Improving the Output

Author: N.H. Crowhurst

The waveform of the positive-feedback oscillator can be improved by adding negative feedback. Positive feedback that provides 100% of the required input at the one frequency will provide at least half as much voltage at double this frequency. If the amplifier produces any non-linearity that would cause second harmonic distortion, there will be at least a 50% positive feedback of the second harmonic as well as 100% feedback of the fundamental. If the tube causes third-harmonic distortion, there will be at least 33% positive feedback for it as well. For this reason, the positive-feedback oscillator produces considerably more harmonic distortion than either of the other types.

Negative feedback with positive gives lower distortion

By combining frequency-selective positive feedback with negative feedback that is not frequency-selective, distortion can be reduced. Suppose that the negative feedback produces an Ap of 9, while the positive feedback produces an A|3 of 10. The net positive feedback is (10 - 9) or 1 - still 100%, to allow oscillation. At double the fundamental frequency the positive Ap is only 5, while the negative feedback will still be 9. This means that the second harmonic will have a predominant negative feedback of (9 - 5) or 4. Without the negative feedback, there is 50% positive feedback of the second harmonic, resulting in a 6-db increase. The combination of negative with positive feedback results in a 14-db negative feedback, reducing the second harmonic distortion of the oscillator to 1/5, and higher harmonics even more. Thus, the positive and negative feedback arrange--ment inherently produces better sine waves than any other kind of audio oscillator.