Wien Bridge Oscillators


Posted on Feb 5, 2014

A basic Wien Bridge Oscillator using a filament lamp with an op amp. It is a property of filament lamps that the resistance of the tungsten filament increases in a non-linear manner as the filament heats up. The lamp in Fig. 3. 4. 1 is connected in the negative feedback potential divider that sets the gain of the non-inverting ampli


Wien Bridge Oscillators
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fier. The gain of the amplifier is set by: Therefore the greater the resistance of the lamp the lower the amplifier gain. By choosing a suitable lamp, the gain of the amplifier can be automatically controlled over an appropriate range. Usually a lamp with a maximum current flow of around 50mA or less is used, to give an initial gain of more than 3 as the oscillator starts, falling quickly to 3 as the lamp heats up. Hewlett`s original 1939 design used a high voltage vacuum tube (valve) and relatively large lamp, with modern low voltage semiconductors however, it is not easy to find suitable filament lamps that have a suitable voltage range and a low enough current to avoid overloading the amplifier, although useful lamps can still be found, usually of the wire ended T1 or grain of rice` types but even they are becoming more difficult to find in component suppliers catalogues, as LED types become more popular for low voltage lighting. Fig. 3. 4. 2 shows a low current filament lamp designed to work from 5V at 45mA, and Fig. 3. 4. 3 is a graph taken from a typical example, showing how its positive temperature coefficient resistance varies with voltage. (Note that resistance without any temperature dependant characteristics would be a straight line). The useful area of the lamp characteristic, where the largest change in resistance occurs is shaded green, the oscillators amplitude is stabilised by making use of this area....




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