HELP for low-power efficiency

Anadigics offers a new technology that more than doubles efficiency at 16dBm to 22 per cent, while offering a threefold reduction in quiescent current to 15mA. Based on typical power distributions, the increased efficiencies and lower quiescent currents cut power amplifier power consumption in half. We call this power-saving technology High-Efficiency-at-Low-Power, or HELP.

At the heart of HELP is the first affordable and practical fabrication process that allows both heterojunction bipolar and pHEMT field-effect transistors to be built on the same InGaP die. This allows the advantages of each process technology to be applied, while minimising the disadvantages for each.

For example, HBTs are used for amplifier stages for the excellent linear operation. pHEMT serves to create efficient switches because of its fast, efficient switching and low noise. The lower turn-on voltage for pHEMTs makes them far more efficient switches than those that can be obtained with HBTs.

The traditional approach to power control in a power amplifier is to use two states, switching between two levels the quiescent current (Icq) to both amplifier stages. The threshold between high and low currents is set at 16dBm.

Because this is the most prevalent method for lowering power consumption, it can be considered the baseline against which other approaches, such as external DC-to-DC converters or HELP technology, are compared.

The next step in lowering power consumption is to use an external DC-to-DC converter to switch the Vcc between two levels. While this approach lowers the average current consumption, it does so by adding external components that consume valuable board space and raise costs.

Since a clear driver in handset design is to save space and reduce costs wherever possible, adding an external DC-to-DC converter runs counter to the trend. A better solution relies on obtaining greater efficiency from the power amplifier itself.

A HELP-enabled amplifier is switched between high- and low-bias modes by the pHEMT switch according to the logic applied to the VMODE pin.

In operation, the amplifier uses bias-mode switching to select the amplifier chain and achieve high efficiency at lower levels. Above +16dBm, VMODE has 0 V applied to select the high-power operation. Below +16dBm, VMODE switches to +2.85V to allow high-efficiency low-power operation.

At low power, quiescent currents are reduced threefold. Average power consumption is also reduced by 50 per cent, surpassing the reduction of using an external DC-to-DC converter but without the additional expense or real estate.

The bias-mode switching of HELP can also be used with a DC-to-DC converter to achieve further efficiencies at power levels under 7dBm. While the external converter supplements the power savings of HELP technology, the incremental savings of converters may not be sufficient to justify the cost in additional components and required board space. HELP alone offers the best balance of improved performance, efficient use of board space, and low bill of materials.

Since PAE is determined by power levels, the reduction in current (and voltage) translates into greater efficiency at lower output levels. HELP can extend battery life by 30 per cent for the average user.

HELP technology is the first step in removing the power amplifiers as a major issue in battery life. The power amplifier may not be the last frontier in extending talk times and battery life, but it is certainly an area where new technologies are giving handset designers innovative options in managing the amplifier for higher efficiency. The end result is a phone with more features and longer talk time to provide mobile users with a rich multimedia experience to serve their digital lifestyle.

Jerry Miller is product line director at Anadigics

www.anadigics.com