Design of Two Wien Bridge Oscillators
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In 1939, William Redington Hewlett, finished his master’s degree thesis entitled ‘A New Type Resistance-Capacity Oscillator’ at the Stanford University. The oscillator made use of the RC network originated by Max Wien in 1891 (Williams, J.: Max wien, mr. hewlett, and a rainy sunday afternoon. In: Jim, W (ed.) Analog Circuit Design: Art, Science, and Personalities. Butterworth-Heinemann, Boston, pp. 43–55 (1991)). One of the main problems to obtain a proper sine wave at the output was the amplitude control: if the gain is too low, no oscillations occur, if the gain is too high, a square wave appears at the output. Hewlett proposed the following solution: ‘For the variable resistance, a small tungsten lamp may be used. It is a well-known property of such lamps that as the current through them increases, the filament warms up, thereby increasing the lamp resistance.’ By putting a light bulb in the feedback network, the gain automatically drops when the amplitude increases. This very first Wien bridge oscillator was commercialized together with David Packard as the HP 200, the very first product HP ever built.