Consider a n-p-n transistor in

**common base****configuration**. In this common base configuration, emitter current I_{E}is the input and collector current I_{C}is the output current.# Common Base Configuration

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**Current Amplification Factor (****α****)**

The ratio of the transistor output current to the input current is called current gain of a transistor.

The ration of the collector current to the emitter current is called dc forward current transfer ratio or the dc current gain. It is designated by α

_{dc}and is known as the alpha dc.

**Common Base**dc current gain, α

_{dc}= I

_{C }/ I

_{E}.

Where I

_{C}and I_{E}are the magnitude of the collector current and emitter currents at a particular point on the transistor characteristics.
As the collector current is always less than emitter current, the value of α

_{dc }is always less than unity.
Typical values of α

_{dc}lies in the range of 0.95 to 0.998.
From equation, α

_{dc}= I_{C }/ I_{E}, We have I_{C}= α_{dc}I_{E}.
We can write α

_{dc}simply as α, then I_{C}=αI_{E}
Since I

_{C}= I_{C(majority)}+ I_{CBO(minority)}, the above equation can be written as
I

_{C}= α_{ }I_{C (majority) + }I_{CBO}
We know that I

_{E}= I_{C}+ I_{B }or
I

_{B}= I_{E}– I_{C}= I_{E}– (αI_{E}+ I_{CBO})
I

_{B}= I_{E}(1-α) – I_{CBO}
Neglecting I

_{CBO}, we can write I_{B}= I_{E}(1-α)
Common-base short circuit (ac) current gain is defined as the ratio of a small change in collector current (ΔI

_{C}) to the corresponding change in emitter current (ΔI_{E}) at constant collector-base voltage. It is denoted by α_{ac}.
α

_{ac}= (ΔI_{C}/ ΔI_{E}) / V_{CB}= constant.
For all practical purpose dc current gain is considered equal to the ac current gain. i.e. α

_{dc}= α_{ac}.
This parameter is extremely important in transistor theory. It should be pointed out that α is not a constant, but varies with emitter current I

_{E}, collector to base voltage V_{CB}and temperature.- It may be note that the current gain of a transistor in common-base configuration is less than unity. But still it is called the current gain; It is due to the fact that the output resistance of common-base transistor is much higher than the input resistance. This produces a large voltage gain and hence the large power gain.

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*Characteristics of Common Base Configuration*

*Characteristics of Common Base Configuration*

- The figure above shows that the experimental arrangement for determining the static characteristic of an n-p-n transistor in common-base mode.
- Two regulated dc power supplies V
_{EE}and V_{CC}are connected in the circuit as shown in the fig. two milli-ammeters and two voltmeters are included in the circuit to not the required current and voltage to draw characteristic.

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**Input Characteristics of common base configuration (V**_{EE}** vs I**_{E}**, V**_{CB}** = Constant)**

- Let the collector to base voltage V
_{CB}(say 2V) be kept constant The emitter to base voltage V_{EB}is varied in small steps, say 0.1s repeated V and the corresponding values of emitter current I_{E}are noted for each value of V_{EB}. - The test is repeated for various values of V
_{CB}. It is found that the increasing levels of V_{CB}results in a reduced level V_{EB}to establish the same current. - The input characteristic of a typical transistor of common-base mode are shown if fig. the emitter current I
_{E}is taken along Y axis and emitter-base voltage V_{EB}along X axis. Note the tight grouping of the curves for the wide range of values for V_{CB}. The average value of the curve appear to begin its rise at about V_{EB}=0.5 for Si transistor a 25^{o}C. - The emitter currents are almost independent of V
_{CB}. As with the semiconductor silicon diode, a first approximation for the forward-biased base-emitter junction in the dc mode would that V_{EB}= 0.7V.

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**Output Characteristic of common base configuration (V**_{CB}** Vs I**_{C}**, I**_{E}** = Constant)**

The emitter current, I

_{E}is kept constant (say 2mA). The collector to base voltage is varied from zero in suitable steps; say IV and corresponding values of I_{C}are noted. The experiment is repeated for different values of I_{E}. The output characteristic curves obtained are shown in fig.
The

**output characteristics can be divided into three distinct regions namely***common base configuration*- Active region
- saturation region
- cut-off region

- The active region is the region that is located to the right of the line V
_{CB}=0 and above the emitter current, I_{E}=0. In this region, the collector current is constant and is almost equal to the emitter current. - The saturation region id the region that located to the left of the line V
_{CB}=0 and above the output characteristic if emitter current I_{E}=0. The collector current, I_{C}increases sharply for a small change in V_{CB}. - In the above figure the emitter current increases above zero, the collector current increases to a magnitude equal to that of the emitter current as determined by the basic transistor-current relations. The curves clearly indicate that a first approximation to the relationship between I
_{C}and I_{E}in the active region is give by I_{C}=I_{E}. - When I
_{E}=0, I_{C}=I_{CBO}, which is the leakage current of the collector base diode. I_{E}=0 is called cut-off region.

The output characteristic curve may be used to determine dynamic output resistance, It is defined as the ration of a small change in collector to base voltage to the corresponding change in collector current are constant emitter current.

- The reciprocal of the slope of the output characteristics gives the dynamic output resistance. The value of R
_{o}is very high and is of the order of mega ohms. - The characteristic may be used to determine small-signal common-base current gain or ac alpha (α
_{ac}) of transistor. This can be done by selecting two points M and N on the characteristic and note down the corresponding values of ΔI_{C}and ΔI_{E}.

α

_{dc}= ΔI_{C}/ ΔI_{E}= 2mA / 2mA = 1 mA