Low Voltage Power Amplifiers for GSM Handsets
As each successive generation of cellular telephones becomes smaller and lighter, so must the battery technologies that power them. The new generation of cellular telephones will incorporate batteries with low operating voltages between 2.8 and 3.5 V. The handset manufacturers also must maintain or improve their talk time in order to offer competitive products. This combination of low drain voltage and high efficiency presents a unique challenge to RF power amplifier (PA) suppliers.
Two new products that address this problem have been introduced for the Global System for Mobile communications (GSM) standard. Both products use a 0.5 mm epitaxy-based MESFET technology capable of providing optimum performance at lower voltages. The model AP112 900 MHz PA is designed for the GSM900 cellular market; the model AP114 PA is tuned externally for use in either the 1800 MHz digital communications system (DCS) band or the 1900 MHz US personal communications service (PCS) band. A single power source solution is achieved for either PA using the model AM100 DC/DC converter. Currently in development, the model AP116 dual-band amplifier can be used for the GSM900 and DCS900 frequency bands.
This article discusses the performance specifications and typical production variations of each of these products, and addresses the integration of the amplifiers in a handset application. The most critical parameters for any GSM PA are output power and efficiency. The output power of the telephone must be at least 33 dBm. Because the signal must incur losses from the filter and switch before reaching the antenna, the PA must deliver greater than 34.5 dBm to meet the desired telephone output performance. Once this output power level has been achieved, the benchmark for PA performance becomes the power-added efficiency (PAE). The higher the PAE, the greater the telephone’s talk time.
Another benchmark is how well the PA performs at reduced battery voltages. As the battery in the telephone wears down, the voltage it produces drops. At some voltage point, the PA will not meet certain minimum performance standards and the telephone will have to be shut down. This power-down capability is particularly important in telephones that use lithium ion batteries since they have a much more gradual decline in voltage over time than the larger NiCd batteries.
Although specified for a minimum of 48 percent efficiency, the AP112-79 GSM PA typically offers 55 percent PAE along with 3.5 V operation. Even at a reduced voltage of 2.8 V, the PA’s efficiency is maintained at a minimum of 50 percent. In addition, the device exceeds the minimum specifications by delivering up to 35 dBm output power with a 50 dB power control range. This low cost GSM PA is designed for the 880 to 915 MHz frequency band and is supplied in an SSOP-16 package with heat sink. Figure 1 shows the PA’s power output, gain and PAE at 902 MHz for 2.8 and 3.5 Vdd.
To improve performance, reduce cost and add flexibility, the output matching circuits for most MMIC PAs are provided externally. By making use of ceramic surface-mount components with better Qs than GaAs matching elements, a lower loss matching network is achieved, resulting in higher power and efficiency for the amplifier. In addition, by keeping these elements external, the GaAs die size and overall cost are reduced. This approach also permits the amplifier to be tweaked for optimum performance at different powers and/or frequencies.
After selecting a PA that meets the critical performance parameters, the handset manufacturer must determine how to control the output power. The recommended power control method is to control the gate voltage with the DC/DC converter and a level shifter circuit. A true single-supply solution is accomplished by implementing this combination of products for power control.
An alternate method of power control is to change the drain voltage from 3.5 to 0 V, which produces a smoother and more repeatable change in power vs. controlling voltage. Figure 2 shows the PA’s power output as Vdd is swept from 0 to 3.8 V with 12 dBm power input at 902 MHz. Changing input power with a variable gain driver amplifier is another available option.
The AP114-79 3.5 V DCS PA delivers the high efficiency, low voltage operation required for the next generation of handsets in the 1800 MHz European band. Its typical PAE of 50 percent is maintained at 3.5 V as well as 2.8 V. With an output power capability greater than 2 W, this amplifier delivers an output power of 1.4 W even at an end-of-life battery voltage of 2.8 V. Figure 3 shows the AP114 PA’s performance at 1785 MHz.
Similar to the AP112 PA, the model AP114 offers a wide power control range of 50 dB and is supplied in a thermally enhanced SSOP package with slug. The unit’s off-chip matching elements deliver better performance and flexibility. Figure 4 shows a typical circuit using the model AP114-79 PA with a level shift and DC converter IC.
In addition, a single model AM100 DC/DC converter can be used to control both amplifiers, eliminating the need for two DC converters for dual-band operation. The AM100 converter changes the gate voltage and adapts intelligently to changes in power and performance.
For a true dual-band solution to GSM and DCS applications, the model AP116-79 PA will provide a high performance, single-package solution. This PA IC will combine the AP112-79 and mirror-image AP114 chips in a single SSOP-16 package, thereby delivering savings in both size and cost. Samples of the AP116-79 PA are available currently.
The complete GSM solution can be achieved by integrating a driver amplifier to power the PA. Currently, NEC offers a 3 V preamp MMIC designed specifically as a PA driver for mobile communications. The IC is supplied in a super minimold package (which is smaller than conventional minimolds). Low input drive three-stage PAs will be offered in the near future. These products will require 0 dBm input drive and eliminate the need for the external driver.
Future Digital Handset Market Considerations
Success in the rapidly changing digital handset market requires the ability to adapt quickly to market changes. The future undoubtedly will continue to demand greater functionality with higher efficiencies and performance. The company that stays ahead of technology with next-generation products will continue to prosper in a market with tremendous growth potential.