# JEE Main & Advanced Physics Semiconducting Devices Transistor Configurations

Transistor Configurations

Category : JEE Main & Advanced

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 ${{R}_{0}}={{\left( \frac{\Delta {{V}_{CE}}}{\Delta {{I}_{C}}} \right)}_{{{I}_{B}}\to \,\text{constant}}}$

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