Grid bias

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Grid bias is a voltage applied to electron tubes (or valves in British English) with three electrodes or more, such as triodes. The control grid (usually the first grid) of these devices is used to control the electron flow from the heated cathode to the positively charged anode.

A small change in grid voltage will result in a large change in anode current, thus creating an amplifying effect. In order to linearly amplify an incoming signal, the output should respond to both the negative-going and positive-going parts of it. But without bias, when the grid voltage reaches zero and starts to become positive, all the electrons from the cathode will flow to the grid (and may cause overheating).

To prevent this happening, a negative bias must be applied to the grid in order to keep its voltage negative at all times during ordinary input signal levels. This causes the output voltage to more accurately reflect an amplified version of the input voltage, although it will be inverted or 180 degrees out of phase.

Various ways of obtaing a bias voltage are: from a battery or power supply, cathode resistor (self-bias), and grid-leak bias.