COMMON BASE CONNECTIONIn this configuration the input is applied between the emitter and base and the output is taken from the collector and the base. Here the base is common to both the input and the output circuits as shown in Fig.
In a common base configuration, the input current is the emitter current. and the output current is the collector current I The ratio of change in collector
current to the change in emitter current at constant collector-base voltage is called current amplification factor,
In a transistor VEB, IE, VCB, and IC are parameters.
- These parameters can be interrelated in a number of ways. In these parameters the input current and the output voltage are taken as independent variables.
- The input voltage and output current are then expressed in terms of these independent variables. And these dependent variables also be expressed in functional relationship.
i.e., VBE= f1 (IE,VCB)
IC= f2(In, VCB)
- Thus the characteristics of a transistor is completely desired by the above two equations. These relationships can be conveniently displaced graphically.
- The curves thus obtained are known as the static characteristics. The most important static characteristics are the input and the output characteristics
COMMON BASE CIRCUIT
- A test circuit for determining the static characteristic of an NPN transistor is shown in Fig In this circuit, base is common to both the input and the output circuits.
- To measure the emitter and the collector currents mull ammeters are connected in series with the emitter and the collector circuits.
- Voltmeters are connected across the input and the output circuits to measure VBE and VCB There are two potentiometers R1 and R2 to vary the supply voltages VCC and VBE.
- It is a curve, which shows the relationship between the emitter current, I and emitter-base voltage V at constant collector-base voltage V This method of determining the characteristic is as follows.
- First by means of R1, a suitable voltage is applied to VCB from VCC. Next, voltages VBE is increased in a number of steps and corresponding values of IE are noted.
- The emitter current is taken on the Y-axis and the emitter base voltage is taken on the X-axis. Fig 2.12 shows the input characteristic for germanium and silicon transistors.
The following points may be noted from the characteristics curves.
1.This characteristic may be used to find the input resistance of a transistor.The input resistance (ri) value is given by the reciprocal of the input characteristic curve.
2.The emitter current IE increases rapidly with small increase in emitter- base voltage. It means that the input resistance is very high.
3.The emitter current is dependent of collector voltage.
- It is a curve which shows the relationship between the collector current IC and the collector-base voltage VCB at constant emitter current IE. This method of determining the characteristic is as follows.
- First, by means of R2 a suitable voltage is applied to the base and the emitter. Next, VCB is increased from zero in a number of steps and corresponding values of IC are noted.
- The above whole procedure is repeated for different values of IE for obtaining family of curves.
- The collector-base voltage is taken the X-axis. Fig shows the family of output characteristics at different emitter current values.
- The following points may be noted from the family of characteristic curves.
- The collector current IC varies with VCB only at very low voltages.
- This characteristic may be used to find the output resistance (rO)
- A very large change in collector-base voltage produces small change in collector current. It means that the output resistance is very high.
- The collector current is constant above certain values of collector-base voltage. It means that IC is independent of VCB and depends upon IE only.
The output characteristics may be divided into three regions
1. The active region
2. Cut-off region
3. Saturation region
Active region: In this region the collector junction is reverse biased and the emitter junction is forward biased. In this region when IE= 0, IC = ICO. This reverse saturation current remains constant and is independent of collector voltage V as long as is below the break down potential. When emitter current flows in the emitter circuit then a fraction (- IE) of this current reaches the collector. Hence IC = - IE + ICO. Thus in the active region the collector current is independent of collector voltage and depends only upon the emitter current. But due to Early effect there is a small increase (0.5%) in IC with increase in VCB
Saturation region: The region to the left of the ordinate VCB = 0 is called the saturation region. In this region both junctions are forward biased. This is also called as bottomed region because the voltage has a fallen near the bottom of the characteristic where VCB = 0. In this region IC increases rapidly with even small increase VCB in as shown in Fig
Cut-off region: The region below the IE= 0 characteristic, for which the emitter and collector junction are both reverse biased, is called cut-off region. This portion of characteristic is not coincident with the voltage axis as shown in Fig.