PA Puts Emphasis On Power Control

Mobile handset design remains one of the most challenging aspects of wireless development. This issue is derived largely from the market's demand for handsets with greater performance and longer battery life. At the same time, consumers are demanding handsets that are smaller in size. They also want them to cost substantially less than current models. In an effort to satisfy these conflicting needs, Analog Devices, Inc. (ADI) recently introduced the ADL-5551 power amplifier (PA).

The ADL-5551 is the first product to be released in the X-PA family of power-amplifier modules (see figure). It is classified as a high-output-power, high-efficiency, mobile-handset PA with integrated RF power control and measurement. Its primary applications are in GSM/GPRS and multi-standard mobile handsets. In the GSM band, the ADL-5551 provides an output power level of greater than 35 dBm while maintaining a typical 55% efficiency rating. In the DCS/PCS band, 50% efficiency is achieved at power levels of greater than 34 dBm and 33 dBm, respectively.

By far, one of the most significant features of the ADL-5551 PA is the integration of RF power-measurement and -control circuitry. With this device, there is literally no need for designers to include an external RF power detector. By integrating a true RF power detector and control loop, the ADL-5551 enables extremely accurate power control. This accuracy provides significant advantages over traditional discrete diode circuit approaches and open-loop voltage- and current-sensing methods. For example, consider that the architecture used by the ADL-5551 results in battery-power savings of more than 20% compared to competing open-loop voltage- and current-sensing approaches.

Furthermore, most traditional gallium-arsenide (GaAs) heterojunction-bipolar (HBT) power amplifiers provide no power-detection capability. Those that do typically use a current sensor that measures the supply current being drawn into the RF power amplifier. The current draw is then used to estimate the power that is actually being delivered by the amplifier. While this isn't a very accurate approach, it works well enough for many existing cell-phone designs. The problem, of course, is that the handset may be drawing twice as much power as needed, depending upon the circumstances. As a result, these low-accuracy control schemes can waste valuable power, which leads to shorter battery life and more frequent recharges.

By contrast, the ADL-5551 PA provides a highly accurate power-detector mechanism that actually measures the RF output power coming directly out of the amplifier. The output power is then controlled using a power coupler. As a result, handset designers no longer need to guess at the power usage by measuring the current and approximating the voltage.

This precise power control is provided over a wide dynamic range. In fact, the architecture of the X-PA family of devices uses logarithmic detection to provide precise exponential (linear-in-dB) output power control over a range of 40 dB. Over this control range, deviations in output power are less than ±0.5 over frequency and supply voltage. They are less than ±1 dB when including changes due to temperature. An additional 10 dB of power-control range is available for ramping at low-power levels. This last bit of range has slightly degraded linearity and temperature stability.

Because power can be both measured and controlled with great accuracy, the ADL-5551 allows engineers to design to much tighter tolerances. For GSM-handset applications, tighter tolerances result in a reduction for multiple levels of calibration. After all, handsets that use the X-PA family of devices, like the ADL-5551, will no longer need to store calibration coefficients for both full- and low-power scenarios. The high-power accuracy of this PA means that only a single power calibration is required. This feature saves even more time and money.

The ADL-5551 power amplifier and a reference radio design are now available. The area of the reference design board's active region measures less than 4 cm². It includes ADI's OthelloOne RF Transceiver, the ADL-5551 PA, and supporting electronics.

The ADL-5551 X-PA complements ADI's Othello GSM RFIC product family. The PA is quad-band E-GSM, GSM-850, DCS and PCS, as well as GPRS-compliant to level 12. It operates with a 2.9- to 4.5-V power supply. The ADL-5551 PA is available in a 10-×-10-×-1.4-mm LCC package. Pricing for the ADL-5551 starts at $5 per unit in 10,000-piece quantities. The device is now in full production.