Modular Design of Power Amplifiers

Anaren Microwave Inc.
E. Syracuse, NY

Power amplifier designers today face increased peak power-handling requirements due to the new digital modulation forms used in wideband CDMA (W-CDMA) systems. At the same time, the power amplifiers must meet stricter linearity requirements and the available RF/microwave design resources are scarce. A modular amplifier design approach provides a way to deal with the insufficient availability of design resources and takes advantage of the work already performed by engineers at semiconductor manufacturing facilities to test the transistors in real-world applications. Application notes from most transistor manufacturers provide a suggested close-to-production-ready board layout. These designs can be easily fit into modules that can be used with a new series of splitters and combiners designed specifically for this purpose.

The new AdrenaLine® splitter/combiner networks are not limited to binary splits. Efficient modular designs are possible through their use of a serial passive coupler network implemented in low loss materials. This modular approach to power amplifier design means that amplifier designers are now free to focus on higher level design tasks, including linearization techniques, that provide the real difference between amplifier systems.

New modulation techniques, such as those used in W-CDMA, set requirements for high peak power, which means that very efficient power combining must be achieved. The AdrenaLine splitter/combiners provide low loss combining and an innovative design approach that enables high performance to be achieved over a frequency range that covers multiple systems. (For example, DCS, PCS and W-CDMA are covered with one part.)

Modular Amplifiers

Modular power amplifiers offer amplifier designers flexibility to change configurations and modify, for example, output power and/or frequency band of operation. Designing this type of power amplifier starts with the design of a module or building block based on a preliminary idea of what the power output from that transistor will be and the number of devices required in parallel. The module or building block typically consists of one or more transistors with bias circuitry, including temperature compensation and decoupling and input and output impedance matching (for the desired gain, linearity and flatness).

Traditionally, amplifier building blocks have been paralleled on a large PCB using hybrid couplers, Wilkinson power dividers, unbalanced-to-balanced transformers (baluns) or some combination of these components. The major advantage of the balun is the added value of not just paralleling transistors, but also impedance transformation. The use of baluns has become more popular since they are now available in reasonable surface-mountable sizes at microwave frequencies for high volume manufacturing. Traditional baluns include the coaxial balun, which requires hand mounting, and the larger printed microstrip balun. However, the popular LDMOS transistors have input/output impedances often well below 1 W, which makes matching to 50 W a challenge and, for example, much more difficult than matching to 25 W using a balun.

The modular design approach proposed here is shown in Figure 1 . The modular amplifier consists of a splitting network feeding a number of amplifier modules, which are then fed into the combining network. The amplifier modules are connected to the splitter/combiner networks using either solder straps or connectors. In addition, DC biasing may be supplied through a connection to the splitter/combiner units.

A modular design approach allows for easy module testing to ensure specification compliance before the complete amplifier is built. This capability can minimize or eliminate costly troubleshooting of an assembled board. In addition, a failed amplifier module can be easily replaced, which is not possible in a nonmodular amplifier.

If a slight configuration change is required in an amplifier, for example, the power output requirement of an amplifier (built with 4 x 20 W modules) changes from 80 to 100 W, another 20 W module can be added just by changing the splitter/combiner from four way to five way. An integrated board amplifier would make this modification very difficult without completely redesigning the PCB. As illustrated, the modular approach amplifier using AdrenaLine splitter/combiners provides the shortest overall line length and minimizes the use of a lossy microstrip line. This modular approach also helps reduce the complete amplifier to an absolute minimum overall size.

Splitting/Combining Techniques

Equal split Wilkinson power dividers and 3 dB hybrid couplers lend themselves to binary splitting and combining (that is, two, four, eight...ways). In a modular-type amplifier, a more flexible technique is desirable where any number of splits is possible. The serial coupler technique offers this type of flexibility.

Consider the case of a desired three-way split, shown in Figure 2 . The input power is first applied to a 4.77 dB coupler that supplies one-third of the power to the first output (that is, -4.77 dB of the input power). The remainder of the power (two-thirds) is fed into a 3.01 dB coupler that splits the remaining power into two halves. Therefore, an equal three-way split has been performed. Note that the phase of each of the inputs to the amplifiers is different but has a constant slope between outputs (for example, 0°, -90° and -180° assuming a 0° interconnect length). The outputs from the splitter are then input to each of the three amplifier modules whose outputs are fed to a three-way combiner. The combiner is the same type of network used on the splitter side except that the 0"‑ split output is connected to the -180° combine port, the -90° is connected to the -90° combine port and the -180° split is connected to the 0° combine port. Therefore, the phase length through any one of the three paths is identical. If a four-way combiner is desired, the first coupler would be 6.02 dB followed by a 4.77 dB coupler and then a 3.01 dB coupler. This configuration can be extended to provide the desired number of splits, limited only by the processing variations used to build the circuitry.

In addition to the added flexibility of any number of splits, the layout of a serial coupler also provides the shortest interconnect lines, since couplers are connected directly back-to-back, thereby minimizing loss. The serial approach also provides the desired form factor of a long and narrow splitter/combiner network. An analysis of loss due to amplitude and phase imbalance for Wilkinson power dividers, 3 dB hybrid couplers and the serial coupler networks has been published previously.1

Serial coupler networks are implemented in stripline packages with backward-wave couplers where two quarter-wavelength lines are coupling to each other. A typical stackup for a low loss stripline package is shown in Figure 3 . The stripline circuit consists of an upper and lower RF layer bonded to the RF center layer where the actual circuit runs are printed. The RF outputs from the couplers on the RF center layer are fed to the top via feedthroughs, while the isolated ports are connected to high power terminations with solder straps. Alternatively, the RF outputs can be connected through a vertical launch connector.

Since the splitter/combiners are typically used in high power applications, the entire stripline package is mounted on a copper plate to heat sink the terminations and circuitry. Additional layers can be added on the top for DC distribution. On the top layer, decoupling capacitors also can be mounted or additional circuitry can be integrated.

Table 1 lists a number of standardized AdrenaLine splitter/combiner networks that are available with either pads for solder straps or SMA connectors. The solder tab versions (in three-, four- and five-way configurations for 0.8 to 1.0 GHz and 1.8 to 2.2 GHz) are production ready and include a DC distribution layer with decoupling electrolytic capacitors. The connectorized units (in three-, five- and eight-way configurations for 0.8 to 1.0 GHz and 1.8 to 2.2 GHz) are targeted more at prototype testing where the size of the amplifier modules can be arbitrary since they are not mounted directly to the splitter/combiners. If the total number of modules required for a final system is not known, an eight-way network may be used for experimentation since not all input/outputs are required for use in laboratory purposes. All unused ports may simply be terminated and the total power output adjusted by a simple known insertion loss calculation to determine the actual output power when an appropriate size splitter/combiner set is used.

Conclusion

A modular power amplifier concept using AdrenaLine splitter/combiners that permit amplifier designers to achieve a higher degree of re-use of designs or simply to take advantage of modules provided by semiconductor manufacturers has been demonstrated. This time savings allows for more time to concentrate on higher level design tasks such as linearization techniques. Any number of amplifiers can be paralleled using a serial coupler network implemented in a stripline package that provides low insertion loss and a high potential for further integration. Commercially available AdrenaLine splitter/combiner networks with three- to eight-way splits are offered with tabs for solder straps or SMA connectors. Additional information may be obtained from the company's Web site at www.anaren.com.

Reference

1.         J. Merrill, "Considering Loss in RF Amplifier Splitter/Combiners," Microwaves & RF, August 1998. Anaren Microwave Inc., E. Syracuse, NY (315) 432-8909.