A voltage applied to the gate (or base) of a transistor or vacuum tube, which causes the device to operate in its conductive state. When the control voltage (input voltage) is applied to the gate, it is added to the bias, causing the resultant voltage to be higher or lower, based on the sum of the two.

Forward and Reverse Bias
Forward bias is a voltage that brings the transistor or tube into or closer to its conductive state. For example, if the gate requires positive voltage to conduct, forward biasing adds positive voltage.

In contrast, reverse bias holds the device in a non-conductive state until the sum of the control voltage and bias is sufficient to bring it to the conductive state. For example, if the gate requires positive voltage to conduct, reverse biasing adds negative voltage.

Bias is widely used in analog devices, such as an audio amplifier, to keep the input voltage constantly within the conductive region of the transistor or tube. It is also used in digital circuits to reach a certain threshold and open or close the switch faster.