Cathode Bias - Overcoming Problems

Overcoming Problems

To overcome this problem, the bias resistor is typically shunted by a capacitor. In general, the capacitor value is selected such that the time constant of the capacitor and bias resistor is an order of magnitude greater than the period of the lowest frequency to be amplified. The capacitor thus acts as a dynamic battery, and makes the bias constant through input signal swings.

In some designs, the degeneratve feedback inherent in cathode bias may be desirable. In this case, two carefully designed successive stages may be employed, such that the distortion introduced by the first stage is exactly cancelled by that introduced in the second. This technique is not recommended, as the design considerations become very complex. Other degenerative feedback techniques are easier to design, and should be used.

An exception to the general rule may be made in the case of "push-pull", or balanced circuits. A pair of tubes, driven by identical signals 180 degrees out of phase, may share a common unbypassed cathode resistor. Slight differences in tube conduction are then dynamically balanced by bias variations that tend to reduce distortion. This technique is useful in the input circuits of balanced line receivers or push-pull power output circuits.

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