Increase Talk Time in CDMA Mobile Handsets
RF PAs are designed to work with the latest CDMA transceiver chipsets to reduce the total current consumption from the phone board.
Prasanth Perugupalli, Eric Garloff, Andrew Prudente, Tom Arell, Henry Liwinski, Rattapon Lertpiriyapong, Dick Frey, Anadigics Inc.
Various new features on cellular phones, such as high quality video, a high resolution screen, and a video camera consume large amounts of battery power, and call for
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Figure 1. Probability distribution charts showing the probability that a phone transmits at a given power in several types of network settings. |
means to reduce power consumption elsewhere on the phone board. Advancements in battery technology have not kept pace with the power requirements for the latest 2.5G and 3G handsets. On the contrary, the popularity of modern sleek handsets requires smaller battery profiles. Power consumption poses one of the biggest challenges for present-day cellular phone design.
The RF power amplifier in the radio section of the mobile handset consumes maximum current during the transmission of encoded audio and data bits to the cellular base station. The farther away the user equipment (UE) is from the base station, the higher power the UE transmits at, and hence causes larger drain of battery power.
As cellular phones continue to proliferate, more and more cellular base stations (BST) are being deployed to address the increasing need for capacity. In urban areas with high population densities, operators deploy dense cellular networks to reduce congestion and maintain high quality of service. Hence, it isn’t very uncommon that a cellular phone is in close proximity of a BST most of the time. However, in rural areas, base stations are spread further apart, requiring the phones to transmit at higher power. Figure 1 shows the typical characteristics of Urban, Suburban and a newer Typical Modern network, that define the probability of a mobile handset transmitting at a given output power. These charts, often referred to as CDG profiles, are prescribed by the CDMA Development group, and are used as an industry metric to calculate total current consumption for the PA during the normal use of a cellular phone.
Anadigics has developed a line of power amplifiers (PAs) for next-generation cellular handsets that reduces the current requirements of the RF PA. This technology works in conjunction with CDMA Transceiver chipsets to significantly increase the overall talk time for mobile handsets. Typically, gain is 27 to 28 dB and efficiency is 37 to 38% across all band classes and frequency ranges.
Device Technology
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Figure 2. Pin diagram for a typical ZeroIC PA shows the internal construction of the amplifier. |
Based on InGaP-plus™ process technology, these GaAs structures combine a vertically integrated heterojunction bipolar transistor (HBT) and a pseudomorphic high electron mobility FET (pHEMT) epitaxial structure on the same chip. This technology facilitates the design of a best in class RF active device for linear amplification and also high performance RF switches that can be utilized to attain high efficiencies from the PA.
The HBT and pHEMT structures are decoupled from each other, enabling independent device optimization without performance compromises. To minimize cost and maximize yield, this process flow utilizes a number of shared fabrication steps. Extensive DC and RF characterization shows that the HBT and pHEMT devices fabricated through the BiFET™ process exhibit same performance and reliability characteristics as any other stand-alone transistors.
PA Architectures — A Comparison
Power amplifiers used for cellular phone applications typically use HBTs that are biased relatively deep into class AB region to achieve best efficiency even as they meet spectral mask requirements. Whenever the transistor is in ON state, there is a flow of a minimal current, called Quiescent current (Icq). Hence, this current is constantly being sourced from the battery whenever the mobile equipment is switched ON, contributing to the drainage of battery charge. Handset designers constantly strive to increase talk time of the equipment by reducing battery discharge, especially when the phone is not being used to make an active voice or data call.
The most common architecture currently used in the industry relies on switching between two independent power amplification stages within the same package based on the output power needs. Termed HELP™ PA (High Efficiency at Low Power), it provides significant increase in power added efficiency at higher power ( <28 dBm) as well as lower power ( <16 dBm ) levels. BiFET
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Table 1. Probability weighted current consumption comparison for PCS band CDMA PAs highlights the advantage of ZeroIC PA over HELP™ PAs. |
technology provides the low loss switches necessary to switch between the high and low power paths in real time. HELP PAs have Icq levels at 15 mA compared to 50 mA from a traditional PA, and provide almost twice the efficiency at 16 dBm power level than the latter. However, even at low output power levels (<㪢 dBm), the HELP PA still draws 15 mA of current (Icq). The efficiency of such an amplifier can be improved by either reducing the Icq of the low power stage further, or by switching the PA completely OFF when no amplification is necessary.
The new-generation PA (dubbed ZeroIC) is designed such that the normal operation provides for 27 dB amplification in the high power mode (10 to 28 dBm). In the bypass mode, RF power is routed through the second input pin (RFIN_Switch) directly to the output pin through a passive switch network, without any amplification. In this mode, the main amplifier is completely switched OFF, and hence there is no Icq current drawn by the PA. This technique reduces the overall current consumption of the phone by almost 60%, and thus helps greatly to increase the talk time of the phone.
Other features of the ZeroIC™ PA family include a built-in voltage regulator and a minimum 32 dB isolation between RFOUT and RFIN_Switch pins. This allows for reduction in BOM cost, and also safe operation of the ZeroIC PA into load mismatch without affecting the characteristics of the driver amplifier within the transceiver chipset.
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Figure 3. Schematic of application circuit for AWT6331R shows only 7 SMD components required, thus minimizing cost and space for phone board design. |
All ZeroIC PAs are characterized for GPS band noise requirements, and for optional low Vcc operation at 1.5 V using an external DC/DC converter to reduce current consumption further.
The architecture of this type of power amplifier provides significant reduction in overall current consumption compared to other prevalent PA designs. This is evident by considering the probability distribution profiles from Figure 1,
which show that the output power level from the phone is less than 10 dBm for almost 95% of the time for any type of CDMA network. Depending on the output power from the transceiver chipset, the ZeroIC PAs can be switched into bypass mode typically about 10 dBm antenna power. This causes the overall current consumption to be drastically reduced because all active stages of the PA have been turned OFF. The following table shows the current savings obtained compared to HELP2 PA family from Anadigics. This directly translates into increased talk time for CDMA phones utilizing the same battery power.
Ease of Use
All ZeroIC PAs incorporate RF matching networks and signal conditioning circuitry inside the package to minimize the number of surface mounted components (SMD) that take up large board area around traditional power amplifiers. This provides for both cost and space savings for new phone designs. Input DC blocking capacitors, some decoupling capacitors, input and output matching elements are realized inside the PA, either on the GaAs substrate or on the laminate.
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Figure 4. Block diagram shows the implementation of a dual band Phone for BC0 and BC1 bands. |
ZeroIC PAs are currently being utilized in Dual and Triple band mobile handsets for all CDMA networks around the world. These PAs have been designed for total compatibility with the latest CDMA chipsets currently available in the market place. Figure 4 shows a typical implementation of a Dual band phone for the US Cellular and PCS bands.
Conclusions
ZeroIC family of power amplifiers provides significant improvement in talk time for the latest CDMA mobile handsets. These PAs are designed to work with the latest CDMA transceiver chipsets to reduce the total current consumption from the phone board. The novel PA architecture in a 3 mm × 3 mm × 1 mm over-molded plastic package provides superior performance in one of the smallest foot prints.About the Author
Prasanth Perugupalli manages the CDMA line of Power amplifiers; Eric Garloff and Andrew Prudente are senior members of Customer Applications Engineering team; Tom Arell, Henry Liwinski and Rattapon Lertpiriyapong are product design engineers in the CDMA development team; Dick Frey leads the wireless power amplifier design and development team.
ANADIGICS, Inc.
35 Technology Dr.
Warren, NJ, 07059
© 2008 Advantage Business Media
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