Load Tolerance In Specifying RF Power Amplifiers For Immunity / Susceptibility TestingSource: AR
The salient specifications that are used to characterize RF power amplifiers are well known, and generally accepted without question. The list includes gain, frequency response, and of course, RF output power. There are, however, lesser understood amplifier characteristics, such as noise figure, gain stability, distortion, etc. that are considered when specifying a power amplifier. This note covers load tolerance, a major and often overlooked power amplifier characteristic. Depending upon the application, the ability to provide power to loads that vary from an ideal 50 O can be crucial. Such variations in load impedance are commonplace, especially in immunity testing applications. In applications of this nature, load tolerance ultimately determines the effectiveness of the power amplifier.
This Application Note addresses the use of RF power amplifiers in applications characterized by mismatched loads. VSWR variations encountered in immunity testing are discussed in relation to amplifier design. The concept of Minimum Available Power (MAP) is introduced as a quantitative measure of an amplifier's capability to supply power into a poor load VSWR.
Real life applications rarely involve driving an ideal 50Ù load. More likely, the load varies with frequency over a wide range of impedance. A case in point is immunity testing. This application is plagued by widely varying load impedances due to variations in antenna characteristics, room reflections and resonances, imperfect cables and connectors, and reflections from the device under test. Specifically, starting with a typical antenna VSWR of 2.5:1 and factoring in room and signal path effects, it is not uncommon to experience a VSWR in excess of 5.0:1. The problem stated quite simply is, "Most RF power amplifiers are not capable of providing full rated power to loads that vary considerably from an ideal 50Ù." The market thrust at AR Worldwide over the last 35 years has been RF immunity testing. EMC Test Engineers have had to deal with the problem of delivering power into significant mismatches on a regular basis, and as a result, AR Worldwide has developed a line of Class A linear amplifiers with characteristics that lend themselves to this difficult application in particular.
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Application Note: Load Tolerance In Specifying RF Power Amplifiers For Immunity / Susceptibility Testing