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# Bipolar Junction Transistor

A bipolar junction transistor (BJT) has three operating regions:
1. Cut off ( for NPN BJT)
2. Active region ( for NPN)
3. Saturated ( for NPN)

In active region, for silicon BJT, and for Germanium BJT.

In saturated region, .

Common realizations of BJT are shown in 10.1

In active region:
Three configurations in the active region are shown in figure 10.2. For active region, the specified biasing condition is satisfied.

When transistor is used for switching purposes, it works in either cut-off or saturation mode.

In active region, the base and collector currents satisfy the condition (DC Current gain. Ratio of absolute values). is a constant for a particular transistor, which varies from to for different transistors. Note that this condition does NOT hold for saturation and cut-off operations of the BJT.

Now we address the problem of circuit design, in which we find appropriate values of resistances and voltages in figure 10.3 to ensure BJT in active region. The problem assumes importance as many transistor applications are those in which it is in active region.

In cut-off, , as . If becomes less than , the transistor is in saturation. We need to ensure that the BJT is not in these states.

In active region, as ,

The last equation shows that the transistor, in this mode (active), is basically a current amplifier.

Let . Then, . Suppose the BJT has . .

Also, we need to ensure , so that BJT is not in saturation. In the limiting case, , just when the BJT is entering saturation from active region. (In active region, ).

Thus, . That is, for ensuring BJT in active region.

Suppose we increase to . Then, . Thus, the current gain .

Cut off and saturation are used in switching application. For the circuit shown in figure 10.4, we find conditions for operating BJT as a switch.

When , , , and , since BJT is in cut-off.

Now find such that the BJT is in saturation.

 Thus, we get:

Thus, for , the BJT is in active region.

Two different biasing strategies are shown in figure 10.6 and 10.7.

Subsections

Next: CE Characteristics Up: Introduction to Electronics Previous: LM 317: Regulator   Contents
ynsingh 2007-07-25