This is a Wien-bridge sine-wave oscillator circuit. This circuit uses negative-feedback stabilization to make sure the gain won’t go higher than unity to prevent signal distortion. The D2 and D1 will conduct when Negative peaks in excess of -8.5 V. That action will charges C4. The amplifier gain is determined by the biased Q1 that is charged by C4. Offset voltage/current errors never being multiplied because of C3. Beside that, C3 also gives low-frequency roll-off in the feedback network. Here is the schematic diagram of the circuit.
The Q1 is operated at a small negative gate bias because the negative feedback loop is adjusted by R5.