HELP™ Technology – A Revolution in Power Amplifiers
By Mahendra Singh, WCDMA Product Line Director, ANADIGICS, Inc.

This article covers the progression of HELP™ (High Efficiency at Low Power) technology using ANADIGICS patented BiFET (Bipolar and Field Effect Transistor) process. It also shows how ANADIGICS used this unique process and HELP technology to create multi gain state PA modules that achieve greater than 70% reductions in average current consumption as compared to earlier conventional approaches.

Why HELP™ Power Amplifiers were needed?
Based on the increased consumer demand for high speed data and multimedia services, mobile telecommunications operators worldwide continue to transition their networks from 2G to 3G technologies and beyond. This trend has profound implications for mobile device design.

Handsets that offer the 3G services must deliver higher power, greater linearity, and improved efficiency at competitive prices. Most important, and most problematic, they have to deliver longer talk times, because 3G subscribers spend more time actively using their handsets.

Even with the improvements in battery technology over the past few years, however, handset power sources continue to lag behind the need for extended operation. Designers have to find ways to reduce the handset’s current consumption if they are to satisfy the conflicting demands for higher power output and longer talk time.

The burden for achieving this falls directly on the handset’s semiconductor devices. Since the power amplifier (PA) is one of the biggest current consumer in the phone, it makes sense to focus on reducing the current drain from this device by managing its power usage.

Accordingly, a lot of effort has gone into developing power management that can be integrated into the PA die. Integrated power management not only addresses the issue of current consumption, it opens the way to a more efficient handset design. On-die integration allows handset designers to optimize power management and achieve longer talk time without using separate DC/DC converters and bypass capacitors, which would add complexity and cost as well sacrificing valuable board space.

The Need to Optimize at Low Power Levels
One way to manage current consumption in a PA is to optimize for high efficiency across a broad power range. This can be done by carefully evaluating the CDG (CDMA Development Group) or GSMA (GSM Association) power profiles for handsets on 3G networks and optimizing PA efficiency where it’s needed most. The GSMA profile used by 3G handset manufacturers is shown in Figure 1 as an example.

Obviously, the handset spends most of its time at or around a fairly low power level of -4dBm, measured at the antenna. Assuming about 3dB of loss in the circuitry between the PA and the antenna, the power output of the PA at that signal level is about -1dBm.

At such lower power levels (below 0dBm) the PA is essentially running at its quiescent current. The quiescent current at -1dBm is typically about 50mA. By reducing the quiescent current and increasing the PA’s efficiency at this level, designers can achieve considerable savings in current consumption.

Until recently, however, this was not feasible. The typical single-device, two-state PA used for handsets could be internally optimized only at maximum rated power. This produced very poor efficiency at the low power levels where the handset spends most of its time.

Of course, it is possible to optimize single-device PAs for low-power operation by adding an external DC/DC converter and analog bias control to the handset, thereby achieving increased talk time. But as noted above, a DC/DC converter adds size and cost to the handset. In addition, it complicates the design, because the handset must be calibrated at each power setting dictated by the analog control.

BiFET Process – Technology for HELP™ PA
InGaP-Plus™ technology1, introduced by ANADIGICS, solves the optimization problem by allowing designers to create PAs with two devices and multiple gain paths. This opens the way to independent optimization of the PA at different power levels.

Generically known as the BiFET process, InGaP-Plus integrates pHEMT (pseudomorphic High Electron Mobility FET) and HBT (Heterojunction Bipolar Transistor) devices onto same wafer (Figure 2).

By letting high-performance RF switches (pHEMT) co-exist on same die as the PA (HBT), the BiFET process flow enables the creation of multi-gain-state power amplifiers, with each gain state optimized independently for linearity and efficiency. InGaP-Plus thus allows the designers to achieve the best possible PA performance.

HELP™ PA – Using BIFET Process
The initial implementation of this technology, called HELP™ (High Efficiency at Low Power), created a two-state (high and low power) PA. Unlike single-device PAs, which also have two gain states, an InGaP-Plus™ PA can be internally optimized at both high and low power states. The single-device PA cannot.

Internal optimization of a HELP™ PA can increase handset talk time by more than 25%. Of course, like a single-device PA, it can be used in conjunction with an external DC/DC converter to conserve more current. But the additional savings are minimal, and not worth the extra cost and board space.

The first generation of the HELP™ PAs using the BiFET process was designed as two gain state PA. The efficiency of these PAs were optimized at High power (~28dBm) and Medium power (~16dBm). (Figure 3).

HELP2™ took advantage of BiFET process by integrating a voltage regulator on the PA die, a requirement posed by the major chipset manufacturers. PA vendors without the BiFET process were forced to add this functionality in their PA module by adding another chip for this functionality which was not suitable for lowering the PA module size.

HELP3™ (Figure 4) PA added a 3rd gain state, low power mode, by lowering the current in the medium power mode down to 7mA. This further improved PA average power consumption by 65% over the conventional technology.

HELP4™ (Figure 4) adds a low current stage for the low power mode. The ICQ in the low power mode for HELP4™ PA is ~3mA compared to ~8mA for the HELP3™ power amplifiers. The average current consumption for the HELP4™ is ~20% lower then HELP3™ technology and by 72% lower than conventional PAs using GSMA Profile (Figure 5).

Advantages of HELP™ PA over Competitor’s “HELP Like” PAs
HELP™ PA were designed leveraging ANADIGICS patented BiFET process. The integrated switch technology on the PA has many advantages in designing high efficiency PAs. The integrated switch offers isolation needed between the high power and the medium/low power modes to avoid any circuit oscillations. This allows the designers to independently optimize the high power mode and the medium/low power mode without sacrificing any RF performance. The other benefits are that with BiFET process voltage regulator and other functionality can be added on the die, which helps reduces the size of the PA module and still offer extra functionality. The BiFET technology also allows the integration of “daisy chainable” couplers on the PA die.

The integration has another advantage: it enables manufacturers to design PA modules with all of these features in smaller form factors. ANADIGICS now offers the industry’s first 3x3mm single band and 3x5mm dual band WCDMA HELP3™ power amplifiers.

With handset manufacturers moving toward low-voltage logic, HELP4™ technology will offer PA with 1.8V CMOS logic. This is all possible with the BiFET process.

Conclusion
Pioneered by ANADIGICS, the InGap-Plus™ process is the foundation of its HELP™ PA technology. This process allows independent optimization to achieve the lowest current consumption in High, Medium and Low power modes.. ANADIGICS has used this technology to offer the industry’s first 3x3 mm2 single band and 3x5 mm2 dual band WCDMA HELP3™ PA products while integrating many functionality from the phone board to the PA die, thus reducing customer bill of material, cost and phone board space.

About the Author
Mahendra Singh is the director of WCDMA products at ANADIGICS. He holds a M.S. in Electrical Engineering from the University of Connecticut. He can be reached at msingh@anadigics.com

References
1. A. Gupta, B. Peatman, M. Shokrani, W. Krystek, T. Arell, ”InGaP-Plus™– A Major Advance in GaAs HBT Technology, 2006 IEEE CSIC Symposium Tech. Dig, p. 179.

ANADIGICS, Inc.
www.anadigics.com
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