In this configuration, the input is applied between the base and the
emitter and the output is taken from the collector and the emitter. In
this connection, the emitter is common to both the input and the output
circuits as shown in Fig. In the common emitter configuration the input
current is the base current IB and the output current is the collector
current IC. The ratio of change in collector current to the change in
base current at constant collector-emitter voltage is called base
current amplification factor ( ).
COMMON EMITTER CIRCUIT
A test circuit for determining the static characteristic of an NPN
transistor is shown in Fig In this circuit emitter is common to both
input and output circuits. To measure the base and collector current
milli ammeters are connected in series with the base and the output
circuits. Voltmeters are connected across the input and the output
circuits to measure VBE and VCE There are two potentiometers R1 and R2
to vary the supply voltages VCC and VBB.
Circuit arrangement to determine static characteristic of common emitter
Input Characteristics
It is a curve which shows the relationship between base current IB and
the emitter-base voltage, VBE at constant VCE. The method of determining
the characteristic is as follows.
Common emitter input characteristic curve
First, by means of R1 suitable voltage is applied from VCC, Next,
voltage VBE is increased in number of steps and corresponding values of
IB are noted. The base current is taken on the Y-axis and the
base-emitter voltage is taken on the X-axis.
Fig shows the input characteristic for common emitter configuration. The following points may be noted from the characteristic.
1. The input resistance of the transistor is equal to the reciprocal of the slope of the input characteristic curve.
2. The initial portion of the curve is not linear.
3. The input resistance varies considerable from a value 4 kilo ohm to a value of 600 ohms.
4. In the case of silicon transistor the curves break away from zero
current for voltage in the range of 0.5 to 0.6 volt whereas for
germanium transistor the break away point is in the range 0.1 to 0.2V
Output Characteristics
It is a curve which shows the relationship between the collector IC and
the collector- emitter voltage VCE. This method of determining the
characteristic is as follows.
First by means of R1 a suitable base current IB is maintained. Next VCE
is increased from zero, in a number of steps and corresponding values of
IC are noted. The above whole procedure isrepeated for different values
of IB. The collector current is taken on the Y-axis. Fig shows the
family of output characteristics at different base current values. The
following points may be noted from the family of characteristic curves.
Common emitter characteristic curve
1.The collector current IC increases rapidly to a saturation level for
fixed value of IB. But at the same time VCE increases from zero.
2.A small amount of collector current flows even when IB=0 the current
is called ICEO. Now main collector current is zero and the transistor is
cut-off.
3.The output characteristics may be divided into three regions.
The active region
Cut-off region
Saturation region
Active region: In this region the collector is
reverse biased and the emitter is forward biased. The collector current,
IC response is most sensitive for changes in IB. Since = /(1- ) and
also is very close to unity. (I - ) is very small. Therefore, a slight
change in a produces very large change in b and so the collector
current,
is changed substantially
Cut-off region:
When IE= 0 and IC = ICO, the cut-off condition of the transistor is
reached. It is necessary that emitter junction has to reverse biased
slightly i.e., 0.1 V for germanium and 0 volt for silicon.
In this region
Saturation region:
In this region incremental change, in IB do not produce corresponding
large changes in IC. The region is also refer to as bottomed region
because the voltage has fallen near the bottom of the characteristic. In
this configuration saturation is entered while collector is still
reverse biased.