Transformer-coupled and complementary topologies are also used. The drain voltage and current waveforms of selected ideal PAs are shown in Fig. 6.
A. RF-Power Transistors
RF PAs utilize a wide variety of active devices, including bipolar-junction transistors (BJTs), MOSFETs, JFETs (SITs), GaAs MESFETs, HEMTs, pHEMTs, and vacuum tubes [6],
The power-output capabilities range from tens of kilowatts for vacuum tubes to hundreds of watts for Si MOSFETs at HF and VHF to hundreds of milliwatts for InP HEMTs at MMW
frequencies. Depending upon frequency and power, devices are available in packaged, chip, and MMIC form. Virtually all RF-power transistors are n-p-n or n-channel types because the
greater mobility of electrons (versus holes) results in better operation at higher frequencies. While the voltages and currents differ considerably, the basic principles for power amplification
are common to all devices.
B. Methods of Amplification
Class A: In class-A amplification, the transistor is in the active region at all times and acts as a current source controlled by the gate drive and bias. The drain–voltage and drain–current waveforms are sinusoids. This results in linear amplification with an output power of , where output voltage on load cannot exceed supply voltage . The dc-power input is constant, hence, the instantaneous efficiency (Fig. 7) is proportional to the power output and reaches 50% at PEP. The average efficiency is inversely proportional to the peak-to-average ratio (e.g., 5% for 10 dB) and backoff (Fig. 8). For amplification of amplitude-modulated signals, the quiescent current can
be varied in proportion to the instantaneous signal envelope. The utilization factor is 1/8. Class A offers high linearity, high gain, and operation close to the maximum operating frequency of the
transistor.
Class B: The gate bias in a class-B PA is set at the threshold of conduction so the transistor is active half of the time and the drain current is a half-sinusoid. Since the amplitude of the
drain current is proportional to drive amplitude, class B provides linear amplification. The instantaneous efficiency varies linearly with the RF-output voltage and reaches (78.5%) at PEP for an ideal PA. For low-level signals, class B is significantly more efficient than class A, and its average efficiency can be several times that of class A at high peak-to-average ratios (e.g., 28% versus 5% for dB). The utilization factor is the same 0.125 of class A. Class B is widely used in broad-band transformer-coupled PAs operating at HF and VHF. It is finding increasing use in microwave PAs, including experimental PAs using complementary devices.Class C: The gate of a classical (true) class-C PA is biased below threshold so that the transistor is active for less than half of the RF cycle. Linearity is lost, but efficiency can be increased arbitrarily toward 100% by decreasing the conduction angle toward zero. Unfortunately, this causes the output power (utilization factor) to decrease toward zero and the drive power to increase toward infinity. A typical compromise is a conduction
angle of 150 and an ideal efficiency of 85%. When driven into saturation, efficiency is stabilized and the output voltage is locked to supply voltage, allowing linear high-level amplitude modulation. Classical class C is widely used in high-power vacuum-tube transmitters, but is generally impractical for solidstate PAs.
Class D: Class-D PAs use two or more transistors as switches to generate square drain–voltage (or current) waveforms. A series-tuned output filter passes only the fundamental- frequency component to the load, resulting in a power outputs of for the transformer-coupled configuration. Current is drawn only through the transistor that is on, resulting in a 100% efficiency for an ideal PA. The utilization factor is the highest of any PA. If the switching is sufficiently fast, efficiency is not degraded by reactance in the load. Practical class-D PAs suffer from losses due to saturation, switching speed, and drain capacitance. Finite switching speed causes the transistors to be in their active regions while conducting current. Drain capacitances must be charged and discharged once per RF cycle, resulting in power loss that is proportional to [8] and increases directly with frequency. Class-D PAs with power outputs of 100 W to 1 kW are readily implemented at HF, but are seldom used above lower VHF because of losses associated with the drain capacitance. Recently, however, experimental class-D PAs have been tested with frequencies of operation as high as 1 GHz [9].
Class E: Class E employs a single transistor operated as a switch [10]. The drain–voltage waveform is the result of the sum of the dc and RF currents charging the drain-shunt capacitance. In optimum class E, the drain voltage drops to zero and has zero slope just as the transistor turns on. The result is an ideal efficiency of 100%, elimination of the losses associated with charging the drain capacitance in class D, reduction of switching losses, and good tolerance of component variation. Optimum class-E operation requires a drain shunt susceptance of and a drain series reactance . It delivers a power output of for an ideal PA with a utilization factor of 0.098. Variations in load impedance and shunt susceptance cause the PA to deviate from optimum operation, but the degradations in performance are generally no worse than those
for classes A and B.
The capability for efficient operation in the presence of significant drain capacitance makes class E useful in a number of applications. High-efficiencyHFPAswithpower levels to1 kWcan be implemented using low-cost MOSFETs intended for switching rather thanRFuse [11]. ClassEhas been used for high-efficiency amplification at frequencies as high as -band [12].
Class F: Class F boosts both efficiency and output by using harmonic resonators in the output network to shape the drain waveforms. The voltage waveform includes one or more odd harmonics and approximates a square wave, while the current includes even harmonics and approximates a half sine wave. Alternately ("inverse class F"), the voltage can approximate a half sine wave and the current a square wave. As the number of harmonics increases, the efficiency of an ideal PA increases from the 50% (class A) toward unity (e.g., 0.707, 0.8165, 0.8656, 0.9045 for two, three, four, and five harmonics, respectively) and the utilization factor increases from 1/8 toward [13]. The required harmonics arise naturally from nonlinearities and saturation in the transistor. While class F requires a more complex output filter than other PAs, the impedances at the "virtual drain" must be correct at only a few specific frequencies.
A variety of modes of operation in-between classes C, E, and F are possible. The maximum achievable efficiency [13] dependsupon the number of harmonics. The utilization factor depends upon the harmonic impedances and is highest for ideal class-F operation.
C. Load–Pull Characterization RF-power transistors are characterized by breakdownvoltages and saturated drain currents. The load impedance for maximum power results in drain voltage and current excursions from near zero to nearly the maximum values. The load impedances corresponding to delivery of a given amount of RF power with a specified maximum drain voltage lie along parallel-resistance lines on the Smith chart. The impedances for a specified maximum current analogously follow a series-resistance line. For an ideal PA, the resultant constant-power contour is football shaped [14].
In a real PA, the "virtual drain" is embedded behind the drain capacitance and bond-wire/package inductance. Transformation of the ideal drain impedance through these elements causes
the constant-power contours to become rotated and distorted. With the addition of second-order effects, the contours become elliptical. As shown in the example of Fig. 9, the power and efficiency contours are not necessarily aligned, nor do maximum power and maximum efficiency necessarily occur for the same load impedance. Sets of such "load–pull" contours are widely used to facilitate design tradeoffs. Load–pull analyses are generally iterative in nature, as changing one parameter may produce a new set of contours. A variety of different parameters can be plotted during a load–pull analysis, including not only power and efficiency, but also gain, distortion, and stability. Harmonic impedances as well as drive impedances can also be varied. The variable impedance required for load–pull testing can be obtained by mechanical, electrical, or active techniques.D. Microwave PAs
At microwave frequencies, lumped elements (capacitors, inductors) become unsuitable as tuning components and are used primarily as chokes and bypasses. Matching, tuning, and filtering at microwave frequencies are, therefore, accomplished with distributed (transmission-line) networks. Proper operation of PAs at microwave frequencies is achieved by providing the required drain–load impedance at the fundamental and a number of harmonic frequencies.
Class F: Typically, a transmission line between the drain and load provides the fundamental-frequency drain impedance of the desired value. A stub that is a quarter-wavelength at the harmonic of interest and open at one end provides a short circuit at the opposite end. The stub is placed along the main transmission line at either a quarter or a half-wavelength from the drain
to create either an open or short circuit at the drain [15]. The supply voltage is fed to the drain through a half-wavelength line bypassed on the power-supply end or alternately by a lumped-element choke. When multiple stubs are used, the stub for the highest controlled harmonic is placed near the drain. Stubs for lower harmonics are placed progressively further away and their lengths and impedances are adjusted to allow for interactions.
Typically, "open" means 3–10 times the fundamental-frequency impedance, and "shorted" means no more 1/10 to 1/3 of the fundamental- frequency impedance [13]. Dielectric resonators can be used in lieu of lumped-element traps. A wide variety of class-F PAs have been implemented at UHF and microwave frequencies. Generally, only one or two harmonic impedances are controlled. In one -band PA [16], for example, the output circuit provides a match at the fundamental and a short circuit at the second harmonic. The third-harmonic impedance is high, but not explicitly adjusted to be open. The3-dB bandwidth of such an output network is about 20%, and the efficiency remains within 10% of its maximum value over a bandwidth of 15%.
Class E: The drain–shunt capacitance and series inductive reactance required for optimum class-E operation result in a drain impedance of at the fundamental frequency, at the second harmonic, and proportionately smaller capacitive reactances at higher harmonics. At microwave frequencies, class-E operation is approximated by providing the drain with the fundamental frequency impedance and preferably one or more of the harmonic impedances [17]. An example of a microwave approximation of class E that provides the correct fundamental and second harmonic impedances [16], [17] is shown in Fig. 10. The stub immediately to the right of the FET is a quarter-wavelength long at the second harmonic so that the open circuit at its upper end is transformed to a short at its lower end. The line at the drain in combination with drain capacitance and inductance is also a quarter-wavelength to translate the short on its right end to an open at the virtual drain. The remaining lines provide the desired impedance at the fundamental. This circuit uses an FLK052 MESFET to produce 0.68 W at -band with a drain efficiency of 72% and PAE of 60%.
Methods exist for providing the proper impedances through the fourth harmonic [18]. However, the harmonic impedances are not critical [13], and many variations are, therefore, possible. Since the transistor often has little or no gain at the higher harmonic frequencies, those impedances often have little or no effect upon performance. A single-stub match is often sufficient to provide the desired impedance at the fundamental while simultaneously providing an adequately high impedance at the second harmonic, thus eliminating the need for an extra stub
and reducing a portion of the losses associated with it. Most microwave class-E amplifiers operate in a suboptimum mode. Demonstrated capabilities range from 16Wwith 80% efficiency at UHF (LDMOS) to 100 mW with 60% efficiency at 10 GHz[10], [17], [19].Comparison: Classes AB and F have essentially the same saturated output power, but class F has about 15% higher efficiency and class E has the highest efficiency [19]. Gain compression occurs at a lower power level for class E than for class F. For a given efficiency, class F produces more power. For the same maximum output power, the third-order IMD products are about 10 dB lower for class F than for class E. Lower power PAs implemented with smaller RF-power devices tend to be more efficient than PAs implemented with larger devices.
Power Amplifiers and Transmitters for RF and Microwave (2/4)
http://www.ee.pucrs.br/~decastro/pdf/RF&MicrowavePowerAmp&XMTRs.pdf
CRF_Zambrano C. Jaydi D.
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