CE Transistor Characteristics:

Common Emitter Circuit – Figure 4-26 shows a circuit for determining CE Transistor Characteristics. The input voltage is applied between the B and E terminals, and the output is taken at the C and E terminals. The emitter terminal is common to both input and output. Voltage and current levels are measured as shown.

Input Characteristics of Common Emitter Configuration:

To prepare a table of values for constructing the Input Characteristics of Common Emitter Configuration, V_CE is held constant, V_BE is set at convenient levels, and the corresponding I_B levels are recorded. I_B is then plotted versus V_BE, as shown in Fig. 4-27. It is seen that the Input Characteristics of Common Emitter Configuration (like the common base input characteristics) are those of a forward-biased pn-junction. It should be remembered that I_B is only a small portion of the total current (I_E) that flows across the forward-biased BE junction.

Figure 4-27 also shows that, for a given level of V_BE, I_B is reduced when higher V_CE levels are employed. This is because higher V_CE produces greater depletion region penetration into the base, reducing the distance between the CB and EB depletion regions. Consequently, more of the charge carriers from the emitter flow across the CB junction, and fewer flow out via the base terminal.

Common Emitter Output Characteristics:

To prepare a table of values for plotting the Common Emitter Output Characteristics, I_B is maintained constant at several convenient levels. V_CE is adjusted in steps at each I_B level, and the I_C level is recorded at each V_CE step. For each I_B level, I_C is plotted versus V_CE to give a family of CE Transistor Characteristics as shown in Fig. 4-28.

Because I_E is not held constant (as it is for the common-base output characteristics) the shortening of the distance between the depletion regions (when V_CB is increased) draws more charge carriers from the emitter to the collector. Thus, I_C increases to some extent with increasing V_CE although I_B is held constant. So, the slopes of the Common Emitter Output Characteristics are much more pronounced than those of the common base characteristics.

On Fig. 4-28 note that I_C reduces to zero when V_CE becomes zero. This is because the horizontal axis voltage is V_CE, which equals (V_CB + V_BE), (see Fig. 4-29). At the knee of the CE Transistor Characteristics, the CB junction voltage (V_CB) has been reduced to zero, (because V_CE = V_BE). Further reduction in V_CE causes the CB junction to be forward biased. The forward bias repels the minority charge carriers, thus reducing I_C to zero. The dashed lines in Fig. 4-28 show that, if V_CE exceeds a maximum safe level, I_C increases rapidly and the device may be destroyed. As for common-base configuration, this is due to punch-through.

Common Emitter Current Gain Characteristics:

The Common Emitter Current Gain Characteristics are (output current) I_C plotted versus (input current) I_B for various fixed levels of V_CE. Like the common base current gain characteristics, they can be obtained experimentally, or derived from the output characteristics. To experimentally obtain the table of I_C and I_B values, V_CE is held at a selected level, and the base current I_B Is adjusted in steps. The corresponding I_C level is recorded at each step of I_B.