You sometimes read of "thermal distortion" in power amplifiers supposed to be due to cyclic changes in device parameters due to varying heating over that cycle. Until recently I was unimpressed by the likelihood of its existence, reasoning that it must cause rising distortion with falling frequency, and this was not observable in measurements to 0.001% THD. (Changes in distortion due to long-term thermal changes in the output stage quiescent conditions are another matter and clearly do exist.) Improvements in test gear allowing 0.0002% measurements and the use of higher powers discloses that THD can rise with falling frequency. The component concerned is identified, and methods given for both confirming the presence of thermal distortion and reducing it.
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