How is Bias applied in an amplifier?

[FAQ Team]

Bias is applied by two different means. Firstly, it is possible to configure a dedicated negative supply from the amp's power unit and use this to set the grid negative. This is usually how bias is set for output valves and since valves differ in their characteristics, a measure of adjustment is usually provided for each valve so that the bias voltage can be adjusted to give identical quiescent currents in each valve. This is very important in push-pull circuits as any imbalance can result in output transformer core saturation and hence distortion.

Bias may also be applied by putting a suitable resistor between the valve's cathode and earth. The current flow through the valve causes a voltage drop across this resistor, placing the cathode at a positive potential. This means that the grid will be negative with respect to the cathode, providing bias. The presence of a resistor in the cathode circuit reduces the gain of the stage, so it is normally bypassed with a capacitor so that alternating current (the signal) sees a low impedance to earth, maintaining the AC gain of the stage. This type of bias is self-regulating, since an increase of valve current will cause a larger voltage drop across the resistor which will reduce the current. This method is invariably used in audio applications for biasing small signal valves.

The two bias schemes can be used together in a compound bias arrangement as in WADs KEL34 and KiT/Kat88 output stages.