Harmonic Distortion versus Feedback Voltage
Author: J.B. Hoag
If a C-bias voltage is used instead of a grid condenser and resistor, it is necessary to adjust its value to a proper operating point on the characteristic curve of the tube in order to avoid distortion of the wave-form of the oscillations.
For example, if the grid bias is adjusted to too high a value, and if the voltages fed back from the plate to the grid are strong, the lower halves of the plate current will be cut off. This unsymmetrical wave-form is equivalent to an alternating current of a certain frequency, plus harmonics of that frequency, plus a d.c. component of current. Under these conditions, a d.c. milliammeter in the plate circuit will show an increase when the oscillations start. On the other hand, if the grid is not sufficiently negative, and if the feedback voltages are large, the tops of the plate current curves will be squared off at saturation. This is also due to the fact that the positive grid extracts electrons from the plate current. Then, a d.c. meter in the plate circuit will decrease when oscillations start. Again, if the grid bias is properly established, near the center of the characteristic curve, but the feedback voltages are very strong, the tops and bottoms of the plate current will be squared off; the oscillations will not be of pure sine-wave form, but will contain harmonics. The maximum amplitude of currents in the plate circuit is obviously equal to one-half the saturation or emission current. The discussion of large-amplitude oscillation given in the preceding paragraph must not be applied to transmitters where the tuned couplings strengthen the fundamental and reject harmonics born of distortion. This is also the case in frequency meters, where the harmonics are useful rather than detrimental.