Transistor: Common Base Configuration (or) Characteristics of a Common Base

COMMON BASE CONFIGURATION (OR) CHARACTERISTICS OF A COMMON BASE

In this configuration, emitter current (I_E) is the input current and collector current (I_C) is the output current. The ratio of the collector current to the emitter current is called alpha (α) of a transistor.

In the transistor has two important characteristics. They are input characteristics and output characteristics.

Input Characteristics:

This curve give the relationship between the emitter current (I_E) and the emitter. to base voltage (V_EB) for a constant collector to base voltage (V_CB).

Output Characteristics:

This curve give the relationship between the collector current (I_C) and the collector to base voltage (V_CB) for a constant emitter (I_E).

These characteristics obtained by the circuit diagram as shown in fig.3.44. In this circuit, the NPN transistor is connected in a common-base configuration. The collector base voltage can be varied by changing the position of R,. While the emitter- base voltage can be varied by changing the position of potentiometer R,. The ammeter and voltmeters are connected in the emitter and collector circuit of a transistor to measure the voltage and current.

Input Characteristics of a Common-Base Configuration

The curve may be obtained by using the circuit diagram shown in fig.3.44. The following steps are carried out to obtain the characteristics

(i) The collector-to base voltage (V_CB) fixed one constant voltage.

(ii) The increasing the emitter to base voltage (V_EB) in small suitable steps in the order of 0.1 volts.

(iii) Record the corresponding values of emitter current (I_E) at each steps.

(iv) Plot a graph with V_EB at x-axis and I_E  in y-axis.

 (v) Similar procedure use may be used to obtain curve at different V_CB

From the input characteristics the following information's are obtained.

 (i) There exists a threshold voltage (ór) offset voltage (or) knee voltage as indicated by the region OA, below which the emitter current is negligibly small. The value of knee voltage (V_K) is approximately 0.5V for silicon and 0.1 V for germanium transistors.

(ii) For fixed collector to base voltage (1, 5, 10, etc) the emitter current (I_E) increases rapidly with a small increase in emitter to base voltage (V_EB).

(iii) The collector to base voltage (V_CB) is increased above one volts, the curves shift upwards. It occurs due to a phenomenon called base-width modulation (or) early effect.

(iv) The input characteristics may be used to determine the value of AC input resistance. Its value at any point on the curve is given by the ratio of a change in emitter to base voltage to the resulting change in emitter current (ΔI_E) for a constant collector-to-base voltage (V_CB).

R_i = ΔV_EB /ΔΙ_Ε |_VCB constant

Output Characteristics of a Common-Base-Configuration

The following steps are carried out to obtain the output characteristics

(i) Adjust the emitter to base voltage (V_EB) to get a suitable value of emitter current (steps of 2 mA, 4 mA, 6 mA.. etc) keeping the emitter current constant.

(ii) Increase the collector-to-base voltage (V_CB) from zero in a number of suitable steps

(iii) Record the corresponding collector current (I_C)

(iv) Plot the graph V_CB in x-axis and I_C in y-axis. It is shown in fig.3.46

The output characteristics gives the information about the following important points.

(i) The curve may be divided into three important regions namely saturation region, active region and cut-off region. The saturation and cut-off regions are shown by the shaded areas while the active region is the region between the saturation and cut-off region.

(ii) The collector to base voltage (V_CB) is negative for a NPN transistor. It means that collector-base junction of a transistor is also forward biased in the saturation region. In this region, a small change in V_CB results in a large value of current.

 (iii) The active region, the collector current is constant and is equal to the emitter current. The cut-off region is the region along the horizontal axis. It corresponds to the curve marked I_E = 0. In cut-off region, both the junctions of a transistor are reverse-biased.

(iv) A small collector flows even when emitter current (I_E) is zero. This current is called collector leakage current and is designated as I_CBO

(v) The characteristics may be used to determine the common-base-transistor AC output resistance. Its value at any point its given by the ratio of a change in collector to base voltage. (ΔV_CB) the resulting change in collector current.(ΔI_C) for a constant emitter current (I_E).

R_o = ΔV_CB /ΔI_C |I_E = Constant

(vi) The characteristics may be used to determine small-signal common-base current gain (or) a.c alpha (α₀) of transistor. This done by selecting two point R and S on the characteristics and note down the corresponding values of ΔI_C and I_C thus if ΔI_C = 6-4 = 2 mA and I_E = 6-4 = 2 mA, then small signal common base current gain

Α₀ = ΔI_C/ΔΙ_Ε = 2/2 =1