A bridge oscillator is an electronic oscillator that generates a large range of frequencies in the form of sine waves. Oscillators are circuits that produce repetitive electronic signals. They are often used to transmit television or radio signals or produce sound for an electronic device. Sounds produced by an oscillator cannot be heard on their own, but must be converted into audible sound through the use of speakers or other devices. The bridge oscillator is often used as an audio frequency oscillator because it is easy to tune and has low distortion.
The word bridge refers to the structure of the circuit used. A bridge circuit is an electrical circuit in which two circuit branches are bridged or crossed by a third. A circuit branch is any electrical circuit that is divided into two or more parallel circuits. This results in each circuit receiving half of the current coming from the main line. The bridge is made up of four resistors and two capacitors.
The structure of the bridge oscillator circuit is based on a network developed by German physicist Max Wein in 1891. As a result, a bridge oscillator is sometimes referred to as a Wein bridge oscillator, even though Wien did not actually create it. The technology necessary for oscillators did not exist until decades after Wein created his circuit.
Credit for creation of the bridge oscillator actually goes to William Hewlett. He first described this type of oscillator in the Master's degree thesis he wrote for Standford University in 1939. That same year, Hewlett founded the information technology company Hewlett-Packard (HP) with David Packard. Hewlett and Packard used the technology described in his thesis to create HP's first product, the HP2001A. Hewlett was granted a patent for the design of the bridge oscillator in January of 1942.
One of the main advantages of this type of oscillator is its low distortion. This is possible only because the amplitude, or magnitude of change created, is well stabilized. Hewlett achieved this stabilization by using an incandescent light bulb as a thermistor.
A thermistor is a resistor that varies its resistance depending on the temperature. The resistance of a light bulb increases as the temperature increases. A light bulb is therefore a positive temperature coefficient thermistor. Since Hewlett's first bridge oscillator, light bulbs have been replaced with diodes or other types of thermistors that are less sensitive to vibration.