View source for RF power amplifier

{{Short description|Type of electronic amplifier}}
{{Distinguish|Audio power amplifier}}{{use dmy dates|date=August 2021}}
[[File:Graduate School Project (2870679632).jpg|thumb|right|An RF power amplifier]]
[[File:MFR317 FM amp.jpg|thumb|Class C VHF power amplifier based on the transistor MRF317]]
A '''radio-frequency power amplifier''' ('''RF power amplifier''') is a type of [[electronic amplifier]] that converts a low-power [[radio-frequency]] (RF) [[signal (electrical engineering)|signal]] into a higher-power signal.<ref name=APITech-RF-Amps/> Typically, RF power amplifiers are used in the final stage of a [[radio transmitter]], their output driving the [[Antenna (radio)|antenna]]. Design goals often include [[gain (electronics)|gain]], power output, bandwidth, power efficiency, linearity (low [[Gain compression|signal compression]] at rated output), input and output impedance matching, and heat dissipation.
==Amplifier classes==
RF amplifier circuits operate in different modes, called "classes", based on how much of the cycle of the sinusoidal radio signal the amplifier (transistor or vacuum tube) is conducting current. Some classes are [[Class-A amplifier|class&nbsp;A]], [[Class-AB amplifier|class&nbsp;AB]], [[electronic amplifier#Class B and AB|class&nbsp;B]], which are considered the linear amplifier classes in which the active device is used as a controlled current source, while [[electronic amplifier#Class C|class&nbsp;C]] is a nonlinear class in which the active device is used as a switch.  The [[bias (electrical engineering)|bias]] at the input of the active device determines the class of the amplifier.
A common trade-off in power amplifier design is the trade-off between efficiency and linearity. The previously named classes become more efficient, but less linear, in the order they are listed. Operating the active device as a switch results in higher efficiency, theoretically up to 100%, but lower linearity.<ref name=Lee-2003-CMOS/> Among the switch-mode classes are  [[Class-D amplifier|class&nbsp;D]], [[Power amplifier classes#Class F|class&nbsp;F]] and [[Power amplifier classes#Class E|class&nbsp;E]].<ref name=Cloutier-ClassE-amps/> The class&nbsp;D amplifier is not often used in RF applications because the finite switching speed of the active devices and possible charge storage in saturation could lead to a large I-V product,<ref name=Lee-2003-CMOS/> which deteriorates efficiency.
==Solid state vs. vacuum tube amplifiers==
Modern RF power amplifiers use [[power semiconductor device|solid-state devices]], predominantly [[MOSFET]]s (metal–oxide–semiconductor field-effect transistors).<ref name=Balinga-2005-SiRF-MOSFETs/><ref name=MFJ-ALS-1300-tmos/><ref name=Perugupalli-Leighton-etal-2001/> The earliest MOSFET-based RF amplifiers date back to the mid-1960s.<ref name=Austin-Dean-etal-1966-11/> [[Bipolar junction transistor]]s were also commonly used in the past, up until they were replaced by [[power MOSFET]]s, particularly [[LDMOS]] transistors, as the standard technology for RF power amplifiers by the 1990s,<ref name=Balinga-2005-SiRF-MOSFETs/><ref name=Perugupalli-Leighton-etal-2001/> due to the superior RF performance of LDMOS transistors.<ref name=Perugupalli-Leighton-etal-2001/> Generally speaking, solid-state power amplifiers contain four main components: input, output, amplification stage and power supply.<ref>{{Cite web |last=UTE I. VON MEHLEM, ROBERT E. WALLIS |date=1989 |title=SOLID-STATE POWER AMPLIFIERS FOR SATELLITE RADAR ALTIMETERS |url=https://secwww.jhuapl.edu/techdigest/Content/techdigest/pdf/V10-N04/10-04-Mehlem.pdf |website=Johns Hopkins University}}</ref>
MOSFET transistors and other modern [[solid-state (electronics)|solid-state]] devices have replaced [[vacuum tube]]s in most electronic devices, but tubes are still used in some high-power transmitters (see [[Valve RF amplifier]]). Although mechanically robust, transistors are electrically fragile{{snd}} they are easily damaged by excess voltage or current. Tubes are mechanically fragile but electrically robust{{snd}} they can handle remarkably high [[overvoltage|electrical overloads]] without appreciable damage.
==Applications==
The basic applications of the RF power amplifier include driving to another high-power source, driving a transmitting [[Antenna (radio)|antenna]] and exciting [[microwave cavity]] resonators. Among these applications, driving transmitter antennas is most well known. The [[Transceiver|transmitter–receivers]] are used not only for voice and data communication but also for weather sensing (in the form of a [[radar]]).<ref>{{Cite journal |last=W.Q. Lohmeyer, K. Cahoy |title=Space weather radiation effects on geostationary satellite solid-state power amplifiers |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/swe.20071 |journal=Space Weather|date=2013 |volume=11 |issue=8 |pages=476–488 |doi=10.1002/swe.20071 }}</ref>
RF power amplifiers using [[LDMOS]] (laterally diffused [[MOSFET]]) are the most widely used [[power semiconductor device]]s in [[wireless telecommunication]] networks, particularly [[mobile network]]s.<ref name=Balinga-2005-SiRF-MOSFETs/><ref name=Asif-2018-SGMob/><ref name=Perugupalli-Leighton-etal-2001/> LDMOS-based RF power amplifiers are widely used in digital mobile networks such as [[2G]], [[3G]],<ref name=Balinga-2005-SiRF-MOSFETs/><ref name=Perugupalli-Leighton-etal-2001/> and [[4G]]<ref name=Asif-2018-SGMob/> and the good cost/performance ratio make them the preferred option for [[amateur radio]].<ref>{{Cite web |title=A 600W broadband HF/6m amplifier using affordable LDMOS devices |date=27 October 2019 |url=https://qrpblog.com/2019/10/a-600w-broadband-hf-amplifier-using-affordable-ldmos-devices/}}</ref>
==Wideband amplifier design==
[[Electrical impedance|Impedance]] transformations over large [[Bandwidth (signal processing)|bandwidth]] are difficult to realize, so conventionally, most [[wideband]] [[amplifier]]s are designed to feed a 50&nbsp;Ω output load. [[Transistor]] output power is then limited to
: <math> P_\text{out} \le \frac{(V_\text{br} - V_\text{k})^2 }{8 Z_\text{o}},</math>
where
: <math>V_\text{br}</math> is defined as the [[breakdown voltage]],
: <math>V_\text{k}</math> is defined as the knee voltage,
: <math>Z_\text{o}</math> is chosen so that the rated power can be met.
The external load is, by convention, <math>Z_\text{L} = 50~\Omega.</math> Therefore, there must be some sort of [[impedance matching]] that transforms from <math>Z_\text{o}</math> to <math>Z_\text{L} = 50~\Omega.</math>
The loadline method is often used in RF power amplifier design.<ref name=Ozalas-2015-01-14-HowTo/>
==See also==
* [[FET amplifier]]
* [[Power electronics]]
==References==
{{reflist|30em|refs=
<ref name=APITech-RF-Amps>
{{Cite web
|title=RF Amplifiers
|website=info.apitech.com
|lang=en
|url=https://info.apitech.com/rf-amplifiers-va
|access-date=2021-05-18
}}
</ref>
<ref name=Asif-2018-SGMob>
{{cite book
|last=Asif |first=Saad
|date=2018
|title=5G Mobile Communications: Concepts and Technologies
|page=134
|publisher=[[CRC Press]]
|isbn=9780429881343
|url=https://books.google.com/books?id=yg1mDwAAQBAJ&pg=PT134
}}
</ref>
<ref name=Austin-Dean-etal-1966-11>
{{cite journal
|last1=Austin |first1=W. M.    |last2=Dean |first2=J. A.
|last3=Griswold |first3=D. M.  |last4=Hart |first4=O. P.
|date=November 1966
|title=TV Applications of MOS Transistors
|journal=IEEE Transactions on Broadcast and Television Receivers
|volume=12 |issue=4 |pages=68–76
|doi=10.1109/TBTR1.1966.4320029
}}
</ref>
<ref name=Balinga-2005-SiRF-MOSFETs>
{{cite book
|last=Baliga |first=B. Jayant |author-link=B. Jayant Baliga
|year=2005
|title=Silicon RF Power MOSFETs
|page=1
|publisher=[[World Scientific]]
|isbn=9789812561213
|url=https://books.google.com/books?id=StJpDQAAQBAJ&pg=PA1
}}
</ref>
<ref name=Cloutier-ClassE-amps>
{{cite web
|last=Cloutier |first=Stephen R. (WA1QIX)
|title=Class E, AM transmitter descriptions, circuits, etc.
|website=www.classeradio.com
|publisher=WA1QIX
|url=http://www.qrz.com/db/WA1QIX
|access-date=6 June 2015  |via=qrz.com
}}
</ref>
<ref name=Lee-2003-CMOS>
{{cite book
|last=Lee |first=Thomas
|year=2003
|title=The Design of CMOS Radio-Frequency Integrated Circuits
|pages=494–503
|location=Cambridge, UK
|publisher=Cambridge University Press
}}
</ref>
<ref name=MFJ-ALS-1300-tmos>
{{cite web
|title=Ameritron ALS-1300: 1200&nbsp;Watt no-tune TMOS-FET amplifier
|series=product information downloads
|publisher=[[MFJ Enterprises]]
|url=http://www.ameritron.com/Downloads/index.php?productid=ALS-1300&filename=ALS-1300.pdf&company=ameritron
|access-date=6 June 2015 |url-status=dead
|archive-url=https://web.archive.org/web/20140423115819/http://www.ameritron.com/Downloads/index.php?productid=ALS-1300&filename=ALS-1300.pdf&company=ameritron
|archive-date=2014-04-23
}}
</ref>
<ref name=Ozalas-2015-01-14-HowTo>
{{cite AV media
| first = Matthew | last = Ozalas
| date = 14 January 2015
| title = How to design an RF power amplifier – the basics
| medium = short how-to video
| url = https://www.youtube.com/watch?v=WAingaHfBMs
| access-date = 2015-02-10   | via = youtube.com
}}
</ref>
<ref name=Perugupalli-Leighton-etal-2001>
{{cite book
|first1=Prasanth |last1=Perugupalli  |first2=Larry |last2=Leighton
|first3=Jan |last3=Johansson         |first4=Qiang |last4=Chen
|year=2001
|chapter=LDMOS RF power transistors and their applications
|editor-last1=Dye |editor-first1=Norman
|editor-last2=Granberg |editor-first2=Helge
|title=Radio Frequency Transistors: Principles and practical applications
|publisher=[[Elsevier]]
|isbn=9780080497945
|pages=259–92
|chapter-url=http://160592857366.free.fr/joe/ebooks/Electronics%20and%20Electrical%20Engineering%20Collection/DYE,%20N.%20(2000).%20Radio%20Frequency%20Transistors%20-%20Principles%20and%20Practical%20Applications%20(2nd%20ed.)/Radio_Frequency_Transistors_2E.pdf#page=275
}}
</ref>
}} <!-- end of "refs=" -->
==External links==
{{Commons category|RF power amplifiers}}
*{{cite report
|first=Carlos |last=Fuentes
|date=October 2008
|title=Microwave Power Amplifier Fundamentals
|place=Bern, Switzerland
|publisher=[[European Centre for Nuclear Research]]
|url=http://cas.web.cern.ch/cas/Denmark-2010/Caspers/AN-GT101A-Microwave%20Power%20Amplifier%20Fundamentals%2008-10-27%20%20CAS2010.pdf
|access-date=2013-03-05
}}
*{{cite web
|last=Khanifar |first=Ahmad
|date=2014-12-01
|title=Wideband RF Power Amplifier Design Guidelines / RF Power Amplifier Design for Digital Predistortion
|series=Technical support
|place=Laguna Hills, CA
|publisher=Linamp Technologies, Inc.
|website=linamptech.com
|url=http://www.linamptech.com/technical-support/rf-power-amplifier-design-for-digital-predistortion
|access-date=1 December 2014
}}
{{DEFAULTSORT:Rf Power Amplifier}}
[[Category:Electronic amplifiers]]
[[Category:Radio electronics]]
[[Category:MOSFETs]]