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A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies. The oscillator is based on a bridge circuit originally developed by Max Wien in 1891 for the measurement of impedances. The bridge comprises four resistors and two capacitors. The oscillator can also be viewed as a positive gain amplifier combined with a bandpass filter that provides positive feedback. Automatic gain control, intentional non-linearity and incidental non-linearity limit the output amplitude in various implementations of the oscillator. The circuit shown to the right depicts a common implementation of the oscillator, with automatic gain control, using modern components. Under the condition that R1=R2=R and C1=C2=C, the frequency of oscillation is given by: and the condition of stable oscillation is given by == Background == There were several efforts to improve oscillators in the 1930s. Linearity was recognized as important. The "resistance-stabilized oscillator" had an adjustable feedback resistor; that resistor would be set so the oscillator just started (thus setting the loop gain to just over unity). The oscillations would build until the vacuum tube's grid would start conducting current, which would increase losses and limit the output amplitude. Automatic amplitude control was investigated. Terman states, "The frequency stability and wave-shape form of any common oscillator can be improved by using an automatic-amplitude-control arrangement to maintain the amplitude of oscillations constant under all conditions." In 1937, Meacham described using a filament lamp for automatic gain control in bridge oscillators. Also in 1937, Scott described audio oscillators based on various bridges including the Wien bridge. Terman at Stanford University was interested in Black's work on negative feedback, so he held a graduate seminar on negative feedback. William Hewlett attended the seminar. Scott's February 1938 oscillator paper came out during the seminar. Here is a recollection by Terman: :Fred Terman explains: "To complete the requirements for an Engineer’s degree at Stanford, Bill had to prepare a thesis. At that time I had decided to devote an entire quarter of my graduate seminar to the subject of 'negative feedback' I had become interested in this then new technique because it seemed to have great potential for doing many useful things. I would report on some applications I had thought up on negative feedback, and the boys would read recent articles and report to each other on current developments. This seminar was just well started when a paper came out that looked interesting to me. It was by a man from General Radio and dealt with a fixed-frequency audio oscillator in which the frequency was controlled by a resistance-capacitance network, and was changed by means of push-buttons. Oscillations were obtained by an ingenious application of negative feedback." In June 1938, Terman, Buss, Hewlett and Cahill gave a presentation about negative feedback at the IRE Convention in New York; in August 1938, there was a second presentation at the IRE Pacific Coast Convention in Portland, OR; the presentation became an IRE paper. One topic was amplitude control in a Wien bridge oscillator. The oscillator was demonstrated in Portland.〔; Packard remembers first demonstation of the 200A in Portland.〕 Hewlett, along with David Packard, co-founded Hewlett-Packard, and Hewlett-Packard's first product was the HP200A, a precision Wien bridge oscillator. The first sale was in January 1939. Hewlett's June 1939 engineer's degree thesis used a lamp to control the amplitude of a Wien bridge oscillator.〔 states, "Hewlett may have adapted this technique from Meacham, who published it in 1938 as a way to stabilize a quartz crystal oscillator. Meacham's paper, "The Bridge Stabilized Oscillator," is in reference number five in Hewlett's thesis."〕 Hewlett's oscillator produced a sinusoidal output with a stable amplitude and low distortion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wien bridge oscillator」の詳細全文を読む スポンサード リンク
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