Transmitter power amplifiers (PAs) consume more power than any other circuit in today’s wireless applications. In some cases, PAs swallow up more than 50% of the power budget. Also, their inefficiency produces excessive wasted power as heat. With cellular basestations joining the green trend and cell phones including ever more features and multiple radios that shorten battery life, the industry is turning its attention to the PA.
One main factor causes the PA’s inefficiency: It’s a linear amplifier that inherently operates at lower efficiencies. The reason is there’s essentially no other way to get the linearity and broadband characteristics needed with the newest radio technologies like Long Term Evolution (LTE).
POWER-AMPLIFIER DESIGNS Virtually all wireless and cellular technologies require linear power amplification (see “Key Power-Amplifier Specifications”). These include cdma2000, WCDMA, HSPA, and LTE broadband multicarrier designs. Even EDGE needs linear amplification with its eight-phase shift-keying (8PSK) modulation.
Linearity is essential to the fidelity of the modulation as well as minimizing intermodulation distortion (IMD) and harmonics. And that’s not easy to achieve, especially at UHF and microwave frequencies. But technology has prevailed and we now have plenty of new products and methods to meet current and future needs.
Class A amplifiers provide the best linearity. With their maximum possible efficiency of 50%, though, actual efficiency is much less. That’s why most PA designers use a class AB design. With some quiescent current flowing, the more linear region of the devices is accessible, and crossover distortion can be eliminated in push-pull designs. It’s also possible to improve efficiencies.
Potential efficiency is about 78%. However, that’s rarely achieved in practice. Real efficiencies of 25% to 40% or so are possible. Today, most IC PAs are class AB designs, as are most higher-power basestation amplifiers. Other techniques beyond class AB are also being deployed to improve efficiency while maintaining linearity.
IC POWER AMPS Integrated PAs can be found in all but the highest-power amplifiers, which use discrete components (see “A Word About RF Power Transistors”). Typical uses include cell phones, Wi-Fi, Bluetooth, WiMAX, and other transceivers. General power range is approximately 15 to 28 dBm. Most are class AB types and are made from gallium arsenide (GaAs), indium gallium phosphide (InGaP), and silicon germanium (SiGe).
Anadigics offers a whole slew of new PAs, such as the AWM6433. This power amp targets WiMAX applications in the 3.4- to 3.6-GHz European band (Fig. 1). It can serve the fixed and mobile versions of the 802.16 standard, and it’s expected to find homes in laptops, PC cards, USB dongles, and even some handsets eventually.
Typical output power is 24 dBm with an efficiency exceeding 20% with a 3.3-V supply (see “Thinking In dBm”). It can also accommodate a 4.2-V supply. Overall gain is 30 dB. Common error vector magnitude (EVM) performance is less than 3 dB at 22-dBm output with a 3.3-V supply.
Measuring 4.5 by 4.5 by 1 mm, the AWM6433 squeezes in an integrated 25-dB step attenuator, an output-power detector, and input and output impedance-matching circuits. Anadigics’ complete line of PAs for WiMAX includes higher-power versions as well as models for the 2.3- to 2.7-GHz U.S. and Asian bands.
Also, the Anadigics AWL9966 dual-band Wi-Fi front-end IC incorporates low-noise amplifiers (LNAs), PAs, transmit/receive (Tx/Rx) switches, and all matching components for both the 2.4- and 5-GHz frequency assignments. It should greatly reduce design time, component count, and bill of materials (BOM) while bringing greater range and reliability to any Wi-Fi product.
Typical LNA specs include a 2.6-dB noise figure with 12-dB gain at 2.4 GHz and a 3-dB noise figure with 14-dB gain at 5.5 GHz. Linear PA gain is 31 dB with output power levels of 18 dBm in the 5-GHz band and 20 dBm in the 2.4-GHz band. EVM is less than 3%. The AWL9966 also integrates a Bluetooth RF switch path to enable both Bluetooth and Wi-Fi operation with a shared antenna and no need for external switching. The device comes in a 4- by 4- by 0.6-mm package.
Front-end modules are rapidly becoming the “hot” wireless product because of the great benefits they bring. The “front end” is considered the receiver LNA, the transmitter PA, the Tx/Rx switch, and any additional impedance matching or filter components. In past designs, these individual components required extra design attention, not to mention extra board space. They were added to the BOM, increasing cost and making procurement more complex. Now, putting all of these parts in one chip is a blessing for designers.
The California Eastern Laboratories UPG2253T6S front-end module, which targets the Bluteooth and IEEE 802.15.4/ZigBee products space in the 2.4-GHz band, includes all but the LNA (Fig. 2). It’s targeted at laptops, netbooks, cell phones, and headsets, as well as industrial applications using 802.15.4/ZigBee modules for automatic meter reading, wireless security, cable replacement, lighting systems, and other monitor and control uses in homes or commercial buildings.
The PA puts out 19 dBm with a power-added efficiency (PAE) of 28%. The second harmonic is down –25 dB, and the third harmonic is down –40 dB. The Tx/Rx switch consists of two single-pole double-throw (SPDT) units. The IC’s through/PA feature is a bypass that can include the PA for high power and bypass if it isn’t needed. It automatically switches to a low-power mode when greater battery savings are needed. The through/bypass path can also be used as the Rx path. A low-pass filter is included as well. The UPG2253T6S comes in a 3- by 3- by 0.7-mm package and operates from 3 V.
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