RF Power Amplifiers are used in a wide variety of applications including Wireless Communication, TV transmissions, Radar, and RF heating. The basic techniques for RF power amplification can use classes as A, B, C, D, E, and F, for frequencies ranging from VLF (Very Low Frequency) through Microwave Frequencies.
RF Output Power can range from a few mW to MW, depend by application.
The introduction of solid-state RF power devices brought the use of lower voltages, higher currents, and relatively low load resistances.
This article describes improvements in device technology and design techniques that will enable power amplifiers with higher efficiency and better linearity performance — at higher frequencies
Agrowing number of semiconductor technologies are being applied to RF power transistor applications. These technologies include Si LDMOS FET, SiGe HBT, InGaP HBT, GaAs MESFET, AlGaAs pHEMT, SiC MESFET and AlGaN/GaN HEMT. The dependencies of linearity and efficiency of such technologies are often common, such as transconductance derivatives, capacitance variations, breakdown effects and parasitic resistances. This article overviews the work that has been achieved to date to maximize linearity and efficiency in the most promising technologies, as related specifically to infrastructure applications. The article also addresses the increasing number of device and circuit level techniques that are being used to enhance these two important parameters as required for IM3, ACPR and ACLR suppression in 3G systems such as W-CDMA/UMTS.
This article focuses on high power (that is greater than 10 watt) RF transistor technologies where digital modulation techniques are demanding higher and higher peak-to-average ratios (PARs) and thus higher peak powers. Peak and average DC-to-RF efficiencies have become critical parameters, and much attention is being focused in decreasing multi-carrier intermodulation distortion, adjacent channel power ratios (ACPRs) and adjacent channel leakage ratios (ACLRs). Unfortunately, improving transistor linearity often leads to decreased efficiency which directly affects overall system efficiency, heat removal, size and cost.
Competing Technologies The generation of solid state RF power has been in existence since the late 1960s when silicon bipolar transistors were introduced by such companies as TRW and RCA (ref. 1). Today there are a range of technologies available, including silicon bipolar, silicon LDMOS FET, GaAs MESFET, GaAs pHEMT, AlGaAs/InGaAs HFET, GaAs, InP, InGaP and SiGe HBT as well as wide bandgap transistors such as SiC MESFET and AlGaN/GaN
Of particular interest today are wide bandgap transistors such as silicon carbide (SiC) MESFETs and gallium nitride (GaN) HEMTs. Such transistors exhibit very high RF power densities (watts per mm of gate width) compared to any other technologies (by a factor of 10 over GaAs MESFET for example) (ref. 3 and 4).
Wide band-gap transistors fabricated from 4H-SiC and AlGaN/GaN offer superior RF performance, particularly at elevated temperatures, compared to comparable components fabricated from GaAs or Si. RF output powers on the order of 4 to 7 W/mm and 10-12 W/mm are achievable from SiC MESFETs and AlGaN/GaN HFETs respectively. Achievement of higher power densities is a priority for RF power technologies as it reduces size, which is important in both fixed and mobile platforms. It also provides higher working impedances, which are important for wider bandwidth operation, simpler circuits and easier manufacture.
Figure 2 shows a comparison of the input and output impedances of a 20 mm GaN HEMT delivering greater than 100 watts CW peak power with a commercially available Si LDMOS FET of similar power capability. Clearly, the GaN HEMT has much more convenient impedance levels which can also result in easier packaging whereby no internal pre-matching is needed (Figure 3). The higher gain of the GaN device requires lower drive drive. Initial linearity measurements show similar performance for the two device technologies.
Both SiC MESFETs and GaN HEMTs show promising efficiencies and linearities. For example, Figure 4 shows the peak efficiencies of a GaN HEMT as a function of drain-tosource voltage over a range of 10 to 40 volts. Note that the drain efficiency remains almost constant at greater than 60 percent over the complete voltage range which enables efficiencies to be optimized at reasonable back-off powers (e.g. up to 10 dB). Figure 5 shows an example of the promising linearity that can be obtained from such wide bandgap transistors. The figure shows a comparison between a 1.2 mm gate width GaAs pHEMT and a 1 mm gate width AlGaN HEMT. Although these transistors have comparable gate widths the AlGaN HEMT provides >10 dBm more output power with improved third order intermodulation distortion.
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· Most important parameters that defines an RF Power Amplifier are:
1. Output Power
5. DC supply voltage
Choosing the bias points of an RF Power Amplifier can determine the level of performance ultimately possible with that PA. By comparing PA bias approaches, can evaluate the tradeoffs for: Output Power, Efficiency, Linearity, or other parameters for different applications.
· The Power Class of the amplification determines the type of bias applied to an RF power transistor.
· The Power Amplifier’s Efficiency is a measure of its ability to convert the DC power of the supply into the signal power delivered to the load.
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Power amplifierdesign papers free download
An Integrated 2GHz 500mW Bipolar Amplifier,Dr. S. Weber, G. Doing,ISSCC/RFIC 1998, Denver.
Modeling for Si-Bipolar Power Amplifiers,Dr. S. Weber, AACD 1998, Kopenhagen.
First Integrated Bipolar RF PA Family for Cordless Telephones,Dr. Stephan Weber, ESSCIRC 1999 Duisburg.
Power Controller for Dual Band TDMA Power Amplifiers,Dr. Stephan Weber, RFIC/IMS 2001, Phoenix.
PA Design,Danilo Gerna, Alexandre Giry, CRAFT, EUROPEAN PROJECT N.25710, September First 1999.
MONOLITHIC TRANSFORMER-COUPLED RF POWER AMPLIFIERS IN SI-BIPOLAR,Werner Simbuerger, D. Kehrer, A. Heinz, H.D. Wohlmuth, M. Rest, K. Aufinger, A.L. Scholtz, presentation at AACD 2001.
MONOLITHIC TRANSFORMER-COUPLED RF POWER AMPLIFIERS IN SI-BIPOLAR,Werner Simbuerger, D. Kehrer, A. Heinz, H.D. Wohlmuth, M. Rest, K. Aufinger, A.L. Scholtz, presentation at AACD 2001.Presentation slides.
RF Power Amplifier Design,Markus Mayer & Holger Arthaber, Department of Electrical Measurements and Circuit Design, Vienna University of Technology, June 11, 2001
Report on HF MOST model benchmarking through key blocks validation, the simulated performance of a simple 900 MHz Power Amplifier is compared with its measured performance,” R. van Dongen, European Project Project Nr. 25710 August 26, 1999.
DECT power amplifier chip,” in : Wireless Trench technology for portable wireless applications, Ericsson Review No. 01, 2001Ted Johansson.
A 2.4-GHz, 2.2-W, 2-V Fully-Integrated CMOS Circular-Geometry Active-Transformer Power Amplifier
A MONOLITHIC 5.8 GHZ POWER AMPLIFIER IN A 25 GHZ FT SILICON BIPOLAR TECHNOLOGY,W. Simbürger e.a., Infineon Technologies, Corporate Research, Otto-Hahn-Ring 6, D-81730 Munich, Germany.