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Ultralinear Wireless Amplifiers Using Low Cost Building Blocks
Linear power amplifiers (LPA) play a very important role in high performance wireless base stations. Modern multicarrier LPAs improve cell site performance and capacity significantly. Older base station equipment used single channels, each amplified individually using high power, nonlinear class-C amplifiers. The signals were improved by filtering the harmonic and spurious signals with expensive, bulky narrow-band filters. The various individual channels were then combined and fed to the system antenna for transmission.
Modern second- and third-generation digital base stations combine various channels in a wideband low power combiner and feed the multitone signal to a high power linear amplifier that amplifies the channels simultaneously. This system configuration permits any of the channel frequencies to be changed to optimize the signal traffic pattern for best utilization of the cells' coverage area.
These digital networks, using CDMA transmission standards, are very noise sensitive, and the high power amplifiers can introduce a significant amount of noise into the system. A highly linear amplifier is capable of introducing a minimum amount of noise when amplifying the signal to very high levels without distortion. These types of amplifiers are necessary for modern base station systems and normally are constructed using extremely complex linearization techniques such as predistortion or feedforward configurations.
A New Approach
The new generation of linear base station amplifiers used in microcell, picocell and repeater cell stations now can be designed using multiple modular building blocks. This new approach is based on using parallel and serial combining techniques to produce low cost, high power base stations that are simple to manufacture, require no tuning or adjustments and are easy to maintain in the field.
The modular approach offers an excellent performance-to-price ratio, is easy to design, test and manufacture using lower skilled labor (no complex linearization) and features excellent input and output SWR with soft fail capability without the need for expensive isolators. The technique utilizes ultralinear, class-A, high power, low noise amplifiers with improved thermal management because the heat sources are spread over a large area, resulting in lower life-cycle costs. In addition, the amplifiers feature easy alarm circuit implementation for fault location and repairability.
The models PA1124C (cellular) and PA1125C (PCS) high power connectorized amplifier assemblies provide an efficient and simple way to achieve a high level of distortion-free performance over a wide frequency range without the need for complex and costly feedforward or predistortion linearization techniques. These new LPAs are a result of new transistor technologies, advanced circuit topologies and CAD tools, and high volume production techniques. Figure 1 shows typical cellular and PCS output spectra produced by the two respective power amplifiers.
THE MODULAR AMPLIFIERS
Essentially, the LPAs are ultralinear FET power amplifier modules that are designed specifically to amplify the multitone signals of today's newer-generation wireless communications systems with very low intermodulation distortion. The modules are used to drive the high power feedforward amplifiers used in cellular, PCS and wireless local loop base stations.
The new amplifiers feature excellent gain flatness and phase linearity, and low SWR. In addition, the devices incorporate internal temperature compensation and are equipped with all of the necessary DC bias networks to make them easy to integrate into the system's architecture. Unconditional stability over the full temperature range is guaranteed, thus operating into reactive loads poses no oscillation problems. The self-contained amplifiers require only the application of a DC supply voltage, resulting in faster time to market for the new systems.
The amplifiers utilize a combination of unique FET selection and proprietary matching circuit design to provide superior intermodulation performance. In addition, mirror-image designs provide an efficient way to combine these amplifier modules with stripline combiners. Thus, the architecture provides a method of combining very high linearity, lower power amplifiers that achieve feedforward performance at a much lower complexity level.
The PA1124C and PA1125C power amplifiers utilize eight individual amplifier modules that are assembled in four combined amplifier pairs. The PA1124C amplifier assembly uses standard PA1131 and PA1131L (L denoting the mirror-image amplifier configuration) modular blocks, while the PA1125C amplifier assembly uses PA1132 and PA1132L blocks. The mirror-image amplifier pairs are fed from a new four-way serial combiner that provides a more streamlined final assembly layout. The serial combiner has the advantage of allowing odd numbers of amplifiers to be combined to achieve any desired output power level.
The serial combiner, shown in Figure 2 , consists of 6, 4.77 and 3 dB stripline couplers to drive each of the amplifier balanced pairs with one-quarter the input RF power. The output of each balanced amplifier pair is serially combined with 3, 4.77 and 6 dB couplers to provide the assembly's full output power.
The amplifier coupler design features low noise, high dynamic range and good input and output SWR. The coupler design permits the units to work into open or shorted loads without permanent damage, which eliminates the need for costly and bulky isolators. Both units are capable of operating over the full -40° to +85°C case temperature range with good performance. Table 1 lists performance specifications for the individual amplifier modules and power amplifier assemblies.
The two power amplifiers also provide a high level of phase linearity across the entire operating band, making the assemblies suitable for use in feedforward configurations as error amplifiers or main amplifier drivers. The amplifiers' phase insertion can be easily corrected by adding a coaxial cable to compensate for the added delay in the feedforward loop.
In addition, the modular amplifier approach allows distribution of the heat sources over a larger area of the overall metal housing. In comparison, thermal management of power transistors used in conventional high power amplifiers is more difficult to achieve because the heat is concentrated in point sources. For this reason, the modular approach allows higher temperature operation with longer lifetimes.
The two amplifier assemblies have also found use as laboratory amplifiers to extend the performance of CDMA and TDMA multitone test stations. These amplifiers are suitable for amplifying low level signals from wireless testers and multitone signal generators up to a 3 W composite power level. By using these amplifiers, the system designer is provided with a clean, low cost, high power signal to test their designs. The good broadband capability, low SWR and high load pull capability eliminate the need for expensive bulky isolators in the test system.
A novel low cost approach for ultralinear high power amplifier architecture has been developed. The new amplifiers display superior linearity performance using a simple modular construction that is easy to manufacture and maintain in the field.
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