It has been suggested that this article be merged into Biasing. (Discuss) Proposed since March 2014.
Grid bias is a DC 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. Bias point in small-signal applications is set to minimize distortion and achieve sufficiently low power draw. In high-power applications, biasing is typically set for maximum available output power or voltage, with a secondary target of either low distortion or high efficiency.
In a typical voltage amplifier, including power stages of most audio power amplifiers, DC bias voltage is negative relative to cathode potential. Instant grid voltage (sum of DC bias and AC input signal) should never rise above cathode potential to prevent grid-to-cathode currents that overload preceding amplifier stages and may cause severe even-order distortion. High transconductance tubes develop significant grid currents even with small negative bias; in these cases, maximum instant voltage ceiling is lowered to -1.0..-0.5 Volt.
An external voltage source (fixed bias) - a battery or a dedicated DC power supply. When the cathode potential is raised above ground (as in cascode circuits), bias voltage is obtained by tapping into main (positive) plate power supply.
Automatic bias or self bias - using a cathode resistor to raise cathode potential above grid (tied to ground) and stabilize plate current;
Grid leak bias - diverting DC grid current through a high value grid resistor.