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Amplitude and Phase of Intermodulation Distortion

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It is shown that amplitude intermodulation distortion (AIMD) in a feedback amplifier is associated primarily with signal-induced changes in the low-frequency gain of the forward path, whereas phase intermodulation distortion (PIMD) is associated primarily with signal-induced changes in the gain-bandwidth product. The usual nonlinearity in transistor ß therefore contributed AIMD but not PIMD. It is shown that both AIMD and PIMD involve intermodulation products at the same frequencies. For comparable nonlinearities other amplitudes of the intermodulation products are roughly equal, but their phases are different by -/2. An intermodulation specification based on the amplitude of the intermodulation products includes both AIMD and PIMD. In contrast, a specification based on the depth of an amplitude modulation neglects PIMD.

JAES Volume 31 Issue 5 pp. 298-304; May 1983
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AES - Audio Engineering Society