What is baised?

Answer 1

Transistor is an active device. Active devices (unlike the passive devices such as resistors and capacitors) you have to bias them so that they can function properly. A resistor behaves well no matter what voltage or current you apply to them (it still obeys I=V/R). But a transistor has to be biased at the right voltages in order to operate as what it is supposed to be.

For example, in bipolar npn, to have it as signal amplifier (e.g. common emitter), VBE (voltage across Base and Emitter) has to be larger than certain voltage (e.g. 0.7V) and VCE (voltage across collector and emitter) has to be larger than another voltage (e.g. 0.2V). Otherwise, the transistor won't be able to amplify the signals with high gain.

In a MOSFET (e.g. NMOS), similarly, we have to bias VGS (gate to source) to be larger than the threshold voltage and VDS (drain to source) to be larger than the saturation voltage.

In conclusion, biasing is to use voltage to set the active element at the "right operating region"! The above are just examples. And depends on what you want the transistors to do, you need different "biasing condition". E.g. if you want to make the MOSFET behave like a resistor, you make VDS smaller than the saturation voltage.

So, after biasing, how can they be used? Usually, a small voltage (signal) will be coupled to the biased transistors for signal processing.