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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