The Wien Bridge Oscillator (sometimes mistakenly misspelled Wein) uses negative feedback to generate a very pure sine wave. The circuit can generate a sine with distortion so low it is difficult to measure. The schematic I´ve attached does not necessarily have such very low distortion, but it is simple – three transistors – and generates a nice sine wave for doing audio work. You can find out how this circuit works on Wikipedia.
This circuit uses a #385 lamp in the negative feedback loop, to stabilize the oscillation point. As the output becomes greater, the higher power is fed back through R5, R6 and C3 to the lamp and it gets a little higher in resistance. The higher resistance reduces the gain of Q1 and stabilizes the oscillation point.
Nowadays with the proliferation of LEDs, lamps are getting harder to find. I have seen Chicago Miniature lamps for under a dollar in the Mouser online catalog, P. 116 (.PDF; if this has expired, go to mouser.com and search for CM385 lamp). The filament never gets hot enough to burn out so no socket is needed – just solder it in – and the lamp will never have to be replaced. You could also use the CM7009 which has wire leads.
If you don´t like to use a lamp, there is another version from a Japanese ham, JF1ozl. This uses a FET to control the feedback.
Error! This circuit is NOT a Wien Bridge Oscillator, it is a Phase Shift Oscillator.
Back to experimenting…