Q. What is meant by biasing transistor?
The purpose of dc biasing of transistor is to obtain a certain dc collector current at a certain dc collector voltage. These values of current and voltage are expressed in terms of operating point. To obtain the operating point we make use of some circuits called biasing circuits. Only the fixing of a suitable operating point is not sufficient It must also be ensured that it remains there. In a transistor the operating point shifts with the use of circuit. Such a shift of operating point may drive the transistor into undesirable region. There two reasons for the shift of operating point. Firstly the transistor parameters are temperature dependent. Secondly the parameters (like beta) change from unit to unit. The requirements of biasing a transistor are: establish the operating point in the active region of the characteristics, so as that on applying the input signal the instantaneous operating point does not move to saturation or cut off region. Stabilize the collector current against temperature variations.
Make the operating point independent of transistor parameters so that it does not shift when the transistor is replaced by another of the same type of circuits.