In this version of the oscillator, Rb is a small incandescent lamp. A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of sensitivity of wheatstone bridge pdf. The circuit shown to the right depicts a once-common implementation of the oscillator, with automatic gain control using an incandescent lamp.
There were several efforts to improve oscillators in the 1930s. The oscillations would build until the vacuum tube’s grid would start conducting current, which would increase losses and limit the output amplitude. 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. 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.
Hewlett’s June 1939 engineer’s degree thesis used a lamp to control the amplitude of a Wien bridge oscillator. Hewlett’s oscillator produced a sinusoidal output with a stable amplitude and low distortion. Schematic of a Wien bridge oscillator that uses diodes to control amplitude. The oscillator at the right uses diodes to add a controlled compression to the amplifier output.
For a linear circuit to oscillate, it must meet the Barkhausen conditions: its loop gain must be one and the phase around the loop must be an integer multiple of 360 degrees. The linear oscillator theory doesn’t address how the oscillator starts up or how the amplitude is determined. The linear oscillator can support any amplitude. In practice, the loop gain is initially larger than unity. Random noise is present in all circuits, and some of that noise will be near the desired frequency.
A loop gain greater than one allows the amplitude of frequency to increase exponentially each time around the loop. With a loop gain greater than one, the oscillator will start. Ideally, the loop gain needs to be just a little bigger than one, but in practice, it is often significantly greater than one. A larger loop gain makes the oscillator start quickly. A large loop gain also compensates for gain variations with temperature and the desired frequency of a tunable oscillator. For the oscillator to start, the loop gain must be greater than one under all possible conditions. A loop gain greater than one has a down side.
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In theory, the oscillator amplitude will increase without limit. In a stable oscillator, the average loop gain will be one. Although the limiting action stabilizes the output voltage, it has two significant effects: it introduces harmonic distortion and it affects the frequency stability of the oscillator. The amount of distortion is related to the extra loop gain used for startup.
If there’s a lot of extra loop gain at small amplitudes, then the gain must decrease more at higher instantaneous amplitudes. The amount of distortion is also related to final amplitude of the oscillation. Although an amplifier’s gain is ideally linear, in practice it is nonlinear. The nonlinear transfer function can be expressed as a Taylor series.