Audio Oscillator schematic

Posted on Jul 30, 2012

The oscillator circuit (see Figure 1) involves two unity gain phase shift stages, A1 and A2, in tandem and a gain stage, A3, with back to back diodes and resistor network providing non-linear negative feedback. At a particular frequency (determined by RT and CT - the timing components) A1 and A2 provide 90 degrees phase shift each, 180 degrees in total and the circuit begins oscillating, since A3 and its non linear network has more than unity gain for small signals. As the oscillation level increases the diodes conduct and limit the gain of A3 stabilising the output at the desired level, in this case a little over 1V RMS. However, some distortion of the sine wave peaks is caused by the diodes.

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The principle is quite simple. Using two phase shift networks, the phase is rotated by 180° at one frequency. The final amplifier provides an additional inversion (effectively 180° phase shift), so the circuit will oscillate. The limiter (whether diodes, thermistor or something else) is used to prevent the amplitude from building up to the point where the signal is grossly distorted. Normally, diodes are not at all effective in preventing distortion, but that's where the fourth stage comes into its own. The fourth stage, A4, is the real secret of the design since it combines the outputs of the three preceding stages using a feedforward* approach. This is done in such a way as to reduce the third and higher odd harmonic distortion products generated in those stages due to the back to back diodes used for level stabilisation. Because the diodes are symmetrical in their effect they cause only third and higher odd harmonics of the sine wave output. The disadvantage of this simple circuit (especially if diodes are used) is that it will be almost impossible to get distortion below around 5% along with reliable oscillation. Even if you can find a suitable thermistor, the distortion will be no better than a typical Wien bridge oscillator, but with more active parts and a restricted frequency range due to the limitations of the opamps. The net effect of A4 is to remove at least 90% of these unwanted harmonics from the output over...

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