Rf = 100 k preset - The slider is set about a quarter of the way round. This is used to control the gain. The gain should be as small as possible consistent with the oscillator starting up reliably. The zener diodes limit the oscillator output by decreasing the amplifier gain if the output rises above their breakdown voltage. They also decrease th

e accuracy of the sine wave shape. However if the diodes are not there, the amplifier limits and the distortion is much worse. For a much smaller audio output, replace the Zener diodes in series with normal diodes in parallel. The output will now be about +/- 0. 7 volts, suitable for headphones or the input of an audio amplifier. This circuit is designed to have a gain of exactly 1. If the gain were less, the oscillations would die away. If the gain were more, the pure sine wave would get bigger until the op amp limited and the wave would become clipped. If the output is too small, the diodes don`t conduct. This makes the gain higher. If the output is too large, the diodes to conduct and this negative feedback decreases the gain. When the output is just right, the gain is one. The resistors and capacitors (R and C) provide positive feedback. At the frequency of oscillation, the positive feedback is in phase and this reinforces the oscillations. At other frequencies, this is not the case so the oscillator frequency is controlled. Rf provides negative feedback. With too much negative feedback, the oscillator would fail to start. With too little feedback, the output would increase until the oscillator limited. The following article will show you how to use two op amps to make a simple sine wave generator. DO NOT USE A HIGH SPEED OP AMP - USE A CHEAP SLOW ONE, TRANSISTOR NOT CMOS. A high speed one would just give you...

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