In the circuit the frequency trimming component is arranged so that the voltage across it is in quadrature with the voltage V0 from the bridge so that as it is adjusted the attenuation of the bridge only changes a little, avoiding the need for a two gang component. The range of variation of frequency is very limited.
By using a high gain amplifier and metal film feedback resistors the loop gain can be set so that the unit just oscillates and the use of an automatic gain setting component, a thermistor for example, is eliminated.
Wien Bridge Oscillator - Don
Basic Electronics IIT Kharagpur Part 29 of 38
741 sine wave (Wien bridge) oscillator (typical problems)
ElectronicsWeekly.com (blog) I can remember a colleague working for weeks on a sinewave generator board for a piece of test equipment. He came up with a with several refinements that had wonderfully low distortion. To be honest, I can't actu
EDN.com You can't measure 90-dB distortion and noise using sources that are “60 dB-ish.” So rather than try to find and keep an almost-ideal programmable signal generator, I decided to build up a low-distortion Meacham-bulb-st