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Power-Amp Modules from Agilent Extend Mobile-Phone Battery Life by 15%

E-pHEMT power amps provide industry's best power-added efficiency for CDMA and GSM phones.

The manufacturer says . . .

ChipCenter's Paul O'Shea says . . .

Agilent Technologies Inc. introduced a new series of power-amplifier (PA) modules designed for next-generation CDMA and GSM mobile handsets. Agilent's new E-pHEMT (enhancement-mode pseudomorphic high-electron-mobility transistor) PA modules provide the industry's best power-added efficiency (PAE) for CDMA and GSM phones, and allow manufacturers to increase mobile-phone battery life by as much as 15%, a critical feature in today's multi-function phones. Agilent's E-pHEMT process provides the industry's best PAE, a measure of how much power it takes to amplify a signal for transmission to a base station. Higher PAE increases battery life, especially "talk time." Agilent estimates that the E-pHEMT PA modules can increase talk time by as much as 30 minutes on a single battery charge. Agilent also has engineered the new devices to deliver industry-leading reliability, using a new passivation technique in the manufacturing process. The process yields products that offer 100,000 years mean time to failure during continuous use at +90°C. "The industry-best power-added efficiency of our new E-pHEMT power modules allows our customers to offer additional features while maintaining battery life, or to offer phones with much longer battery life," said Bryan Ingram, vice president and general manager of Agilent's Wireless Semiconductor Division. "Along with our FBAR filters, embedded camera modules, infrared transceivers, and blue LEDs for backlighting, the new PA modules position Agilent as a major player in future mobile-handset designs. These E-pHEMT components are a product of our technical innovation at Agilent Labs." Agilent announced three E-pHEMT PA modules, two for CDMA/AMPS handsets and one for GSM. Of the CDMA modules, the ACPM-7813 addresses the 800 MHz dual-mode band, while the ACPM-7833 operates in the 1900 MHz (PCS) range for handsets and wireless data-terminal applications. The ACPM-7891 PA module is targeted at tri-band GSM/GPRS phones. The CDMA modules offer an industry-best PAE rating of 40%, with E-pHEMT technology that requires only a single positive power supply. Higher efficiency also permits the use of smaller batteries to make room for additional features in handsets, data cards, and PDAs, such as GPS emergency location, embedded camera modules, and multimedia services. In addition, the PA provides extremely high signal quality, which is a function of the amplifier's linearity. This is vital to CDMA because its complex signal transmission requires highly accurate signal reproduction. Further, E-pHEMT has excellent low-voltage operation and is well-positioned for next-generation 3G systems that may use lower battery voltages. Agilent's GSM tri-band PA module also offers a superior PAE of up to 60% in the GSM band and 56% in both the DCS and PCS bands. The module is also capable of Class 12 operation for 2.5G applications such as GPRS service. Agilent has already begun volume shipments of both CDMA and GSM modules to multiple customers. To keep up with the demand for high-efficiency PA modules, FBAR filters, and other E-pHEMT products, Agilent is quickly ramping its new state-of-the-art six-inch wafer fab in Ft. Collins, Colorado. The fab offers high-volume manufacturing capacity, and leverages state-of-the-art CMOS statistical process control techniques to optimize the performance of advanced analog/mixed-signal devices. The Agilent ACPM-7813 CDMA PA module (4 × 4 × 1.1 mm SMT package) operates over the 824 MHz to 849 MHz frequency range, and features +28.5 dBm linear output power at 3.4 V. Specification-compliant linear operation can be realized from supply voltages between 3.2 V and 4.2 V, with minimum performance reduction at 3 V. The Agilent ACPM-7833 CDMA PA module (4 × 4 × 1.1 mm SMT package) operates over the 1850 to 1910 MHz PCS frequency range with a +28.5 dBm linear output power at 3.4 V. The Agilent ACPM-7891 GSM PA module (6 × 12 × 1.8 mm SMT package) operates over the 900 MHz EGSM, 1800 MHz DCS, and 1900 MHz PCS frequency range. The module features +35 dBm linear output power at 2.2 V in the EGSM mode and +32.5 dBm linear output power at 2.2 V in the DCS/PCS mode. The PA operates from supply voltages between 3.0 V and 5.3 V, with minimum performance reduction at 2.7 V. Agilent Semiconductor Products Group Tel: (800)235 0312 Web: www.agilent.com

The introduction of the E-pHEMT power amplifier for GSM and CDMA is a very significant announcement for Agilent. According to the company, this power amplifier has the best power-added efficiency of anything on the market, 40% for CDMA and 60% for GSM. The major benefit will be increased talk time and transmit time, or maintaining talk time with added features. The company says that E-pHEMT power amps extend battery life by 15%, and increase talk time by 30 minutes. I found this to be a significant introduction not only because Agilent can extend the battery life but also because the company can work with the customer to decide the optimal amount of integration. Additionally, the company can work with the customer to determine the best impedance match—you aren't forced to using typical 50 W impedances. Agilent says they are one of the few companies that have both active and passive components in-house. The FBAR technology is easily integrated with active components because it is silicon-based. For example, if you have different impedance needs and different requirements for thermal expansion, you don't have to worry about that with FBAR. You can find more information on the FBAR technology by reading a review I wrote a few months ago (Agilent Technologies Announces High-Performance FBAR Filter for U.S. PCS Band Mobile Phones and Data Cards). Many different semiconductor technologies are currently being used for power amplifiers (PAs) that include a mix of silicon and GaAs devices such as silicon bipolar, silicon MOSFET, GaAs MESFET, GaAs HBT, and GaAs pHEMT. Agilent uses the enhancement-mode pHEMT (E-pHEMT) process for its PA designs, while most competitors use a GaAs HBT technology. However, the leakage current has not usually been a strength of enhancement-mode FET devices. The Agilent PAs using E-pHEMT technology may have changed this because the technology is based on a very low drain-source current consumption of less than 10 µA at room temperature. This performance has been achieved by a combination of using a buried gate, using the appropriate crystal orientation, reducing the second recess depth, and optimizing the InGaAs channel with a selective etch process. The following table is a good reference point for the power-added efficiency (PAE) of this E-pHEMT technology compared to similar technologies. click for full-size table One of the drawbacks to the conventional depletion-mode pHEMTs, despite the high performance, is the conduction of current at zero gate bias, or when the drain current reaches a saturated level at a gate-source voltage of 0 V DC. However, E-pHEMT devices conduct only a small amount of leakage current at zero gate bias, so that the drain current is approximately 0 at a gate source voltage of 0 V DC. Therefore, it can operate without a negative voltage supply or drain switch. The quiescent current is controlled through a regulated voltage line, with a variable control voltage level of 1.0–2.7 V. Agilent CDMA power amps have lower quiescent current consumption using the E-pHEMT process. To maximize talk time, a low-power mode option was incorporated into the PA modules to improve the quiescent current below 30 mA at a 1.2 V control voltage and 70 mA at a 2.5 V control voltage, compared to a 100 mA quiescent current using the GaAs HBT process. Power-added efficiency is a good criterion for how much talk time you get per battery charge. There are a couple of key reasons why Agilent's E-pHEMT PAs have excellent PAE when operated in both linear modes (CDMA) and in saturated modes (GSM). First of all, the low RDSon gives a high drain efficiency while high gain reduces the input drive requirement. Secondly, the high frequency allows for very fast switching to reduce power dissipation in the device. When operated in the linear mode, the PAE is typically 8.5% at medium power-out (16 dBm) and over 40% at maximum power-out (28.5 dBm) in comparison to 7% and 32% of GaAs HBT PAs at the same conditions. As a saturated amplifier, E-pHEMT has demonstrated industry-leading PAE levels. A performance efficiency of 60% in the EGSM band and 57% in the DCS/PCS bands is realizable in production. Power amplifiers typically require a bias buffer to properly set the bias point. For PAs based on bipolar technologies, this results in having two PN junctions (1.2 V) in series as part of the circuit. The sum of these drops limits the amount of available bias-network headroom, especially as the battery voltage drops below 3 V. E-pHEMT devices have very low turn-on voltages and therefore do not have the same limitation in bias headroom. This allows for superior power-out and performance efficiency at bias levels less than 3 V in comparison to HBT devices. Additionally, bipolar devices cannot perfectly match the entire active geometry for PA designs. This can allow for small areas to become "hotter" than other local areas. This imbalance in thermal regions can cause thermal runaway. Therefore, the GaAs HBT devices need to add ballast resistors to equalize this imbalance. These ballast resistors result in lower gain and output power. The inherent increase in RDSon of hotter areas in E-pHEMT devices provides intrinsic ballasting. These E-pHEMT devices do not require ballast resistors to eliminate thermal runaway and don't sacrifice power-out or PAE performance. The HBT device doesn't have a secondary breakdown mechanism. The imbalance and hot spots resulting from localized breakdown effects can cause excessive heat that can fuse the electrodes of HBT devices. Therefore, HBT devices require the addition of a clamp such as zener diodes on the collector to limit peak output voltage. This extra circuitry can add some loss and generate spurious signals when clipped. This secondary breakdown does not exist in E-pHEMT because of the intrinsic ballasting effect of increasing RDSon compared to the temperature. Another aspect of this design that I found interesting has to do with impedance matching. Power amps are often subjected to, and must survive, large variations between input and output impedance. It's not unusual to have 10:1 variations. The removal of the antenna, the presence of large ground planes, and operation in a charger can create large voltage variations at the output of the power amp. Devices will exhibit a breakdown when subjected to a large potential difference between the output and input (drain-to-gate for field-effect transistors and collector-to-base for bipolar junction transistors). In a bipolar device, the increased potential difference causes the base to get very thin, resulting in extremely high beta values. Consequently, any injection of current in the high-potential state causes a "punch-through" breakdown to the emitter. Conversely, with the same high potential difference in a field-effect transistor (FET), the source is shut off completely and there is no breakdown path from the drain to the source. The level for the breakdown in a FET is determined by the potential from the drain to the gate, or the BVGD. The BVGD of a FET device is much higher than the potential difference that causes a collector to emitter "punch through" in a bipolar device. The higher breakdown level in the FET results in more robust performance. The Idd or current level for E-pHEMT is 130 mA at a typical power point of 14 dBm, which is good but as you would expect, and when you get up to a high power of 28.5 dBm, the current increases to 500 to 520 mA. Agilent provides both typical and high power values because a lot of designers want to know what the value is at a typical point since it won't be running at high power all the time. There will be a lot of interest for this product from handset manufacturers, and Agilent rightly expects to be a player in this market over the next couple of years. It will start out with small numbers sold, but it will ramp up to a significant number over the year, perhaps gaining the company a single-digit market share. The new Agilent PA modules are available now for sampling and ordering. Pricing for the ACPM-7891 for GSM handsets is less than $2.00 each in high-volume quantities. Pricing for the ACPM 7813 and ACPM-7833 for CDMA handsets is less than $2.00 each in 100,000-unit lots. Agilent also expects to begin sampling a high PAE W-CDMA (Wideband CDMA) PA module by mid-year 2003. Datasheets ACPM-7813—CDMA 800 MHz dual-mode band power-amplifier module ACPM-7833—CDMA 1900 MHz PCS power-amplifier module ACPM-7891—tri-band GSM/GPRS power-amplifier module

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