Harmonic Distortion

Author: N.H. Crowhurst

The basic purpose of an amplifier is to amplify the audio input voltages without distorting them in any way. The output audio voltage should be an exact replica of the input voltage, except that it is very much larger - 1000 or even a greater number of times. Practical amplifiers never completely achieve this exactness, although they may get very close to it. There is always some distortion that makes the output waveform a trifle different from the input waveform.

When the word distortion is used without qualification, it is usually taken to mean the kind of distortion due to curved or nonlinear characteristics in the amplifier. The fact that a change in audio voltage at the input is not accompanied by an exactly corresponding change at the output, at different points on the waveform is a form of distortion.

The output of an amplifier should be an exact replica of the input voltage, only much larger. Any difference in wave shape is called distortion

Top: distortion due to 2nd or even-numbered harmonics; bottom: distortion due to 3nd or odd-numbered harmonics

If we consider what this kind of curvature does to the amplification of a wave of single frequency, we can see how it introduces harmonic distortion - the presence of overtones of the fundamental frequency that are not present at the input.

A sharpening or flattening of both tops and bottoms of the waves is equivalent to the addition of third or other odd-numbered harmonics of the original frequency. A flattening at one peak and a sharpening at the other is equivalent to the addition of second and other even-numbered harmonics of the original frequency. If the waveform goes lopsided, that is, the upward slope is different from the downward slope, this is also due to second or other even-numbered harmonics added to the original frequency.

These are the principal kinds of harmonic distortion. Any real example will usually consist of one or a combination of two or more of them.