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 for the measurement of impedances. 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. Automatic gain control, intentional non-linearity and incidental non-linearity limit the output amplitude in various implementations of the oscillator. The circuit shown to the right depicts a once-common implementation of the oscillator, with automatic gain control using an incandescent lamp. Under the condition that R1=R2=R and C1=C2=C, the frequency of oscillation is given by: and the condition of stable oscillation is given by Background There were several efforts to improve oscillators in the 1930s. Linearity was recognized as important. The "resistance-stabilized oscillator" had an adjustable feedback resistor; that resistor would be set so the oscillator just started (thus setting the loop gain to just over unity). The oscillations would build until the vacuum tube's grid would start conducting current, which would increase losses and limit the output amplitude. Automatic amplitude control was investigated. Frederick Terman states, "The frequency stability and wave-shape form of any common oscillator can be improved by using an automatic-amplitude-control arrangement to maintain the amplitude of oscillations constant under all conditions." In 1937, Larned Meacham described using a filament lamp for automatic gain control in bridge oscillators. Also in 1937, Hermon Hosmer Scott described audio oscillators based on various bridges including the Wien bridge. Terman, at Stanford University, was interested in Harold Stephen Black's work on negative feedback, so he held a graduate seminar on negative feedback. Bill Hewlett attended the seminar. Scott's February 1938 oscillator paper came out during the seminar. Here is a recollection by Terman: 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. I would report on some applications I had thought up on negative feedback, and the boys would read recent articles and report to each other on current developments. This