The arrangement of building blocks is known as the architectureof 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.
A. Linear Architecture
The conventional architecture for a linear microwave transmitter consists of a baseband or IF modulator, an up-converter, and a power-amplifier chain (Fig. 13). The amplifier chain consists of cascaded gain stages with power gains in the range of 6–20 dB. If the transmitter must produce an amplitude-modulated or multicarrier signal, each stage must have adequate linearity. This generally requires class-A amplifiers with substantial power backoff 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.
B. Power CombinersWhether to use a number of smaller PAs versus a single larger PA is one of the most basic decisions in selection of an architecture [14]. Even when larger devices are available, smaller devices often offer higher gain, a lower matching 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.
In the corporate architecture (Fig. 14), power is split and combined in steps of two. Hybrid combiners isolate the two PAs from each other and allow one to continue operating if the other fails. Quadrature combiners insert a 90 phase shift at the input of one PA and a 90 phase shift at the output of the other.
This provides a constant input impedance, 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). TheWilkinson combiner is fabricated using quarter-wavelength lines and can be extended to include more than two inputs or outputs.
C. Stage Bypassing and Gate SwitchingStage-bypassing and gate-switching techniques reduce power consumption and increase efficiency by switching between large and small amplifiers (e.g., the driver) according to peak signal level. This can significantly increase the transmitter efficiencywhen operating well into backoff, as shown in Fig. 8 ("GS") for
ideal class-B PAs. These techniques are particularly effective
for mobile handsets that operate over a large dynamic range,
and improvement of the average efficiency from 2.1% to 9.5%
has been demonstrated [20].
D. Kahn Technique
The Kahn envelope elimination and restoration (EER) technique(Fig. 15) combines a highly efficient, but nonlinear RFPA with a highly efficient envelope amplifier to implement a high-efficiency linear RF PA. In its classic form, a limiter eliminates the envelope, allowing the constant-amplitude phase modulated carrier to be amplified efficiently by class-C, class-D, class-E, or class-F RF PAs. Amplitude modulation of the final RF PA restores the envelope to the phase-modulated carrier creating an amplified replica of the input signal. EER is based upon the principle that any narrow-band signal can be produced by simultaneous amplitude (envelope) and phase modulations. In a modern implementation, both the
envelope and phase-modulated carrier are generated by a DSP. In contrast to linear amplifiers, a Kahn-technique transmitter operates with high efficiency over a wide dynamic range and, therefore, produces a high average efficiency for a wide range of signals and power (backoff) levels. Average efficiencies three to five times those of linear amplifiers have been demonstrated from HF to -band [21]. Transmitters based upon the Kahn technique generally have excellent linearity because linearity depends upon the modulator rather than RF-power transistors. The two most important factors affecting the linearity are the envelope bandwidth and
alignment of the envelope and phase modulations. The envelope bandwidth must be at least twice the RF bandwidth and the misalignment must not exceed one-tenth of the inverse of the RF bandwidth [22]. In practice, the drive is not hard limited andfollows the envelope, except at low levels [23]. At higher microwave frequencies, the RF-power devices exhibit softer saturation characteristics and larger amounts of amplitude-to-phase conversion, necessitating the use of predistortion.
The most widely used high-level modulator is class S (Fig. 16). A transistor and diode or a pair of transistors act as a two-pole switch to generate a rectangular waveform with a switching frequency several times that of the output signal. The width of pulses is varied in proportion to the instantaneousamplitude of the desired output signal, which is recovered by a low-pass filter. Class S is ideally 100% efficient and, inpractice, can have high efficiency over a wide dynamic range. The switching frequency must typically be six times the RF bandwidth. A switching frequencies of 500 kHz is readily achieved with discrete components, and 10 MHz is achievable
in IC implementations. Class-G and split-band modulators canbe used in wide-band applications.
E. Envelope Tracking
The envelope-tracking architecture is similar to that of the Kahn technique. The supply voltage is varied dynamically to conserve power, but with sufficient excess ("headroom") to allow the RF PA to operate in a linear mode. The RF drive contains both amplitude and phase information, and the burden of providing linear amplification lies entirely on the final RF PA. Typically, the envelope is detected and used to control a dc–dc converter. While both buck (step-down) oboost (step-up) converters are used, the latter is more common as it allows operation of the RF PA from a supply voltage higher than the dc-supplyvoltage. This configuration is also more amenable to the use ofn-p-n or n-channel transistors for fast switching. The result is a minimum corresponding to the dc-supply voltage and tracking of larger envelopes with a fixed headroom. If the RF PA is operated in class A, its quiescent current can also be varied.
The efficiency is significantly better than that of a linear RF
PA operating from a fixed supply voltage, but lower than that of the Kahn technique. The efficiency of a system based upon an ideal converter and class-B RF PA with headroom that is
10% of peak is included in Fig. 7 ("ET"). In practice, power consumption by the converter and other circuits further reduces the efficiency at lower output amplitudes.
A high switching frequency in the dc–dc converter allows both a high modulation bandwidth and the use of smaller inductors and capacitors. Converters with switching frequencies of 10–20 MHz have recently been implemented using MOS ASICs[24], GaAs HBTs [25], and RF-power MOSFETs [26]. The average efficiency for CDMA signals is typically increased from that of a conventional linear amplifier by a factor of 1.5–2.
F. Outphasing
Outphasing was invented by Chireix during the 1930s as a means of obtaining high-quality AM from vacuum tubes with poor linearity and was used through about 1970 in RCA "ampliphase" AM-broadcast transmitters. In the 1970s, it came into use at microwave frequencies under the name LINC (i.e., linear amplification using nonlinear components). An outphasing
transmitter (Fig. 17) produces an amplitude-modulated signal by combining the outputs of two PAs driven with signals of different time-varying phases. Basically, the phase modulation
causes the instantaneous vector sum of the two PA outputsto follow the desired signal amplitude. The inverse sine of envelope phase modulates the driving signals for the two
PAs to produce a transmitter output that is proportional to . In a modern implementation, a DSP and synthesizer produce the inverse-sine modulations of the driving signals. Virtually all microwave outphasing systems in use today employ hybrid combiners to isolate the two PAs from each other and to allow them to see resistive loads at all signal levels. However, both PAs deliver full power all of the time. Consequently, the efficiency of a hybrid-coupled outphasing transmitter varies with the output power (as in a class-A PA), resulting in an average efficiency that is inversely proportional to peak-to-average ratio (as in class A). Recovery of the power from the dump port
of the hybrid combiner offers some improvement in the efficiency. Summation of the out-of-phase signals in a nonhybrid combiner inherently results in variable reactive PA-load impedances.If the combiner is untuned, the current drawn from the PAs is proportional to the transmitter output voltage, resulting in an efficiency characteristic that varies with signal amplitude, as in a similar class-B PA. The Chireix technique uses shunt reactances on the inputs to the combiner to tune out the drain reactances at a particular amplitude, which, in turn, maximizes the efficiency
in the vicinity of that amplitude. In the classic implementation, the efficiency is maximized at the level of the unmodulated AM carrier and remains high over the upper 6 dB of the output range
(Fig. 7) and for about 8 dB into backoff (Fig. 8).With judicious choice of the shunt susceptances, the average efficiency can be maximized for any given signal [27]. For example, the average efficiency for a multicarrier signal with a 10-dB peak-to-average ratio can be boosted from the 28% of class B to 52.1%. Simulations suggest that nonhybrid combining of microwave PAs increases both efficiency and distortion [28].
G. Doherty Technique
The classical Doherty architecture (Fig. 18) combines two PAs of equal capacity through quarter-wavelength lines or networks. The "carrier" (main) PA is biased in class B, while the "peaking" (auxiliary) PA is biased in class C. Only the carrier PA is active when the signal amplitude is half or less of the PEP amplitude. Both PAs contribute output power when the signal amplitude is larger than half of the PEP amplitude. Operation of the Doherty system can be understood by dividing it into low-power, medium-power (load-modulation), and peak-power regions [29]. In the low-power region, the
speaking PA remains cut off and appears as an open circuit. The carrier PA, therefore, sees a 100- load and operates as an ordinary class-B amplifier. The instantaneous efficiency increases linearly with output, reaching the 78.5% of ideal class B at saturation of the carrier PA at 6 dB from transmitter PEP. As the signal amplitude increases into the medium-power region,
the peaking PA becomes active. The additional current sent to the load by the peaking PA causes the apparent load
impedance at to increase above the 25 of the low-power region. Transformation through the quarter-wavelength line results
in a decrease in the load presented to the carrier PA.
carrier PA remains in saturation and acts as a voltage source. It operates at peak efficiency and delivers an increasing amount of power. At PEP output, both PAs see 50- loads and each delivershalf of the system output power. The PEP efficiency is ideally the 78.5% of class-B PAs.
Power Amplifiers and Transmitters for RF and Microwave (3/4)
http://www.ee.pucrs.br/~decastro/pdf/RF&MicrowavePowerAmp&XMTRs.pdf
CRF_Zambrano C. Jaydi D.
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