In this circuit is added a jfet transistor to the classic wien bridge schematic. A jfet in the feedback network let the gain vary over a small range to ensure consistent oscillation. At startup, the JFET gate voltage is zero, the drain-source impedance is very low. This produces a gain enough to get the circuit going. Once oscillations build up, the rectified negative transitions of the output provide a turn-off voltage for the JFET’s gate. This reduces the gain, and the circuit settles down to stable oscillation.
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