Wien bridge oscillator

In this version of the oscillator, Rb is a small incandescent lamp. Usually R1 = R2 = R and C1 = C2 = C. In normal operation, Rb self heats to the point where its resistance is Rf/2. 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.
Wien bridge oscillator - schematic

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. The modern circuit is derived from William Hewlett `s 1939 Stanford University master`s degree thesis. Hewlett figured out how to make the oscillator with a stable output amplitude and low distortion. [2] [3] Hewlett, along with David Packard, co-founded Hewlett-Packard, and Hewlett-Packard`s first product was the HP200A, a precision Wien bridge oscillator. 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...

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