Common Base Transistor Characteristics
The graph between voltages and currents when the base terminal of a transistor is common to input and output circuit are known as common base transistor characteristics of a transistor.
The circuit diagram to study common base characteristics of a transistor is shown in the figure.
The graph showing the variation of emitter current ( IE ) with the variation of emitter-base voltage ( VEB ) when a collector-base voltage ( VCB ) is kept constant is known as input characteristics of a transistor.
It can be written as VEB = f ( VCB ,IE)
A set of input characteristics are shown in the figure.
A transistor in fact consists of two diodes in series i.e. a PN junction diode in contact with another and np junction diode. So when the emitter-base junction is forward biased, the variation of IE with VEB is similar to the forward characteristics of a PN junction diode.
From the input characteristics, it is clear that there is a cutin or offset of threshold voltage below which there is no emitter current or the emitter current is very small. The value of threshold voltage = 0.1 V for Ge transistor and threshold voltage = 0.5 V for Si transistor.
Early effect or base width modulation
When a transistor is not biased, the width of the base region is W as shown in the figure. When the emitter-base junction is forward biased and the collector-base junction is Reverse Biased, emitter current IE increases with an increase in VEB at a fixed VCB.
However, when VCB increasing, the width of the depletion layer at the collector-base junction increases. As a result of this, the effective width of the base region decreases.. if the width of the depletion layer at the collector junction penetrating the base region is W’, then the effective width of the base region is equal to W – W’.
This change of the effective width of the base region by the reverse Bias Voltage of collector junction is known as early effect or base width modulation. Due to this effect, the gradient of the holes concentration in the base region increases with the increase in a reverse bias VCB for a given value of VEB.
Thus, Emitter current IE increases with the increase in VCB.
The Dynamic input resistance = ΔVCB/ΔIE.
Since a small change in VEB causes a large change in IE so the Dynamic input resistance of junction Je is very small. At a certain reverse bias of collector junction, the depletion layer at the collector junction covers the whole base region so the effective width of the base reduced to zero.
As a result of this, the potential barrier at the emitter junction decreases and hence large emitter current flows. This phenomenon is known as punch through. The value of the collector voltage at which punch through takes place is called punch-through voltage.
The variation of collector current IC with the collector-base voltage (VCB ) at constant emitter current (IE ) is called output characteristics of the transistor in the common base configuration. It can be written as Ic = F (VCB, Ic ).
It is clear that the output characteristics of a transistor in the common base configuration are divided into three regions :
- Active region,
- Saturation region,
- Cut off region.
In this region, the emitter-base junction JE is forward biased and collector-base junction JC is reversed biased. If IE = 0, then the transistor behaves as a PN junction diode formed by base and collector part of the transistor. In this case, the collector current IC is equal to the reverse saturation current ICO. The magnitude of ICO is constant and independent of VCB.
Now let us suppose, small emitter current IE flows. Since only a few holes entering the base region from the emitter Recombine with the electrons in the base region, so most of the emitter current reaches the collector. Then the collector current, IC=ICO– α IE
Where α Is the fraction of total emitter current which represents the holes that have traveled from the emitter to the collector through the base.
The collector current IC depends only on the value of IE and independent of VCB.
In this region, both the junction i.e emitter-base junction (JC) and collected base junction (Je) is forward biased. Thus, for pnp transistor, the saturation region of the output characteristics lies to the left of VCB = 0 and above IE =0.
In fact, VCB Is slightly positive. As the collector junction is forward biased, so the collector current IC increases exponentially with VCB just like in PN junction diode. That is why IC increases with a small increase in VCB in this region. The holes from the Collector (P region) cross the junction to enter the base region under the condition of forwarding bias.
Thus, hole current flows from the collector region to the base region. This hole current flows just opposite to the hole current flows from the collector region to the base region. Hence, the net hole current fin the transistor decreases. If the forward bias is high, then the hole current from the collector IC becomes positive.
Cut off region
In this region, both the emitter-base junction and collector-base junction are reverse biased. This region lies IE =0 and to the right side of VCB=0.