Electronic oscillator
There are two types: The most common form of linear oscillator is an electronic amplifier such as a transistor or operational amplifier connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback.
When the power supply to the amplifier is switched on initially, electronic noise in the circuit provides a non-zero signal to get oscillations started.
[5] The noise travels around the loop and is amplified and filtered until very quickly it converges on a sine wave at a single frequency.
The negative resistance port is connected to a tuned circuit or resonant cavity, causing them to oscillate.
[22][23] The switching device periodically charges the storage element with energy and when its voltage or current reaches a threshold discharges it again, thus causing abrupt changes in the output waveform.
[24]: p.20 Although in the past negative resistance devices like the unijunction transistor, thyratron tube or neon lamp were used,[22] today relaxation oscillators are mainly built with integrated circuits like the 555 timer IC.
[25]: p.20.1 Triangle-wave or sawtooth oscillators are used in the timebase circuits that generate the horizontal deflection signals for cathode-ray tubes in analogue oscilloscopes and television sets.
Many amplifiers such as common-emitter transistor circuits are "inverting", meaning that their output voltage decreases when their input increases.
, the active device can no longer be considered a 'pure gain', and it will contribute some phase shift to the loop.
The frequency of RC and LC oscillators can be tuned over a wide range by using variable components in the filter.
Due to the narrow passband of the filter, the response of the circuit to a noise pulse will be sinusoidal, it will excite a small sine wave of voltage in the loop.
Since for small signals the loop gain is greater than one, the amplitude of the sine wave increases exponentially.
[32][27] During startup, while the amplitude of the oscillation is small, the circuit is approximately linear, so the analysis used in the Barkhausen criterion is applicable.
[32] In most oscillators this nonlinearity is simply the saturation (limiting or clipping) of the amplifying device, the transistor, vacuum tube or op-amp.
[34] To achieve the maximum amplitude sine wave output from the circuit, the amplifier should be biased midway between its clipping levels.
A common-emitter transistor amplifier's collector voltage should be biased midway between cutoff and saturation levels.
[43] The amplitude of the sine wave, and the resulting clipping, continues to grow until the loop gain is reduced to unity,
The amount of harmonic distortion in the output is dependent on how much excess loop gain the circuit has:[43][26]: 12 [34][27] An exception to the above are high Q oscillator circuits such as crystal oscillators; the narrow bandwidth of the crystal removes the harmonics from the output, producing a 'pure' sinusoidal wave with almost no distortion even with large loop gains.
Since oscillators depend on nonlinearity for their operation, the usual linear frequency domain circuit analysis techniques used for amplifiers based on the Laplace transform, such as root locus and gain and phase plots (Bode plots), cannot capture their full behavior.
[42] To determine startup and transient behavior and calculate the detailed shape of the output waveform, electronic circuit simulation computer programs like SPICE are used.
In applications where a 'pure' very low distortion sine wave is needed, such as precision signal generators, a nonlinear component is often used in the feedback loop that provides a 'slow' gain reduction with amplitude.
The current through an arc light is unstable due to its negative resistance, and often breaks into spontaneous oscillations, causing the arc to make hissing, humming or howling sounds[50] which had been noticed by Humphry Davy in 1821, Benjamin Silliman in 1822,[51] Auguste Arthur de la Rive in 1846,[52] and David Edward Hughes in 1878.
[54][55][56] An oscillator was built by Elihu Thomson in 1892[57][58] by placing an LC tuned circuit in parallel with an electric arc and included a magnetic blowout.
Duddell demonstrated his oscillator before the London Institute of Electrical Engineers by sequentially connecting different tuned circuits across the arc to play the national anthem "God Save the Queen".
[68][69] Austrian Alexander Meissner independently discovered positive feedback and invented oscillators in March 1913.
[67] In August 1912, Lee De Forest, the inventor of the audion, had also observed oscillations in his amplifiers, but he didn't understand the significance and tried to eliminate it[72][73] until he read Armstrong's patents in 1914,[74] which he promptly challenged.
[75] Armstrong and De Forest fought a protracted legal battle over the rights to the "regenerative" oscillator circuit[75][76] which has been called "the most complicated patent litigation in the history of radio".
[77] De Forest ultimately won before the Supreme Court in 1934 on technical grounds, but most sources regard Armstrong's claim as the stronger one.
[73][75] The first and most widely used relaxation oscillator circuit, the astable multivibrator, was invented in 1917 by French engineers Henri Abraham and Eugene Bloch.
Further advances in mathematical analysis of oscillation were made by Hendrik Wade Bode and Harry Nyquist[84] in the 1930s.
