Designing High Efficiency Overdriven and Saturated Power Amplifiers
here. Requires Adobe Acrobat Reader,
It is common knowledge that efficiency is a major issue with the design of any transmitter. The benefits of high efficiency include power consumption, low power derating and high reliability, among others. In order to improve efficiency several specific amplification classes, such as class E
Devices intrinsic capacitances, packaging parasitics, low gain and intrinsic losses are the main limiting factors of high efficiency modes. The constraints become even more severe if high output power levels are involved. This article will discuss the design of a high efficiency VHF power amplifier (collector efficiency 0
Recent relevant works
High efficiency approximated approach
The main cause precluding class E operation at high frequencies is the large intrinsic capacitances of most power transistors. The reactance of intrinsic transistor capacitances is often very low. So, to obtain optimum class E operation at high frequencies low-load impedances are needed. Unfortunately, most times low loads lead to higher output power levels and currents than specified for safe transistor operation.
Maintaining high efficiency and safe operation forces the use of load impedances different from loads for optimum class E operation. High efficiency can be retained with different loads but they may cause negative collector-to-emitter voltage V
Therefore, the output power P
These effects are illustrated Figure 1. This figure is a simulation of the collector current I
The amplifier presented to illustrate this technique is based on the well known “classical” 2N6083 RF bipolar power transistor. The transistor manufacturer announces collector efficiency 0
In order to design the output load at the fundamental frequency, a linear equivalent value of the output capacitance C
The relationship P
The load and source impedances at the harmonics are set strongly reactive (at least 3 times the load at the fundamental). Discrete three-element matching networks are used to provide both the source an load impedances at the fundamental frequency while keeping strong inductive behavior at harmonic frequencies.
Amplifier testing and optimization
The amplifier was tested and optimized using the custom test-fixture shown in the photo on page 46. It not only provides a fixture for the transistor but also allows the variation of the load and source impedance while the load at the harmonics is kept strongly reactive.
The test-fixture also provides ports for sampling of significant amplifier voltages and currents. This samples are measured in a broadband digital oscilloscope.
Results and measurements
Figure 2 shows collector efficiency, 0
The reason for this behavior is that fast degradation of gain occurs with high driving levels. Figure 3 shows this effect, output power (P
At radio frequencies classical high efficiency amplification classes cannot be easily achieved. However, careful approximations can achieve good results with low efficiency degradation. A high efficiency design using forward and inverse active and saturation regions has been presented. An example has been designed constructed and the results measured. The device was loaded with high reactive source and load impedances at harmonic frequencies. The load at the fundamental frequency was chosen to function in sub-optimum class E with negative collector-to-emitter voltage during part of the duty cycle. This negative V
This operational mode has shown very high collector efficiency and good PAE. with remarkable output power level at high frequencies.
Nathan O. Sokal, Alan D. Sokal, “Class E-A New Class of high efficiency Tuned Single-Ended Switching Power Amplifiers", IEEE Journal of Solid-State Circuits, VOL. SC-10, NO. 3, pp. 168-176, june 1975.
Frederick H. Raab, “Class-E, class-C, and class-F power amplifiers based upon a finite number of harmonics", IEEE Transactions on Microwave Theory and Techniques, Volume: 49, NO. 8, pp. 1462-1468, Aug 2001.
Francisco Javier Ortega-González, Alberto Asensio-López, José Luis Jiménez Martin, Germán Torregrosa-Penalva, “High Efficiency Load-Pull Harmonic Controlled Class E Power Amplifier", IEEE Microwave and Guided Wave Letters, VOL 8, No 10, pp. 348-350, october 1998.
Frederick H. Raab, “Effects of Circuit variations on the Class E Tuned Power Amplifier", IEEE Journal of Solid-State Circuits, VOL. SC-13, NO.2, pp. 239-247, april 1978.
Willian H. Cantrell, “Tuning Analysis for the High-Q Class-E Power Amplifier", IEEE Transactions on Microwave Theory and Techniques, VOL. 48, NO. 12, pp. 2397-2402, december 2000.
Francisco Javier Ortega-González, Alberto Asensio-López, José Luis Jiménez Martin, Effects of Matching on RF Power Amplifier Efficiency and Output Power", Microwave Journal, pp. 60-72, april 1998.
Marian K. Kazimierczuk, Wojiech A. Tabisz, “Class C-E High Efficiency Tuned Power Amplifier,” IEEE Transactions on Circuit and Systems, VOL. 36, NO. 3, pp. 421-428, march 1989.
About the authors
Francisco Javier Ortega González received the Ingeniero de Telecomunicación degree from Universidad Politécnica de Madrid, Madrid, Spain, and the Ph.D. degree at the Dpto. de Señales, Sistemas y Radiocomunicaciones, E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid. His research activities are in the area of high frequency circuit design, electromagnetism and radar systems. Email; firstname.lastname@example.org
Alberto Asensio received the Ingeniero de Telecomunicación degree from the Technical University of Madrid, Madrid, Spain in 1984, and the Ph.D. degree at the Signals, Systems, and Radiocommunications Department of the Technical School of Telecommunication Engineering of the same University in 1990, where he has been Associate Professor since 1991. His research activities are in the area of high frequency circuit design and radar systems. E-mail: email@example.com
Alberto Martin Consuegra received the Ingenierio Técnico de Telecomunicación degree from de Universidad Politecnica de Madrid, Madrid, Spain. Currently is working toward its Ingeniero Electrónico degree at the Alcala de Henares University, Madrid. His research activities are in the area of electronic and high frequency circuit design.
Juan Angel Ruiz was born in Madrid, Spain. Currently he is working toward its Ingeniero Técnico de Telecomunicación degree at the Universidad Politecnica de Madrid, Madrid. His research activities are in the area of high frequency circuit design and wireless networks.
Germán Torregrosa Penalva received the Ingeniero de Telecomunicación degree from Universidad Politécnica de Madrid, Madrid, Spain, in 1999, and is currently working toward the Ph.D. degree at the Dpto. de Señales, Sistemas y Radiocomunicaciones, E.T.S.I. Telecomunicación, Universidad Politécnica deMadrid. His research activities are in the area of high frequency circuitdesign and radar systems.
This work was supported by project TIC 2001-3839-C03-01 of the Spanish National Board of Scientific and Technology Research (MCYT).
Want to use this article? Click here for options!
© 2013 Penton Media Inc.
Acceptable Use Policy blog comments powered by Disqus
Most Popular Stories
CTIA Wireless IT & Entertainment 2010
Read the latest from the show...