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Push-Pull Amplifier Harmonics

February 25th Fact-of-the-Day

A simple tuned circuit at the output of an RF amplifier will decrease the ratio of harmonic to fundamental frequency output voltage in inverse proportion to the resonant Q of the tuned circuit (the higher the Q, the lower the harmonic content). However the harmonic attenuation obtainable with a simple tuned circuit of reasonable Q is not usually sufficient with a high power amplifier, so additional filtering usually is needed. Another useful design option for harmonic reduction is to use a push-pull rather than a single-ended amplifier, because even harmonic-frequency output-voltages cancel in a perfectly-symmetrical push-pull amplifier. That provides a considerable advantage, because the second harmonic normally is the highest-amplitude harmonic with a single-ended design. However, it is necessary to maintain near-perfect push-pull symmetry (carefully matched tubes or transistors) to achieve high ratios of even harmonic reduction. ©2005 Martek, International. All rights reserved.


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