- 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.
- R = 15 k - This controls the oscillator frequency.
- C = 10nF - This also controls the oscillator frequency.
- The zener diodes limit the oscillator output by decreasing the amplifier
gain if the output rises above their breakdown voltage. They also decrease
the accuracy of the sine wave shape. However if the diodes are not there,
the amplifier limits and the distortion is much worse.
- This oscillator runs at about 1kHz. Use the formula below ...
- F = 1 / (2 π RC)
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.
The resistors and capacitors (R and C) filter the 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.
Here is a sample layout on protoboard.
