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Wien bridge oscillator
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==Amplitude stabilization== The key to the Wien bridge oscillator's low distortion oscillation is an amplitude stabilization method that does not use clipping. The idea of using a lamp in a bridge configuration for amplitude stabilization was published by Meacham in 1938.<ref>{{Harvnb|Meacham|1938}}. {{Harvnb|Meacham1938a}}. Meacham presented his work at the Thirteenth Annual Convention of the Institute of Radio Engineers, New York City, June 16, 1938 and published in ''Proc. IRE'' October 1938. Hewlett's patent (filed July 11, 1939) does not mention Meacham.</ref> The amplitude of electronic oscillators tends to increase until [[clipping (signal processing)|clipping]] or other [[Gain (electronics)|gain]] limitation is reached. This leads to high harmonic distortion, which is often undesirable. Hewlett used an [[incandescent bulb]] as a power detector, low pass filter and gain control element in the oscillator feedback path to control the output amplitude. The resistance of the light bulb filament (see [[Electrical resistivity and conductivity#Temperature dependence|resistivity article]]) increases as its temperature increases. The temperature of the filament depends on the power dissipated in the filament and some other factors. If the oscillator's period (an inverse of its frequency) is significantly shorter than the thermal time constant of the filament, then the temperature of the filament will be substantially constant over a cycle. The filament resistance will then determine the amplitude of the output signal. If the amplitude increases, the filament heats up and its resistance increases. The circuit is designed so that a larger filament resistance reduces loop gain, which in turn will reduce the output amplitude. The result is a [[negative feedback]] system that stabilizes the output amplitude to a constant value. With this form of amplitude control, the oscillator operates as a near ideal linear system and provides a very low distortion output signal. Oscillators that use limiting for amplitude control often have significant harmonic distortion. At low frequencies, as the time period of the Wien bridge oscillator approaches the thermal time constant of the incandescent bulb, the circuit operation becomes more nonlinear, and the output distortion rises significantly. Light bulbs have their disadvantages when used as gain control elements in Wien bridge oscillators, most notably a very high sensitivity to vibration due to the bulb's [[microphonics|microphonic]] nature [[amplitude modulation|amplitude modulating]] the oscillator output, a limitation in high frequency response due to the inductive nature of the coiled filament, and current requirements that exceed the capability of many [[op-amp]]s. Modern Wien bridge oscillators have used other nonlinear elements, such as [[diode]]s, [[thermistor]]s, [[field effect transistor]]s, or [[photocell]]s for amplitude stabilization in place of light bulbs. Distortion as low as 0.0003% (3 ppm) can be achieved with modern components unavailable to Hewlett.<ref>{{Harvnb|Williams|1990|pp=32β33}}</ref> Wien bridge oscillators that use [[thermistor]]s exhibit extreme sensitivity to ambient temperature due to the low operating temperature of a thermistor compared to an incandescent lamp.<ref>{{Harvnb|Strauss|1970|p=710}}, stating "For acceptable amplitude stability, some form of temperature compensation would be necessary."</ref>
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