Prof. Dr.

Ashis Kumar Bhattacharjee

Chairperson, Department of Physics
Eden Mohila College, Dhaka

Common Emitter Configuration

In this configuration we use emitter as common terminal for both input and output. This
common emitter configuration is an inverting amplifier circuit. Here the input is applied
between base-emitter region and the output is taken between collector and emitter
terminals. In this configuration the input parameters are VBE and IB and the output
parameters are VCE and IC. The common emitter arrangement for NPN and PNP
transistor is shown in the figure -1 below.

Figure – 1: Common Emitter configuration of BJT transistor.

Characteristics of Common emitter (CE) Configuration

The characteristic of the common emitter transistor circuit is shown in the figure -2
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 milli ammeters and voltmeter. On
the basis of these readings, the input and output curve plotted on the curve.

1 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

Figure – 2: Circuit diagram for CE characteristics curve.

Input Characteristic Curve

The curve plotted between base current IB and the base-emitter voltage VEB is called
Input characteristics curve. For drawing the input characteristic the reading of base
currents is taken through the ammeter on emitter voltage VBE at constant collector-
emitter current. The curve for different value of collector-base current is shown in the
figure -3 below.

Figure -3: Input Characteristics curve for CE configuration of BJT transistor.

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

2 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

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

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

Output Characteristic

In CE configuration the curve draws between collector current IC and collector-emitter

voltage VCE at a constant base current IB is called output characteristic. The
characteristic curve for the typical NPN transistor in CE configuration is shown in the
figure - 4 below.

Figure - 4: Output Characteristics curve for CE configuration of BJT transistor.

In the active region, the collector current increases slightly as collector-emitter

VCE 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.

3 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

The value of the collector current IC increases with the increase in VCE at constant
voltage IB, the value β of also increases.

When the VCE falls, the IC 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 IB

In the active region IC = βIB, a small current IC is not zero, and it is equal to reverse
leakage current ICEO.

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 IB is
called output resistance ro.

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

Common Emitter 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 IC, 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).

We Known,

I C
          (1)
I B
I C
          (@)
I E

4 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

Now,

Substituting the value of ΔIE 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 IB and the output current IC are related by the
equation shown below.

5 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

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

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

6 : Electronics-Transistor. Note-3
 Prof. Dr. Ashis Kumar Bhattacharjee
Chairperson, Department of Physics
Eden Mohila College, Dhaka

7 : Electronics-Transistor. Note-3