Common Emitter configuration, Input and Output Characteristics

 Common Emitter configuration:

Common emitter configuration

Two terminals are needed for input and two terminals for output. Transistors have three terminals, so one terminal have to be taken as common terminal for both input and output. In Common Emitter configuration, emitter terminal is taken as common for both input and output. So input is given between base and the emitter terminals and output is taken between collector and emitter terminals. This is the most commonly used configuration.

Circuit diagram of Common Emitter NPN and PNP Transistor:

Common emitter configuration of NPN transistor

Input voltage VBE is applied between base and emitter terminals and output voltage VCE is applied across emitter and collector. The output current IC is taken across the emitter and collector terminals. The input side is forward biased and the output side is reverse biased.

Emitter base region acts like forward biased diode and so the depletion region is very small.  Emitter collector region acts like reverse biased diode and the depletion region is large. The input current IB is measured in µA because the base region is very lightly doped. 

Common emitter configuration of PNP transistor

 The input and output impedance are moderate in common emitter configuration and thus the current and voltage gain is moderate and the power gain is high. So this configuration is widely used for amplification. 

Input Characteristics of Common Emitter Configuration:

Input characteristics of common emitter configuration

 Input characteristics are the relationship between the input current and the input voltage keeping output voltage constant. Here the input current is the base current IB, input voltage is base emitter voltage VBE and the output voltage is collector emitter voltage VCE.

First the output voltage VCE is kept at zero and the input voltage VBE is gradually increased and the input current IB is noted. Then again the output voltage VCE is increased like 10V, 20V and kept constant and by increasing the input voltage VBE, the input current IB is noted.

From the results it is observed that when the input voltage VBE is increased initially there is no current produced, further when it is increased the input current IB increases steeply. When the output voltage VCE is further increased the curve shifts right side.

Output Characteristics of Common Emitter Configuration:

Output characteristics of Common Emitter Configuration

 Output characteristics is the relationship between the output current and the output voltage keeping input current constant. Here the values of output current IC and the output voltage VCE is noted keeping input current IB constant.

In active region when the output voltage is increased there is very slight change in the output voltage. The curve looks almost flat in the active region. Cut off region is the region where the input current is below zero. When both the junctions are forward biased, it is in saturation region.