Gain compression
Reduction in gain due to nonlinearity
Gain compression is a reduction in differential or slope gain caused by nonlinearity of the transfer function of an amplifying device for large-signal inputs.
Overview
When operating an amplifier beyond its linear range, gain compression will occur due to nonlinear circuit characteristics. The output of large-amplitude inputs will be less than expected when using the small signal gain of the amplifier, such that an increase in input will not be matched by a proportional increase in output. Gain compression is the difference between the ideal linear power transfer curve and the real circuit's power transfer curve.
An important gain compression parameter is the OP1dB, which is the power input that results in a 1 dB compression of the output power (OP), corresponding to a gain ratio of 10-1⁄10 = 79.4%.
Harmonic distortion results from nonlinear transfer curves. And once an amplifier's maximum amplitude is reached, signals will be clipped, resulting in even stronger harmonic distortion.
Nonlinearity may be caused by heat due to power dissipation. Also, a transistor's operating point may move with temperature.
Relevance
Gain compression is relevant in any system with a wide dynamic range, such as audio or RF. It is more common in tube circuits than transistor circuits, due to topology differences, possibly causing the differences in audio performance called "valve sound".
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