Wien Bridge Oscillator with AGC and bandpass filter

[Vineyards]

This one is meant for oscillator novices who would like to learn about a few tricks about oscillator design. There are big names in electronics like Max Wien, R.P. Sallen and E. L. Key, Stephen Butterworth. They pretty much did everything in terms of design principles etc. and I did the necessary calculations (and used a filter design software for the filter components).

Here is a brief explanation of the circuit. To the left you will find Wien Bridge which is tuned to oscillate at 1kHZ (in practice it may differ a bit). Wien bridge is a tuned bridge oscillating at a frequency determined by the caps and resistors that form the frequency selective positive feedback circuit. There is a negative feedback path formed by R1 and an AGC circuit comprising the two diodes which rectify the output and feed it into the FET which functions as a VCR (voltage controlled resistor) the changes at the FET gate causes the VCR resistance to go up and down regulating the output current. The thermal stability of this circuit is weak and there are a few tricks to improve it. But we need to study the thermal characteristics of the FET first.