Transistors are solid-state devices similar in some ways to the diodes you have studied. Transistors are more complex and can be used in many more ways. The most important feature of transistors is their ability to amplify signals and act as switches. Amplification can make a weak signal strong enough to be useful in an electronic application. For example, an audio amplifier can be used to supply a strong signal to a loudspeaker.

What you are expected to learn

At the end of this module, you should be able to:

• Describe how a transistor is constructed and its two different configurations.

• Draw and label the schematic symbol for an NPN and a PNP transistor.

• Identify the ways of classifying transistors.

• Identify the function of a transistor using a reference manual and the identification number (2NXXXX).

• Identify commonly used transistor packages.

• Describe how to bias a transistor for operation.

• Explain how to test a transistor with both a transistor tester and an ohmmeter.

• Describe the process used for substituting a transistor.

How to learn from this module

Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some tips for you:

1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.

2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.

3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.

4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.

5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.

What to do before (Pretest)

• A transistor is a three-layer device used to amplify and switch power and voltage.

• A bipolar transistor is also called a junction transistor or simply a transistor.

• Transistors can be configured as NPN or PNP. The NPN types are more widely applied.

• The middle region of the transistor is called the base, and the two outer regions are called the emitter and collector.

• In a BJT, the emitter emits the carriers, the base is the control region, and the collector collects the carriers.

• Normal operation of a BJT requires that the collector-base junction be reverse-biased and the base-emitter junction be forward-biased.

  • Voltage-Divider Biased NPN Transistor

  • Emitter-Biased NPN Transistor

  • Base-Biased NPN Transistor

  • Decibel Gain, Power, and Voltage

• Most of the current carriers coming from the emitter cannot find carriers in the base region with which to combine. This tends to make the base current much less than the other currents.

• The base is very narrow, and the collector bias attracts the carriers coming from the emitter. This tends to make the collector current almost as high as the emitter current.

• Emitters of PNP transistors produce holes. Emitters of NPN transistors produce electrons.

• A transistor is classified according to whether it is NPN or PNP, silicon or germanium, high or low power, and switching or high frequency.

• Silicon transistors are much more widely used than germanium transistors.

• Transistors are identified with a prefix of 2N followed by up to four digits.

• The transistor package provides protection, a heat sink, and a support for the leads.

• Transistor packages are identified with the letters TO (transistor outline).

• In a properly biased transistor, the emitter-base junction is forward biased and the collector-base junction is reverse biased.

• PNP transistor bias sources are the reverse of NPN bias sources.

• A BJT is a normally off device. It is turned on with base current.

• The internal barrier voltage for germanium transistors is 0.3 V and for silicon transistors is 0.7 V.

• The reverse-bias voltage applied to the collector-base junction is higher than the forward-bias voltage applied to the emitter-base junction.

• An analog ohmmeter can check both junctions, identify polarity, identify leads, check gain, indicate leakage, and may even identify the transistor material.

• When a transistor is tested with an ohmmeter, each junction exhibits a low resistance when it is forward biased and a high resistance when it is reverse biased.

• Transistor testers are available for testing transistors in and out of circuit.

Simulation Activity

The principle of a transistor

Transistor application circuit

Self Check