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Gigatronics - Microwave Power Amplifier Fundamentals

Gigatronics - Microwave Power Amplifier Fundamentals - A...

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A Giga-tronics Technical White Paper AN-GT101A 4650 Norris Canyon Road San Ramon, CA 94583 925-328-4650 Ph www.gigatronics.com Microwave Power Amplifier Fundamentals Written by: Carlos Fuentes Member Technical Staff Giga-tronics Incorporated Published: October 2008 Revision: A
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A Giga-tronics Technical White Paper AN-GT101A ©2008 Introduction The Need to Amplify Signals An amplifier is one of the most common electrical elements in any system. The requirements for amplification are as varied as the systems where they are used. Amplifiers are available in a large number of form factors ranging from miniscule ICs to the largest high-power transmitter amplifiers. In the following discussion the focus will be on solid state power amplifiers used at microwave frequencies, particularly in test and measurement applications. Microwave power amplifiers may be used for applications ranging from testing passive elements, such as antennas, to active devices such as limiter diodes or MMIC based power amplifiers. Furthermore, other applications include testing requirements where a relatively large amount of RF power is necessary for overcoming system losses to a radiating element, such as may be found at a compact range, or where there is a system requirement to radiate a device-under-test (DUT) with an intense electromagnetic field, as may be found in EMI/EMC applications. As varied as the system requirements may be, the specific requirements of a given amplifier can also vary considerably. Nevertheless, there are common requirements for nearly all amplifiers, including frequency range, gain/gain flatness, power output, linearity, noise figure/noise power, matching, and stability. Often there are design trade-offs required to optimize any one parameter over another, and performance compromises are usually necessary for an amplifier that may be used in a general purpose testing application. The following discourse includes a description of amplifier topologies introducing the basics of spatially combined distributed amplifiers, a discussion of typical amplifier specifications and a review of performance verification measurements. Broadband Microwave Power Amplifiers There are numerous techniques for designing microwave power amplifiers. These may be broadly split between tube and solid state technologies. For high power requirements (> 100 Watts), typically these are satisfied with tube based designs. Tube amplifiers, such as Traveling Wave Tube Amplifiers (TWTAs), require a high voltage power supply, typically require warm-up time, and have significant aging related issues. For solid state amplifiers to achieve similar performance often requires switching between narrow-band amplifiers, with deleterious effects to the overall linearity and gain/power flatness. The switches themselves embody performance compromises. Mechanical switches, while quite linear and relatively low loss, have switching speed limitations, and are subject to failure after repeated switching cycles. Solid state switches may overcome the speed issue, but are not nearly so linear or low loss.
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