Bias
1.On tubes, bias is a small direct voltage applied to the grid to move the operating point of the device into a more linear range so as to reduce distortion.

2.On FETs, bias is a small direct voltage applied to the gate to move the operating point of the device into a more linear range so as to reduce distortion.

3.One bipolar transistors, bias is a small direct current applied to the base to move the operating point of the device into a more linear range so as to reduce distortion.

4.In magnetic tape recorders, bias is a very high frequency signal (often in the 100 kHz or higher range) mixed with the audio signal during recording. The purpose of bias is to reduce distortion by rapidly saturating the tape in both directions, minimizing the time spent in its non-linear magnetic hysteresis (see the WFTD Archives) curve. In more basic terms, bias makes it easier for the tape to respond to the audio signal in a linear fashion, and reduces distortion.

In all cases, the level of the bias must be carefully adjusted to achieve the best results. Too little bias increases distortion, too much reduces signal level and diminishes high frequencies.