The power amplifier (PA) driving the antenna is traditionally the single largest power consumer in a wireless handset. Reducing power consumption by increasing the efficiency across the spectrum of output power levels can significantly extend battery life.

The first step in achieving increased power efficiency is to evaluate the power levels required for most users in both urban and suburban environments. The CDMA Development Group (CDG), for example, has published graphs showing the power-level distributions for both environments (Fig. 1).

The CDG data shows that most handsets operate at reduced power levels most often. For example, while wireless standards mandate maximum power levels of around +28 dBm at the PA level, over 80% of all calls require less than +10 dBm. Unfortunately, traditional PAs suffer from reduced efficiency at lower power levels, which increases current draw in relation to output power. Increasing efficiency at lower power levels can yield big battery life savings. A standard WCDMA amplifier with a 42% efficiency at +28 dBm typically experiences a precipitous drop off in efficiency to only 8% at +16 dBm. Quiescent current draw is around 50 mA.

Traditional wireless PAs are two-state GaAs devices, switching the current between high and low power levels. The threshold for switching between levels is +16 dBm, which explains why efficiency is specified at this level. A common way to maximize the benefit of having two levels is to use an external dc-to-dc converter to switch the voltage to the amplifier. The drawback is the additional components in terms of bill of materials, cost, and board space consumed.

1. Probability density functions and cumulative density functions from the CDG demonstrate that handsets most often operate at low- and mid-level power.

The recent introduction of a commercially viable BiFET InGaP technology, which combines bipolar and field-effect transistors (FETs) on the same InGaP die lets designers develop products with greater levels of integration and higher performance. For example, newer PAs for WCDMA applications combine heterojunction bipolar transistors (HBTs) and pseudomorphic high-electron-mobility (pHEMT) FETs in one monolithic package.

The resulting BiFET technology allows highly linear amplification stages to be created with HBTs and fast, high-efficiency switches to be built using pHEMTs. The lower turn-on voltage for pHEMTs makes them far more efficient switches than can be obtained with HBTs. Branded as HELP (for High-Efficiency-at-Low-Power) devices, PAs using BiFET technology dispense with the need for external converters to maximize performance at low output power levels.

The pHEMT switches allow different amplifier chains to be chosen in the PA, depending on the output power required. The result is that the efficiency at mid-level power is more than doubled, from 9% to 21% at 16 dBm (Fig. 2). The first generation of HELP-enabled PAs switch the bias mode pin between high and low logic levels. With the increase in efficiency, average power consumption is cut by 50%. By allowing a three-level approach to operation, the third-generation HELP parts (called HELP3) cut power by up to 75%. Quiescent currents are also significantly reduced from 50 to 15 mA for HELP and 7 mA for HELP3 (Fig. 3).

2. Efficiency and current consumption is charted for a HELP PA.

3. Pertinent specifications of the three types of PAs is shown.

How does this play out in application? Consider a typical phone in an urban setting, consuming 125 mA for the receive circuit and baseband section. The transmit circuit power consumption will vary depending on the PA used, while the rest of the transmit circuit has the same power consumption (power consumption in the transmit circuit includes not only the PA, but other components such as the RF driver amplifier). Here are the three cases for current draw in the talk mode:

1. Phone with two-state PA PA current draw = 70 mA or 29% of talk mode current 116 mA (Tx) + 125 mA (BB+Rx) = 241 mA

2. Phone with two-level BiFET (HELP) PA PA current draw = 34 mA or 17% of talk mode current 81 mA (Tx) + 125 mA (BB+Rx) = 206 mA 17% increase in talk-time

3. Phone with three-level BiFET (HELP3) PA PA current draw = 18 mA or 9% of talk mode current 68 mA (Tx) + 125 mA (BB+Rx) = 193 mA 25% increase in talk-time

The 70% and 86% reduction in quiescent current for HELP and HELP3 also represent significant savings of power in the standby mode.

HELP PAs using BiFET technology can also be used in conjunction with a dc-to-dc converter to save additional power. The incremental savings, however, are minimal—only a few percent—so that the additional component costs and real-estate use won't justify the converter. In the choice between an external converter and BiFET technology, the BiFET PA offers better savings. For example, for an urban environment, a standard PA consumes about 70 mA. Current draw falls to 40 mA with the addition of external converter and 34 mA with a HELP PA (Fig. 4).

4. Significant current performance improvements can be achieved using a proprietary HELP architecture.

Of course, power consumption isn't the only reason to pick a PA. System-level performance in terms of linearity, noise, and the ability to support such advanced services as High-Speed Packet Download Access (HSDPA) must certainly be considered. Basic performance specs, after all, must be met.

Power management in the PA begins after basic requirements are met. The move to higher integration levels—adding more functions into the PA package—also saves real estate and reduces the bill of materials. For example, one difference between HELP and HELP3 technology is that HELP3 includes a built-in voltage regulator.

About the author
Jerry Miller is the director of CDMA and WCDMA products at Anadigics. He holds a B.S. in Electrical Engineering from the University of Michigan. Miller can be reached at [email protected].