COMMON EMITTER CONNECTION
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.
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.
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.
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