Common Emitter Connection (or CE Configuration)

Definition: The configuration in which the emitter is connected between the collector and base is known as a common emitter configuration. The input circuit is connected between emitter and base, and the output circuit is taken from the collector and emitter. Thus, the emitter is common to both the input and the output circuit, and hence the name is the common emitter configuration. The common emitter arrangement for NPN and PNP transistor is shown in the figure below.

Base Current Amplification Factor (β)

The base current amplification factor is defined as the ratio of the output and input current in a common emitter configuration. In common emitter amplification, the output current is the collector current I_C, and the input current is the base current I_B.

In other words, the ratio of change in collector current with respect to base current is known as the base amplification factor. It is represented by β (beta).

Relation Between Current Amplification Factor (α) & Base Amplification Factor (β)

The relation between Β and α can be derived as

We Known, 

Now,

Substituting the value of ΔI_E in equation (1), we get,

The above equation shows that the when the α reaches to unity, then the β reaches to infinity. In other words, the current gain in a common emitter configuration is very high, and because of this reason, the common emitter arrangement circuit is used in all the transistor applications.

Collector Current

In CE configuration, the input current I_B and the output current I_C are related by the equation shown below.

If the base current is open (i.e., I_B = 0). The collector current is current to the emitter, and this current is abbreviated as I_CEO that means collector- emitter current with the base open.

Substitute the value ΔI_B in equations (1), we get,

Characteristics of Common emitter (CE) Configuration

The characteristic of the common emitter transistor circuit is shown in the figure below. The base to emitter voltage varies by adjusting the potentiometer R₁. And the collector to emitter voltage varied by adjusting the potentiometer R₂. For the various setting, the current and voltage are taken from the milliammeters and voltmeter. On the basis of these readings, the input and output curve plotted on the curve.

Input Characteristic Curve

The curve plotted between base current I_B and the base-emitter voltage V_EB is called Input characteristics curve. For drawing the input characteristic the reading of base currents is taken through the ammeter on emitter voltage V_BE at constant collector-emitter current. The curve for different value of collector-base current is shown in the figure below.

The curve for common base configuration is similar to a forward diode characteristic. The base current I_B increases with the increases in the emitter-base voltage V_BE. Thus the input resistance of the CE configuration is comparatively higher that of CB configuration.

The effect of CE does not cause large deviation on the curves, and hence the effect of a change in V_CE on the input characteristic is ignored.

Input Resistance: The ratio of change in base-emitter voltage V_BE to the change in base current ∆I_B at constant collector-emitter voltage V_CE is known as input resistance, i.e.,

Output Characteristic

In CE configuration the curve draws between collector current I_C and collector-emitter voltage V_CE at a constant base current I_B is called output characteristic. The characteristic curve for the typical NPN transistor in CE configuration is shown in the figure below.

In the active region, the collector current increases slightly as collector-emitter V_CE current increases. The slope of the curve is quite more than the output characteristic of CB configuration. The output resistance of the common base connection is more than that of CE connection.

The value of the collector current I_C increases with the increase in V_CE at constant voltage I_B, the value β of also increases.

When the V_CE falls, the I_C also decreases rapidly. The collector-base junction of the transistor always in forward bias and work saturate. In the saturation region, the collector current becomes independent and free from the input current I_B

In the active region I_C = βI_B, a small current I_C is not zero, and it is equal to reverse leakage current I_CEO.

Output Resistance: The ratio of the variation in collector-emitter voltage to the collector-emitter current is known at collector currents at a constant base current I_B is called output resistance r_o.

The value of output resistance of CE configuration is more than that of CB