Module 2: Bipolar Junction Transistor - BJT

Module 2: Bipolar Junction Transistor -

BJT

Prepared by: Cesar Mendoza Applied Technology Teacher

Prepared by: CESAR MENDOZA-Applied Technology Teacher

Module ObjectivesUpon successful completion of this module, students should be able to:1. State the advantages of a transistor over vacuum tubes. 2. Define a Bipolar Junction Transistor.3. Differentiate between an NPN and a PNP transistor in terms of their

construction, two-diode analogy and symbols.4. Identify the three different BJT terminals and state the function of

each terminal.5. Recognize the low-power and high-power BJTs.6. Measure transistor currents, and confirm that the emitter current is

the sum of the collector and base currents.7. State the common applications of a Bipolar Junction Transistor.8. Describe the three different BJT configurations with the help of circuit

diagrams.9. Describe the application of a BJT as a switch through a practical

demonstration.10. Build and test an ‘automatic night lamp circuit’ to demonstrate the

function of a BJT as a switch.11. Build and test a simple temperature control system.


OBJECTIVE 1. State the advantages of a transistor over vacuum tubes.

The reasons are obviously its advantages over vacuum tubes such as :

1. compact size, 2. light weight, 3. more resistive to shocks and vibrations, 4. low operating voltage, 5. long life and so on.


OBJECTIVE 2:Define a Bipolar Junction Transistor.A Bipolar Junction Transistor (BJT) is defined as a three-terminal semiconductor device, whose operation depends upon the flow of electric charge carriers within the solid.

The word transistor is derived from the combination of two words, “Transfer-Resistance”. It means that it is a device, which transfers a low resistance into a circuit having high resistance.

Transistors in general are classified as bipolar or unipolar type. The bipolar type has two PN-Junctions, while unipolar types have only one PN-Junction.


OBJECTIVE 3:Differentiate between an NPN and a PNP transistor in terms of their construction, two-diode analogy and symbols.


The three regions are called 1. Emitter (E), 2. Base (B), and 3. Collector (C),

and the two PN-junctions are a. C-B junction and b. E-B junction.


OBJECTIVE 3: Identify the three different BJT terminals and state the function of each terminal.


E

E- Emitter

C- Collector

Base -BEmitter: To emit majority charge carriers into the base.

Base: Base is the middle, extremely thin, and lightly doped region. It controls the electrons flowing to the collector.

Collector: To collect the emitted charge carriers through the base.

The functions of the three regions are as follows:

OBJECTIVE 3: Recognize the low-power and high-power BJTs.

Low–power, small-signal transistors are sealed in a metal, plastic or epoxy package.


High power transistors are usually designed to be mounted onto a metal frame, which acts as a heat sink (to conduct heat away).


OBJECTIVE 4: Measure transistor currents, and confirm that the emitter current is the sum of the collector and base currents.

To use a BJT, we connect it so that:A. Its emitter is its negative terminal.B. The collector is several volts positive

of its emitter.C. The base is 0.7V (or slightly more)

positive of its emitter.Under these conditions, we find that:

1. A small base current flows into the base.2. A much larger current flows into the

collector.3. The base and collector currents flow out

of the emitter


IE = IC + IB

where IE Emitter current, IC Collector current, and IB Base current

OBJECTIVE 4: State the common applications of a Bipolar Junction Transistor

Common applications of a transistor include the following:

1. A switch2. An amplifier


As a Switch

When the switch is closed, a small current flows into the base (B) of the transistor, which is just enough to make LED B glow dimly.


When the switch is open, no base current flows, so the transistor switches off the collector current.

Transistor Amplifier:Amplification is the process of increasing the level of a weak signal. A small change in the base current of a transistor produces a large variation in the collector current


Module 2: Bipolar Junction Transistor - BJT

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