Common Base Transistor Characteristics:

To investigate the Common Base Transistor Characteristics of a diode (a two-terminal device), several levels of forward or reverse bias voltage are applied and the resulting current levels are measured. The characteristics of the device are then derived by plotting the graph of current versus voltage. Because a transistor is a three-terminal device, there are three possible connection arrangements (configurations) for investigating its Common Base Transistor Characteristics. From each of these configurations, three sets of Common Base Transistor Characteristics may be derived.

Figure 4-20 shows a pnp transistor with its base terminal common to both the input (EB) voltage and the output (CB) voltage. The transistor is said to be connected in common base configuration. Voltmeters and ammeters are included to measure the input and output voltages and currents.

Input Characteristics of Common Base Configuration:

To determine the Input Characteristics of Common Base Configuration, the output (CB) voltage is maintained constant, and the input (EB) voltage is set at several convenient levels. For each level of input voltage, the input current I_E is recorded. I_E is then plotted versus V_EB to give the Input Characteristics of Common Base Configuration shown in Fig. 4-21.

Because the EB junction is forward biased, the common-base input characteristics are essentially those of a forward biased pn-­junction. Figure 4-21 also shows that for a given level of input voltage, more input current flows when higher levels of CB voltage are used. This is because larger CB (reverse bias) voltages cause the depletion region at the CB junction to penetrate deeper into the base of the transistor, thus shortening the distance and reducing the resistance between the EB and CB depletion regions.

Output Characteristics of Common Base Configuration:

The emitter current I_E is held constant at each of several fixed levels. For each fixed level of I_E, the output voltage V_CB is adjusted in convenient steps, and the corresponding levels of collector current I_C are recorded. In this way, a table of values is obtained from which a family of output characteristics may be plotted. In Fig. 4-22 the corresponding I_C and V_CB levels obtained when I_E was held constant at 1 mA are plotted, and the resultant Common Base Transistor Characteristics is identified as I_E = 1 mA. Similarly, other characteristics are plotted for I_E levels of 2 mA, 3 mA, 4 mA, and 5 mA.

The Output Characteristics of Common Base Configuration is shown in Fig. 4-22 show that for each fixed level of I_E, I_C is almost equal to I_E and appears to remain constant when V_CB is increased. In fact, there is a very small increase in I_C with increasing V_CB. This is because the increase in collector-to-base bias voltage (V_CB) expands the CB depletion region, and thus shortens the distance between the two depletion regions. With I_E held constant, the increase in I_C is so small that it is noticeable only for large variations in V_CB.

As illustrated in Fig. 4-22, when V_CB is reduced to zero, I_C still flows. Even when the externally applied bias voltage is zero, there is still a barrier voltage existing at the CB junction, and this assists the flow of I_C. The charge carriers which constitute I_C are minority carriers as they cross the CB junction. Thus, the reverse-bias voltage V_CB and the (unbiased) CB barrier voltage assist their movement across the junction. To stop the flow of charge carriers, the CB junction has to be forward biased.

Consequently, as shown in Fig. 4-22, I_C is reduced to zero only when V_CB is increased positively.

The region of the graph for the forward-biased CB junction is known as the saturation region, (see Fig. 4-22). The region in which the junction is reverse biased is named the active region, and this is the normal operating region for the transistor.

If the reverse-bias voltage on the CB junction is allowed to exceed the maximum safe limit specified by the manufacturer, device breakdown may occur. Breakdown, illustrated by the dashed lines in Fig. 4-22, can be caused by the same effects that make diodes breakdown. Breakdown can also result from the CB depletion region penetrating into the base until it makes contact with the EB depletion region, (Fig. 4-23). This condition is known as punch-through, or reach-through, and very large currents can flow when it occurs, possibly destroying the device. The extension of the depletion region is produced by the increase in V_CB. So, it is very important to maintain V_CB below the maximum safe limit specified by the device manufacturer. Typical maximum V_CB levels range from 25 V to 80 V.

Current Gain in Common Base Configuration:

The current gain characteristics (also termed the forward transfer characteristics) are a graph of output current (I_C) versus input current (I_E). They are obtained experimentally by use of the circuit in Fig. 4-20. V_CB is held constant at a convenient level, and I_C is measured for various levels of I_E. I_C is then plotted versus I_E, and the resultant graph is identified by the V_CB level, (see Fig. 4-24).

The CB current gain characteristics can be derived from the CB output characteristics, as shown in Fig. 4-25. A vertical line is drawn through a selected V_CB value, and corresponding levels of I_E and I_C are read along the line. The I_C levels are then plotted versus I_E, and the characteristic is labelled with the V_CB used. Because almost all of I_E flows out of the collector terminal as I_C, V_CB has only a small effect on the current gain characteristics.