Electronic Design

Leading-Edge Products Help Designers Put A Stopper In The Power Amp's Power Drain

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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Startup company RF Axis also offers a front-end chip, the tiny biCMOS RFX2401, which targets Bluetooth class 1 and single-port ZigBee applications in the 2.4- to 2.5-GHz band. The RF front-end integrated circuit (RFeIC) connects directly to the single-chip wireless transceiver via a bandpass filter (Fig. 3). The only other part required is a single bypass capacitor.

The RFX2401’s transmit signal chain gain is 21 dB, and its output power for Bluetooth EDR (enhanced data rate, 3 Mbits/s) with 8PSK modulation is 16 dBm. Its output power for 250-kbit/s Zig- Bee with offset quadrature phase-shift keying (OQPSK) modulation is 20 dBm. The receiver LNA gain is 10 dB with a noise figure of 3 dB. Tx/Rx switch insertion loss is zero. The chip comes in a 3- by 3- by 0.9-mm, 16-pin quad flat no-lead (QFN) package.

The RFX2402, a variation of the RFX2401, is designed for wireless localarea- network (WLAN) applications in 802.11n or 802.11g access points, PC cards, and other Wi-Fi products. The receive signal chain gain is 8 dB, and the transmit power output is 15 dBm using 54-Mbit/s orthogonal frequency-division multiplexing (OFDM) in 11n or 11g applications. Tx power is 19 dBm with 802.11b applications. Another version, the RFX2405, is a dual Rx/Tx device for combined Bluetooth/WLAN applications. A 5-GHz version is in development.

A new PA from Analog Devices can be used as the final amplifier in a product or as a driver for a larger PA. The ADL5604 1-W class A amplifier is made with GaAs heterojunction bipolar transistors (HBTs). Its broadband design can fit almost any application in the 400- to 2700- MHz range. Overall gain is 14.2 dB with power at 1-dB gain compression (P1dB) output of 29 dBm and output third-order intercept point (OIP3) of 42.6 dBm all at 2140 MHz. Power drain at the quiescent level is 325 mA with a 5-V supply. The device comes in a 4- by 4-mm, 16-lead lead-frame chip-scale package (LFCSP). It features some on-chip matching components, on-chip active bias circuits, and a power-down mode. The ADL5604 can be used in cdma2000, WCDMA, WiMAX, GSM, ISM, PCS, and LTE applications.

Avago Technologies offers several linear PAs for Wi-Fi and WiMAX portable products. Designated the MGA-2xx03 series, these ICs are made with GaAs E-pHEMT technology and come in a 3- by 3- by 1-mm package. The MGA22003 covers the 2.3- to 2.7-GHz band and meets the WiMAX Forum 802.16 mask at more than 25 dBm. The MGA23003 covers the 3.1- to 3.8-GHz band and meets the ETSI 802.16 mask at 25 dBm. Both chips have more than 34-dB gain across the bands. The MGA25203 is a PA for the 5.1- to 5.9-GHz range. It delivers 30 dB of gain and a power output of 23 dBm with –34-dB EVM. All ports are 50 O, and the PAs can operate from any dc source in the 3- to 5-V range. Shutdown and low-power modes are provided.

Another new PA is the RF5602 from RF Micro Devices. This InGaP HBT design covers the 2.3- to 2.5-GHz band and targets Wi-Fi and WiMAX customer premise equipment (CPE) and access points. It also fits PCS and WiBro systems. The small signal gain is in the 32- to 34-dB range. Output power spans from 23.5 to 26 dBm with EVM in the 2% to 3% range. Operating voltages run from 3.3 to 5 V. An onchip power detector is on the die.

Not all power amplifiers target wireless service. For instance, RFMD’s D10040300GTH hybrid power doubler amplifier module suits cable-television (CATV) service. It can be used in both line amplification and at the hybrid fiber coaxial nodes. Also, it has a frequency range of 40 MHz to 1 GHz.

The module’s intended market includes cable operators who are upgrading existing networks to 1 GHz to accommodate some of the more bandwidth-intensive services, such as high-speed Internet access, Voice over Internet Protocol (VoIP), and HDTV. The power and linearity are expected to reduce the number of amplifiers needed in the system. The device has a gain of 30.5 dB at 1 GHz and a noise figure in the 3.5- to 4.5-dB range. Current draw with 24 V is 420 to 440 mA.

HIGH-POWER DESIGNS
Higher-power PAs that are intended for basestations utilize discrete power transistors. They additionally operate class A or class AB to achieve the linearity needed for multicarrier upgrades. Several interesting new approaches have been used to achieve higher efficiency while maintaining linearity at power levels from 10 W to more than 100 W. Some solid-state amplifiers employ multiple lower-power designs that are put in parallel, and their outputs are combined into one output.

Feed-forward techniques have been popular for improving linearity at higher power (Fig. 4). The idea is to cancel out any distortion that occurs. The input signal is first split and sent to the main PA, which operates class AB. The input signal also is sent to a delay line that simulates the delay produced by the main PA. The directional coupler then takes a sample of the PA output with distortion, attenuates it, and combines it with the delayed signal. This cancels the main signal, leaving only the distortion products.

The distortion signal is next amplified in another PA comparable with the main PA. This distortion signal combines with the delayed main PA output. The distortion cancels out when the two signals are combined. The technique works well, but it does require some fine-tuning to adjust for phase, amplitude, and delay variations. That and the need for an extra PA make this technique less desirable, but the design is still used in some products.

Another multi-amplifier technique gaining some ground is the Doherty design, which was invented in 1936 for high-power AM broadcast and short-wave transmitters. It’s handy for improving efficiency while providing the linearity needed at high power. Thus, the design is being looked at to boost the efficiency of amplifiers for signals that have high peak to average power ratios, such as CDMA and LTE.

The basic Doherty design uses two amplifiers: a main amplifier, usually class AB, and an auxiliary or peaking amplifier, commonly operating class C (Fig. 5). The main PA supplies the amplification at lower input signal levels, during which time the peaking PA is off. The ?/4 lines are used for impedance matching.

The peaking PA cuts in when a certain input level is reached, and both amplifiers supply power to the load. One common cut-in point is 6 dB down from the maximum composite output power. The result is high linearity at higher power. The cut-in point that can be adapted to the application is determined by the splitter ratio and/or the bias on the peaking amplifier.

More higher-power designs are adopting the Doherty method. For example, NXP Semiconductors recently announced a Doherty reference design using its BLF7G22L-130 Gen 7 LDMOS power transistors. It typically delivers in excess of 47% efficiency at an average power output of 48 dBm with a gain of 15 dB. The peak-to-average-power ratio is 8 dB with a typical WCDMA signal. Peak power is about 55 dBm operating with 28 V dc. This design is useful beyond 2.1 GHz. Further improvements in linearity can be achieved with a pre-distortion feedback technique.

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Digital predistortion (DPD) is also becoming the linearity improvement choice for many PAs. The technique predistorts the input signal as the exact inverse of the distortion to be introduced by the PA. The result is a linear output (Fig. 6).

The multi-carrier digitized signals to be amplified are sent to the digital correction circuit, which uses DSP algorithms to produce the inverse distortion based on feedback from the PA output. The predistorted signals are then sent to the modulator and finally on to a digital-to-analog converter (DAC).

The resulting RF signal is subsequently upconverted to the desired frequency and fed to the PA, usually a high-power class AB circuit or a Doherty in the newer designs. The PA’s output is sampled with a directional coupler, and the signal derived is used as feedback. The digital correction circuit digitizes and uses that signal to optimize the correction factors on the fly. The system continuously adjusts itself adaptively to ensure the most linear output possible. One pre-distortion product, PMC-Sierra’s Paladin PM7820, has a 20-MHz bandwidth and is optimized for use with Doherty amplifiers.

One of the more difficult parts of the DPD system is the signal chain that samples the PA output and creates the feedback to the DSP, which calculates the predistortion data. This circuitry essentially becomes a very critical receiver that terminates in a fast ADC. Linear Technology’s LTM9003 µModule feedback circuit for DPD can be used as a DPD receiver, a transmit observation path receiver, or a PA linearization receiver (Fig. 7).

The LTM9003 µModule includes a 400- MHz to 3.8-GHz input downconverting mixer, a 125-MHz wide bandpass filter with 0.5-dB passband ripple, and a lowpower ADC with a 12-bit resolution and a 250-Msample/s sample rate. With the 250-Msample/s sample rate and 125-MHz bandwidth, harmonics up to the fifth and seventh can be digitized, providing a much better picture of the power-amplifier output and allowing for more accurate predistortion calculations.

The µModule receiver has a –145.5- dBm/Hz input noise floor and a 25.3-dBm third-order intermodulation intercept point (IIP3) spec. It uses an external local oscillator (LO); the LO and RF input ports have a 50- impedance. Total power consumption is 1.5 W. The LTM9003 comes in a 11.25- by 15-mm package.

Powerwave’s G3L multicarrier PAs, which feature a high-power Doherty design with DPD, range in frequency coverage from the 850-MHz band to the 2100-MHz band with power levels of 120 to 170 W. They mount in a rack (Fig. 8a). The company’s tower-mounted antennas, called remote radio heads, are available for cellular and WiMAX with power levels to 40 W (Fig. 8b).

Putting the PA on the tower with the antenna prevents the huge transmission line losses at these frequencies, further improving efficiencies in a basestation. The main issue is convincing cellular operators to use tower-mounted PAs that do improve efficiency but make access and maintenance a real issue.

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