Other Parts Discussed in Thread: TINA-TI
In your publication "Design of Op amp sine wave oscillators" in Analog Applications Journal August 2000 by Ron Mancini on page 35 Figure 5 "Wien Bridge oscillator with AGC" it states that R1 and R2 are chosen to center the bias on JFET Q1 so that (Rg + Rq1) = Rf/2 at the desired output voltage. Q1 changes the resistance keeping the output voltage constant. In previous wien bridge oscillators Rg goes from minus input to ground and Rf goes from the minus input to the op amp output (feedback resistor ). So I guess Rq1 is the resistance from drain to source of the JFET. I need you to confirm this. So the gain has to be 1/3 so the oscillation requires the gain to be 3. When Rf = 2 Rg1 then the amplifier's gain is 3 and oscillation occurs at f=1/(2piRC) . This amp with AGC keeps distortion low.
So how do I calculate Rg1? By hunt and try of components? I plan to make a circuit to - using a specific JFET - calculate the needed bias to produce a specific resistance from drain to source to make Rg1 and then get Rf from that. Is there anything I am missing or do you have anything to add to this? Thank you for your help.