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Home Audio Oscillators Improving the Output  
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Improving the OutputAuthor: N.H. Crowhurst The waveform of the positivefeedback 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 nonlinearity 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 thirdharmonic distortion, there will be at least 33% positive feedback for it as well. For this reason, the positivefeedback oscillator produces considerably more harmonic distortion than either of the other types.
By combining frequencyselective positive feedback with negative feedback that is not frequencyselective, distortion can be reduced. Suppose that the negative feedback produces an Ap of 9, while the positive feedback produces an A3 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 6db increase. The combination of negative with positive feedback results in a 14db negative feedback, reducing the second harmonic distortion of the oscillator to 1/5, and higher harmonics even more. Thus, the positive and negative feedback arrangement inherently produces better sine waves than any other kind of audio oscillator.


Home Audio Oscillators Improving the Output 