What is bias current? Why and when is a bias current applied?

Answer 1

Bias current is usually that current and/or voltage applied to some component, like a transistor, to control its performance. Think of a transistor as a swinging gate, and the bias current that which controls how open or closed the gate is.

Answer 2

Bias current is usually used to set the operating state of an electronic device, like a transistor. In the case of a transistor the bias current can be used to set the operating regime of the transistor, for example for small signal amplification the transistor is biased so that it operates in the linear region. For other applications the tranistor may be biased into the saturation region.