Transistor Characteristics

Transistor Characteristics are the plots which represent the relationships between the current and the voltages of a transistor in a particular configuration. By considering the transistor configuration circuits to be analogous to two-port networks, they can be analyzed using the characteristic-curves which can be of the following types

1. Input Characteristics: These describe the changes in input current with the variation in the values of input voltage keeping the output voltage constant.
2. Output Characteristics: This is a plot of output current versus output voltage with constant input current.
3. Current Transfer Characteristics: This characteristic curve shows the variation of output current in accordance with the input current, keeping output voltage constant.

Common Base (CB) Configuration of Transistor

In CB Configuration, the base terminal of the transistor will be common between the input and the output terminals as shown by Figure 1. This configuration offers low input impedance, high output impedance, high resistance gain and high voltage gain.

Input Characteristics for CB Configuration of Transistor

Figure 2 below shows the input characteristics of a CB configuration circuit which describes the variation of emitter current, I_E with Base-Emitter voltage, V_BE keeping Collector-Base voltage, V_CB constant.

This leads to the expression for the input resistance as

Output Characteristics for CB Configuration of Transistor

The output characteristics of CB configuration (Figure 3) show the variation of collector current, I_C with V_CB when the emitter current, I_E is held constant. From the graph shown, the output resistance can be obtained as:

Current Transfer Characteristics for CB Configuration of Transistor

Figure 4 below shows the current transfer characteristics for CB configuration which illustrates the variation of I_C with the I_E keeping V_CB as a constant. The resulting current gain has a value less than 1 and can be mathematically expressed as:

Common Collector (CC) Configuration of Transistor

This transistor configuration has the collector terminal of the transistor common between the input and the output terminals (Figure 5) and is also referred to as emitter follower configuration. This offers high input impedance, low output impedance, voltage gain less than one and a large current gain.

Input Characteristics for CC Configuration of Transistor

Figure 6 shows the input characteristics for CC configuration which describes the variation in I_B in accordance with V_CB, for a constant value of Collector-Emitter voltage, V_CE.

Output Characteristics for CC Configuration of Transistor

Figure 7 below shows the output characteristics for the CC configuration which exhibit the variations in I_E against the changes in V_CE for constant values of I_B.

Current Transfer Characteristics for CC Configuration of Transistor

This characteristic of CC configuration (Figure 8) shows the variation of I_E with I_B keeping V_CE as a constant.

Common Emitter (CE) Configuration of Transistor

In this configuration, the emitter terminal is common between the input and the output terminals as shown by Figure 9. This configuration offers medium input impedance, medium output impedance, medium current gain and voltage gain.

Input Characteristics for CE Configuration of Transistor

Figure 10 shows the input characteristics for the CE configuration of transistor which illustrates the variation in I_B in accordance with V_BE when V_CE is kept constant.

From the graph shown in Figure 10 above, the input resistance of the transistor can be obtained as

Output Characteristics for CE Configuration of Transistor

The output characteristics of CE configuration (Figure 11) are also referred to as collector characteristics. This plot shows the variation in I_C with the changes in V_CE when I_B is held constant. From the graph shown, the output resistance can be obtained as:

Current Transfer Characteristics for CE Configuration of Transistor

This characteristic of CE configuration shows the variation of I_C with I_B keeping V_CE as a constant. This can be mathematically given by

This ratio is referred to as common-emitter current gain and is always greater than 1.

Lastly, it is to be noted that although the characteristic curves explained are for BJTs, similar analysis holds good even in the case of FETs.