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<title>Gain compression</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Gain compression</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">This article is about reduction in an amplifier's gain caused by circuit imperfections. For a process that intentionally reduces the dynamic range of audio signals, see <a href="Dynamic_range_compression" title="Dynamic range compression">Dynamic range compression</a>. For processes that reduce the size of digital audio files, see <a href="Data_compression" title="Data compression">Audio compression (data)</a>.</div>

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<p><b>Gain compression</b> is a reduction in <i>differential</i> or <i>slope</i> <a href="Gain_(electronics)" title="Gain (electronics)">gain</a> caused by <a href="Nonlinearity" class="mw-redirect" title="Nonlinearity">nonlinearity</a> of the <a href="Transfer_function" title="Transfer function">transfer function</a> of an <a href="Amplifier" title="Amplifier">amplifying</a> device<sup id="cite_ref-:0_1-0" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> for <a href="Large-signal" class="mw-redirect" title="Large-signal">large-signal</a> inputs.

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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

<p>When <a href="Overdrive_(music)" class="mw-redirect" title="Overdrive (music)">overdriving</a> an amplifier beyond its <a href="Linear" class="mw-redirect" title="Linear">linear</a> range, gain compression will occur<sup id="cite_ref-:0_1-1" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> due to <a href="Nonlinear" class="mw-redirect" title="Nonlinear">nonlinear</a> circuit characteristics. The output of large amplitude inputs will be less than expected than using the <a href="Small-signal_model#Differences_between_small_signal_and_large_signal" title="Small-signal model">small signals</a> 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.

</p><p>An important gain compression parameter is the <b>OP1dB</b>, which is the power input that results in a 1 <a href="Decibel" title="Decibel">dB</a> compression of the output power (OP), corresponding to a gain ratio of 10<sup>-<style data-mw-deduplicate="TemplateStyles:r1154941027">

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</style><span class="frac"><span class="num">1</span>⁄<span class="den">10</span></span></sup> = 79.4%.

</p><p><a href="Harmonic_distortion" class="mw-redirect" title="Harmonic distortion">Harmonic distortion</a> results from nonlinear transfer curves. And once an amplifier's maximum amplitude is reached, signals will be <a href="Clipping_(audio)" title="Clipping (audio)">clipped</a>, resulting in even stronger harmonic distortion.

</p><p>Nonlinearity may be caused by heat due to power dissipation. Also, a transistor's operating point may move with temperature.

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<div class="mw-heading mw-heading2"><h2 id="Relevance">Relevance</h2></div>

<p>Gain compression is relevant in any system with a wide <a href="Dynamic_range" title="Dynamic range">dynamic range</a>, such as audio or <a href="RF" class="mw-redirect" title="RF">RF</a>. It is more common in <a href="Vacuum_tube" title="Vacuum tube">tube</a> circuits than <a href="Transistor" title="Transistor">transistor</a> circuits, due to topology differences, possibly causing the differences in audio performance called "<a href="Valve_sound" class="mw-redirect" title="Valve sound">valve sound</a>". The front-end RF <a href="Amplifier" title="Amplifier">amps</a> of <a href="Radio_receiver" title="Radio receiver">radio receivers</a> are particularly susceptible to this phenomenon when overloaded by a strong unwanted signal.<sup id="cite_ref-:0_1-2" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Audio_effects">Audio effects</h2></div>

<p>A tube radio or tube amplifier will increase in volume to a point, and then as the input signal extends beyond the <a href="Linear" class="mw-redirect" title="Linear">linear</a> range of the device, the effective gain is reduced, altering the shape of the waveform. The effect is also present in transistor circuits. The extent of the effect depends on the topology of the amplifier.

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<div class="mw-heading mw-heading2"><h2 id="Radio-frequency_compression">Radio-frequency compression</h2></div>

<p>Gain compression in RF amplifiers is similar to soft clipping. However, in <a href="Narrowband" title="Narrowband">narrowband</a> systems, the effect <i>looks</i> more like gain compression simply because the harmonics are filtered out after amplification. Many <a href="Data_sheet" class="mw-redirect" title="Data sheet">data sheets</a> for RF amplifiers list gain compression rather than distortion figures because it's easier to measure and is more important than distortion figures in nonlinear RF amplifiers.

</p><p>In <a href="Wideband" title="Wideband">wideband</a> and low-<a href="Frequency" title="Frequency">frequency</a> systems, the nonlinear effects are readily visible, e.g. the output is <a href="Clipping_(signal_processing)" title="Clipping (signal processing)">clipped</a>. To see the same thing at 1 <a href="GHz" class="mw-redirect" title="GHz">GHz</a>, an <a href="Oscilloscope" title="Oscilloscope">oscilloscope</a> with a <a href="Bandwidth_(signal_processing)" title="Bandwidth (signal processing)">bandwidth</a> of at least 10 GHz is needed. Observing with a <a href="Spectrum_analyzer" title="Spectrum analyzer">spectrum analyzer</a>, the fundamental compressed and the <a href="Harmonic" title="Harmonic">harmonics</a> picking up.

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<div class="mw-heading mw-heading3"><h3 id="Examples_of_RF_compression">Examples of RF compression</h3></div>

<p>A low-noise <a href="Linear_amplifier" title="Linear amplifier">RF amplifier</a>, if fed by a <a href="Directional_antenna" title="Directional antenna">directional antenna</a> to a consumer 900 <a href="MHz" class="mw-redirect" title="MHz">MHz</a> receiver, should improve the transmission range. It works, but the receiver may also pick up a couple of <a href="UHF" class="mw-redirect" title="UHF">UHF</a> stations around 700 MHz.

</p><p>For example, if channel 54 is transmitting 6 <a href="Megawatt" class="mw-redirect" title="Megawatt">MW</a> of <a href="Amplitude_modulation" title="Amplitude modulation">AM</a>, <a href="Frequency_modulation" title="Frequency modulation">FM</a>, and <a href="Phase_modulation" title="Phase modulation">PM</a>, the RF front end, expecting −80 <a href="DBm" title="DBm">dBm</a>, would be grossly overloaded and generate mixing products. This is a typical effect of gain compression.

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<div class="mw-heading mw-heading2"><h2 id="High-power_loudspeakers">High-power loudspeakers</h2></div>

<p><a href="Power_compression" title="Power compression">Power compression</a> is a form of gain compression that takes place in <a href="Loudspeaker" title="Loudspeaker">loudspeaker</a> <a href="Voice_coil" title="Voice coil">voice coils</a> when they heat up and increase their <a href="Electrical_resistance" class="mw-redirect" title="Electrical resistance">resistance</a>. This causes less power to be drawn from the amplifier and a reduction in <a href="Sound_pressure_level" class="mw-redirect" title="Sound pressure level">sound pressure level</a>.

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<div class="mw-heading mw-heading2"><h2 id="Distinction_with_intentional_dynamic_range_compression">Distinction with intentional dynamic range compression</h2></div>

<div role="note" class="hatnote navigation-not-searchable">Main articles: <a href="Dynamic_range_compression" title="Dynamic range compression">Dynamic range compression</a> and <a href="Limiter" title="Limiter">Limiter</a></div>

<p><a href="Dynamic_range_compression" title="Dynamic range compression">Dynamic range compression</a> is a more general term that typically refers to intentional compression, and may be done in the digital realm or analog realm. <a href="Automatic_gain_control" title="Automatic gain control">Automatic gain control</a> circuits are intentionally designed to actively change the overall gain in response to the level of the input, resulting in a transfer function that may vary over time. Gain compression on the other hand is a consequence of analog amplifier circuit non-linearities that are generally undesired.

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<ul><li><a href="Third-order_intercept_point" title="Third-order intercept point">Third-order intercept point</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFRootXuPedroNunes2018" class="citation cs2">Root, David E.; Xu, Jianjun; Pedro, José Carlos; Nunes, Luís Cótimos, eds. (2018), <span class="id-lock-subscription" title="Paid subscription required"><a rel="nofollow" class="external text" href="https://www.cambridge.org/core/books/nonlinear-circuit-simulation-and-modeling/linear-and-nonlinear-circuits/56C3908478F358C00CA8357E7937CF8E">"Linear and Nonlinear Circuits"</a></span>, <i>Nonlinear Circuit Simulation and Modeling: Fundamentals for Microwave Design</i>, The Cambridge RF and Microwave Engineering Series, Cambridge: Cambridge University Press, pp. <span class="nowrap">1–</span>46, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1017%2F9781316492963.002">10.1017/9781316492963.002</a>, <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-107-14059-2</bdi><span class="reference-accessdate">, retrieved <span class="nowrap">11 March</span> 2022</span></cite></span>

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