Editing Automatic gain control (section)

=== AM radio receivers ===
In 1925, [[Harold Alden Wheeler]] invented automatic volume control (AVC) and obtained a patent.  [[Karl Küpfmüller]] published an analysis of AGC systems in 1928.<ref>K. Küpfmüller, "Über die Dynamik der selbsttätigen Verstärkungsregler", ''Elektrische Nachrichtentechnik'', vol. 5, no. 11, pp. 459-467, 1928. (German) [http://ict.open.ac.uk/classics/2.pdf On the dynamics of automatic gain controllers], (English translation)</ref> By the early 1930s most new commercial broadcast receivers included automatic volume control.<ref>[http://www.nap.edu/openbook.php?record_id=10094&page=281 Memorial Tributes: National Academy of Engineering, Volume 9] (2001) page 281, retrieved 2009 Oct 23</ref>

AGC is a departure from linearity in AM radio [[receiver (radio)|receiver]]s.<ref>F. Langford-Smith (ed.), ''[[Radiotron Designer's Handbook]]'' 4th ed., RCA, 1953, chapter 27 section 3</ref> Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound [[amplitude]], which correlates with loudness, is proportional to the radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the [[carrier wave]]. If the circuit were not fairly linear, the modulated signal could not be recovered with reasonable [[fidelity]]. However, the strength of the signal received will vary widely, depending on the power and distance of the [[transmitter]], and signal path [[attenuation]]. The AGC circuit keeps the receiver's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain the output level within an acceptable range. For a very weak signal, the AGC operates the receiver at maximum gain; as the signal increases, the AGC reduces the gain.

It is usually disadvantageous to reduce the gain of the [[RF front end]] of the receiver on weaker signals as low gain can worsen [[signal-to-noise ratio]] and [[blocking (radio)|blocking]];<ref>[http://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf Automatic gain control in receivers] by Iulian Rosu, VA3IUL</ref> therefore, many designs reduce gain only for stronger signals.

Since the AM detector diode produces a DC voltage proportional to signal strength, this voltage can be fed back to earlier stages of the receiver to reduce gain. A filter network is required so that the audio components of the signal don't appreciably influence gain; this prevents "modulation rise" which increases the effective modulation depth of the signal, distorting the sound. [[Communications receiver]]s may have more complex AVC systems, including extra amplification stages, separate AGC detector diodes, different time constants for broadcast and shortwave bands, and application of different levels of AGC voltage to different stages of the receiver to prevent distortion and cross-modulation.<ref>Langford-Smith 53, page 1108</ref> Design of the AVC system has a great effect on the usability of the receiver, tuning characteristics, audio fidelity, and behavior on overload and strong signals.<ref>Langford-Smith 53, chapter 25 page 1229</ref>