Power Amplifiers Prove Highly Efficient

Recently, power-amplifying devices have taken the cellular-handset world by storm. The improved efficiency of such chips has greatly increased the performance, feature set, and battery life of most cellular phones. This fact has not gone unnoticed by the major handset manufacturers. In fact, they are now ordering record numbers of power-amplifier (PA) chips for their upcoming 2.5G and 3G handset designs.

Take Agilent Technologies, for example. This company is now enjoying brisk sales of its new gallium-arsenide (GA) power amplifiers. More than 500,000 modules are being shipped per month. What makes this announcement newsworthy is that Agilent is using an enhanced-mode, pseudomorphic-high-electron-mobility-transistor (E-pHEMT) process instead of the traditional heterojunction-bipolar-transistor (HBT) approach. Because it requires only a single positive power supply, the E-pHEMT power semiconductor process is well suited for mobile wireless applications. It directly translates into longer mobile-phone battery life or smaller battery sizes, which would make room for new features.

The company's E-pHEMT technology has two main advantages over other processes. The first benefit is low-voltage operation, which stems from the requirement of only one power supply. Secondly, e-pHEMT devices don't have the same thermal-runaway issue as bipolar PAs.

In general, bipolar devices cannot perfectly match the entire active geometry in order to design power amplifiers. This issue can create small areas that become "hotter" than other local areas. In turn, this problem leads to thermal runaway, which occurs when the current increases as temperature rises. In contrast, E-pHEMT devices use a self-regulating function to avoid thermal runaway. The current saturates and drops as temperature increases.

Recently, Agilent Technologies introduced three new power-amplifier modules that flaunt these impressive characteristics. The modules target next-generation CDMA and GSM mobile handsets (see figure). For CDMA designs, the ACPM-7813 addresses the 800-MHz dual-mode band. The ACPM-7833 handles the 1900-MHz PCS range. The ACPM-7891 module, on the other hand, is specifically designed for tri-band GSM/GPRS phones.

The CDMA modules boast a 40% PAE rating, which is the highest power efficiency for this type of device. By comparison, the GSM/GPRS module offers an impressive PAE of up to 60% in the GSM band and 56% in both the DCS and PCS bands. These high PAE ratings result from the need for just one positive power supply. They translate into an increase of up to 15% for mobile-phone battery life.

Both CDMA power amplifiers are the same compact size. In a surface-mount-technology (SMT) package, they measure 4 × 4 × 1.1 mm. The ACPM-7813 operates over the 824-to-849-MHz frequency ranges. It features +28.5-dBm linear output power at 3.4 V. Linear operation can be achieved from supply voltages between 3.2 and 4.2 V. The minimum performance reduction is at 3 V. As for low quiescent current and high power output, they are provided from a single bias-control voltage. If even lower quiescent current is needed, a dynamic bias-control circuit can be used. Here, the control voltages range from 1.2 to 2.5 V.

The ACPM-7833 also is a fully matched CDMA PA module. It operates over
the 1850-to-1910-MHz PCS frequency range. The ACPM-7833 has a + 28.5-dBm linear output power at 3.4 V. The module's characteristics are almost identical to those of the ACMP-7813 described above. In addition to CDMA-handset implementations, both of these PAs can be used in datacard and PDA designs.

For GSM/GPRS handsets, Agilent's ACPM-7891 GSM PA module operates over the following frequency ranges: 900-MHz EGSM, 1800-MHz DCS, and 1900-MHz PCS. It is compact, consisting of a 6-×-12-×-1.8-mm SMT package. The module features +35-dBm linear output power at 2.2 V in EGSM mode. In DCS/PCS mode, it offers +32.5-dBm linear output power at 2.2 V. This PA operates from supply voltages between 3.0 and 5.3 V with a minimum performance reduction at 2.7 V.

Each of these PA modules is now available for sampling and ordering. In high-volume quantities, pricing for the ACPM-7891 is less than $2.00 apiece. Pricing for the ACPM 7813 and ACPM-7833 also is under $2.00 apiece, but in 100,000-unit lots. Agilent expects to begin sampling a high-PAE Wideband-CDMA (W-CDMA) PA module by the middle of this year.