JEE Main & Advanced Physics Semiconducting Devices Transistor Configurations

A transistor can be connected in a circuit in the following three different configurations. Common base (CB), Common emitter (CE) and Common collector (CC) configuration.

(1) CB configurations : Base is common to both emitter and collector .

(i) Input current \[={{l}_{e}}\]                   

(ii) Input voltage \[={{V}_{EB}}\]

(iii) Output voltage \[={{V}_{CB}}\]         

(iv) Output current \[={{l}_{C}}\] With small increase in emitter-base voltage \[{{V}_{EB}},\] the emitter current  \[{{l}_{e}}\] increases rapidly due to small input resistance.

(v) Input characteristics : If \[{{V}_{CB}}\] constant, curve between \[{{l}_{e}}\] and \[{{V}_{EB}}\] is known as input characteristics. It is also known as emitter characteristics

Input characteristics of NPN transistor are also similar to the above figure but \[{{l}_{e}}\] and \[{{V}_{EB}}\] both are negative and \[{{V}_{CB}}\] is positive.

Dynamic input resistance of a transistor is given by

\[{{R}_{i}}={{\left( \frac{\Delta {{V}_{EB}}}{\Delta {{I}_{e}}} \right)}_{{{V}_{CB}}=\,\text{constant}}}\]{\[{{R}_{i}}\] is of the order of \[100\,\Omega \]}

(vi) Output characteristics : Taking the emitter current \[{{i}_{e}}\] constant, the curve drawn between \[{{l}_{C}}\] and \[{{V}_{CB}}\] are known as output characteristics of CB configuration.

Dynamic output resistance \[{{R}_{o}}={{\left( \frac{\Delta {{V}_{CB}}}{\Delta {{i}_{C}}} \right)}_{{{\text{i}}_{\text{e}}}\text{ }=\text{ constant }}}\]

(2) CE configurations : Emitter is common to both base and collector.

The graphs between voltages and currents when emitter of a transistor is common to input and output circuits are known as CE characteristics of a transistor.

Input characteristics : Input characteristic curve is drawn between base current \[{{l}_{b}}\] and emitter base voltage \[{{V}_{EB}},\] at constant collector emitter voltage \[{{V}_{CE}}\].

Dynamic input resistance \[{{R}_{i}}={{\left( \frac{\Delta {{V}_{BE}}}{\Delta {{I}_{B}}} \right)}_{{{V}_{CE}}\to \,\text{constant}}}\]

Output characteristics : Variation of collector current \[{{l}_{C}}\] with \[{{V}_{CE}}\] can be noticed for \[{{V}_{CE}}\]  between 0 to 1 V only. The value of \[{{V}_{CE}}\] up to which the \[{{l}_{C}}\] changes with \[{{V}_{CE}}\] is called knee voltage. The transistor are operated in the region above knee voltage.

Dynamic output resistance \[{{R}_{0}}={{\left( \frac{\Delta {{V}_{CE}}}{\Delta {{I}_{C}}} \right)}_{{{I}_{B}}\to \,\text{constant}}}\]