) to the corresponding change in the collector current ∆ I C ) at constant base current ( I B ) is called output impedance ( r ). r V o CE C I B = ∆ ∆ ( . ) The output impedance for transistor in common emitter configuration is very low. iii) Current transfer characteristics This gives the relationship between the collector current ( I C ) and the base current ( I B ) at constant collector-emitter voltage ( V CE ) and is shown in Figure .
. It is seen that a small I C flows even when I B is zero. This current is called the common emitter leakage current ( I CEO ), which is due to the flow of minority charge carriers. - - - - - - - - Unit electronics and Communication which determine the operating point or the working point of a transistor.
If the operating point is chosen at the middle of the DC load line (point Q ), the transistor can effectively work as an amplifier. The operating point determines the maximum signal that can be obtained without being distorted. For a transistor to work as a open switch, the Q point can be chosen at the cut-off region and to work as a closed switch, the Q point can be chosen in the saturation region. .
. Transistor as a switch A transistor in saturation region acts as a closed switch while in cut-off region; it acts as an open switch. It functions like an electronic switch that helps to turn ON or OFF a given circuit by a small control signal which keeps the transistor either in saturation region or in cut-off region. The circuit is shown in Figure .
. R B 5V V V V I B I E I C R C E C CC in O Figure . Transistor as a switch · When the input is low: When the input is low (say 0V), the base current is zero and transistor is not properly forward biased. It