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

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.

• Transistor Circuit Configuration

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.

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.

• Transistor Current Components

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^oC.
• 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.

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 common base configuration output characteristics can be divided into three distinct regions namely

1. Active region
2. saturation region
3. 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.

                            output Resistance R_o = {ΔV_CB / ΔI_C} I_E = Constant

• Transistor as an Amplifier

• 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

CSV Jun 22, 2016

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