This circuit produces an extremely low
distortion sine-wave, in spite of the non-linear devices used for amplitude limiting (D1
and D2). The reason is first that distortion (harmonics) are fed to the minus input of the
opamp with far less loss than to the plus input, severely attenuating them. Second, a Wien
bridge oscillator requires a gain of exactly 3.00. No more and no less. For lowest
distortion, calculate the minimum and maximum available gain to just above and just below
3.00. In other words, use as little amplitude control as possible.