Anritsu HFE0903 RaabPart3

34 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

RF and Microwave Power Amplifier and Transmitter Technologies —

Part 3

By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington, Zoya B. Popovich, Nick Pothecary, John F. Sevic and Nathan O. Sokal

he building blocks used in transmit-

ters are not only power amplifiers, but a variety of other circuit elements including oscillators, mixers, low-level amplifiers, filters, matching networks, combiners, and circulators. The

arrangement of building blocks is known as the architecture of a transmitter. The classic transmitter architecture is based upon linear PAs and power combiners. More recently, transmitters are being based upon a variety of different architectures including stage bypassing, Kahn, envelope tracking, outphasing, and Doherty. Many of these are actually fairly old techniques that have been recently made practical by the capabilities of DSP.

7a. LINEAR ARCHITECTURE

The conventional architecture for a linear microwave transmitter consists of a baseband or IF modulator, an up-converter, and a poweramplifier chain (Figure 20). The amplifier chain consists of cascaded gain stages with power gains in the range of 6 to 20 dB. If the transmitter must produce an amplitude-modulated or multi-carrier signal, each stage must have adequate linearity. This generally requires class-A amplifiers with substantial power back-off for all of the driver stages. The final amplifier (output stage) is always the most costly in terms of device size and current consumption, hence it is desirable to operate the output stage in class B. In applications requiring very high linearity, it is necessary to use class A in spite of the lower efficiency.

The outputs of a driver stage must be matched to the input of the following stage much as the final amplifier is matched to the load. The matching tolerance for maintaining power level can be significantly lower than that for gain [60], hence the 1-dB load-pull contours are more tightly packed for power than for gain.

To obtain even modest bandwidths (e.g., above 5 percent), the use of quadrature balanced stages is advisable (Figure 21). The main benefit of the quadrature balanced configuration is that reflections from the transistors are cancelled by the action of the input and output couplers. An individual device can therefore be deliberately mismatched (e.g., to achieve a power match on the output), yet the quadrature-combined system appears to be well-matched. This configuration also acts as an effective power combiner, so that a given power rating can be achieved using a pair of devices having half of the required power performance. For moderate-bandwidth designs, the lower-power stages are typically designed using a simple single-ended cascade, which in some cases is available as an RFIC. Designs with bandwidths approaching an octave or

Transmitter architectures is

the subject of this install-

ment of our continuing

series on power amplifiers,

with an emphasis on

designs that can meet

today’s linearity and high

efficiency requirements

Figure 20 · A conventional transmitter.

RF/

Baseband

Exciter

Mixer

3-stage PA

From September 2003 High Frequency Electronics

36 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

more require the use of quadraturebalanced stages throughout the entire chain.

Simple linear-amplifier chains of this kind have high linearity but only modest efficiency. Single-carrier applications usually operate the final amplifier to about the 1-dB compression point on amplitude modulation peaks. A thus-designed chain in which only the output stage exhibits compression can still deliver an ACPR in the range of about –25 dBc with 50-percent efficiency at PEP.

Two practical problems are frequently encountered in the design of linear PA chains: stability and low gain. Linear, class-A chains are actually more susceptible to oscillation due to their high gain, and singlepath chains are especially prone to unstable behavior. Instability can be subdivided into the two distinct categories: Low-frequency oscillation and in-band instability. In-band instability is avoided by designing the individual gain stages to meet the criteria for unconditional stability; i.e., the Rollet k factor [61] must be greater than unity for both in-band and out-of-band frequencies. Meeting this criterion usually requires sacrificing some gain through the use of absorptive elements. Alternatively, the use of quadrature balanced stages provides much greater isolation between individual stages, and the broadband response of the quadrature couplers can eliminate the need to design the transistor

stage itself with k>1. This is another reason for using quadrature coupled stages in the output of the chain.

Large RF power devices typically have very high transconductance, and this can produce low-frequency instability unless great care is taken to terminate both the input and output at low frequencies with impedances for unconditional stability. Because of large separation from the RF band, this is usually a simple matter requiring a few resistors and capacitors.

At X band and higher, the power gain of devices in the 10 W and above category can drop well below 10 dB. To maintain linearity, it may be necessary to use a similarly size device as a driver. Such an architecture clearly has a major negative impact upon the cost and efficiency of the whole chain. In the more extreme cases, it may be advantageous to consider a multi-way power combiner, where 4, 8, or an even greater number of smaller devices are combined. Such an approach also has other advantages, such as soft failure, better thermal management, and phase linearity. However, it typically consumes more board space.

7b. POWER COMBINERS

The need frequently arises to combine the outputs of several individual PAs to achieve the desired transmitter output. Whether to use a number of smaller PAs vs. a single larger PA is one of the most basic decisions in selection of an architec-

ture [60]. Even when larger devices are available, smaller devices often offer higher gain, a lower matching Q factor (wider bandwidth), better phase linearity, and lower cost. Heat dissipation is more readily accomplished with a number of small devices, and a soft-failure mode becomes possible. On the other hand, the increase in parts count, assembly time, and physical size are significant disadvantages to the use of multiple, smaller devices.

Direct connection of multiple PAs is generally impractical as the PAs interact, allowing changes in output from one to cause the load impedance seen by the other to vary. A constant load impedance, hence isolation of one PA from the other, is provided by a hybrid combiner. A hybrid combiner causes the difference between the two PA outputs to be routed to and dissipated in a balancing or “dump” resistor. In the event that one PA fails, the other continues to operate normally, with the transmitter output reduced to one fourth of nominal.

The most common power combiner is the quadrature-hybrid combiner. A 90° phase shift is introduced at input of one PA and also at the output of the other. The benefits of quadrature combining include constant input impedance in spite of variations of input impedances of the individual PAs, cancellation of odd harmonics, and cancellation of backward-IMD (IMD resulting from a signal entering the output port). In addition, the effect of load impedance upon the system output is greatly reduced (e.g., to 1.2 dB for a 3:1 SWR). Maintenance of a nearly constant output occurs because the load impedance presented to one PA decreases when that presented to the other PA increases. As a result, however, device ratings increase and efficiency decreases roughly in proportion to the SWR [65]. Because quadrature combiners are inherently two-terminal devices, they are used in a corporate combining architecture

Figure 21 · Amplifier stages with quadrature combiners.

0º

90º

38 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

(Figure 21). Unfortunately, the physical construction of such couplers poses some problems in a PC-board environment. The very tight coupling between the two quarter-wave transmission lines requires either very fine gaps or a three-dimensional structure. This problem is circumvented by the use of a miniature co-axial cable having a pair of precisely twisted wires to from the coupling section or ready-made, low-cost surface mount 3-dB couplers.

The Wilkinson or in-phase power combiner [62] is often more easily fabricated than a quadrature combiner. In the two-input form (as in each section in Figure 22), the outputs from two quarter-wavelength lines summed into load R

produce an apparent load impedance of 2R0, which is transformed through the lines into at the load impedances R

seen by the individual PAs. The difference between the two PA outputs is dissipated in a resistor connected across the two inputs. Proper choice of the balancing resistor (2R

) produces a hybrid combiner with good isolation between the two PAs. The Wilkinson concept can be extended to include more than two inputs [63].

Greater bandwidth can be obtained by increasing the number of transforming sections in each signal path. A single-section combiner can have a useful bandwidth of about 20 percent, whereas a two-section version can have a bandwidth close to an octave. In practice, escalating circuit losses generally preclude the use of more than two sections.

All power-combining techniques all suffer from circuit losses as well as mismatch losses. The losses in a simple two-way combiner are typically about 0.5 dB or 10 percent. For a four-way corporate structure, the interconnects typically result in higher losses. Simple open microstrip lines are too lossy for use in combining structures. One technique that offers a good compromise among cost, produceability, and performance, uses suspended stripline. The conductors are etched onto doublesided PC board, interconnected by vias, and then sus-

pended in a machined cavity. Structures of this kind allow high-power 8-way combiners with octave bandwidths and of 0.5 dB.

A wide variety of other approaches to power-combining circuits are possible [62, 64]. Microwave power can also be combined during radiation from multiple antennas through “quasi-optical” techniques [66].

7c. STAGE SWITCHING AND BYPASSING

The power amplifier in a portable transmitter generally operates well below PEP output, as discussed in Section 4 (Part 1). The size of the transistor, quiescent current, and supply voltage are, however, determined by the peak output of the PA. Consequently, a PA with a lower peak output produces low-amplitude signals more efficiently than does a PA with a larger peak output, as illustrated in Figure 23 for class-B PAs with PEP efficiencies of 60 percent. Stage-bypassing and gate-switching techniques [67, 68] reduce power consumption and increase efficiency by switching between large and small amplifiers according to signal level. This process is analogous to selection of supply voltage in a class-G PA, and the average efficiency can be similarly computed [69].

A typical stage-bypassing architecture is shown in Figure 24. For low-power operation, switches SA and SB route the drive signal around the final amplifier.

Figure 22 · Multi-section Wilkinson combining architecture.

Figure 23 · Power consumption by PAs of different sizes.

Figure 24 · Stage-bypassing architecture.

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