To study characteristics of common emitters of NPN or PNP transistors.

Introduction

It is vital to get complete details related to the characteristics of common emitters of NPN and PNP transistors through an experiment. The experiment regarding this topic is described below in detail. This experiment is to determine the current and voltage gain values for the transistors.

What is a Transistor?

A semiconductor device that is used to amplify electrical power and signals is known as a transistor. For modern electronics, transistors are considered a major building block. A transistor has semiconductor material that has at least three terminals to connect to an electrical circuit. In amplifiers, switches, oscillators, and other electrical circuits these transistors are used in the form of NPN and PNP bipolar junction transistors.

What is an NPN Transistor?

NPN transistor is a bipolar junction transistor that consists of two n-type semiconductors and one p-type semiconductor in between. Electrons are the majority charge carriers in NPN transistors. Semiconductors materials like silicon and germanium are used in making NPN transistors.

There are three terminals in NPN transistors- base, emitter, and collector. The two diodes are connected back-to-back in the NPN transistor. The first diode which is between the emitter and the base terminal is called the emitter-base diode. The second diode, which is between the collector and base terminal, is called the collector-based diode.

What is a PNP Transistor?

PNP transistor is a bipolar junction transistor that consists of two p-type semiconductors and one n-type semiconductor. It also includes three terminals- emitter, base, and collector. In PNP transistors, the majority of charge carriers are holes and electrons are the minority ones. The emitter in the PNP transistor allows the supply of a majority charge carrier. The collector collects the majority charge supplied by the emitter. The base is the centre section of a transistor and forms two circuits- input circuit with an emitter and it forms output circuit with the collector.

Experiment

Aim

To find the characteristics of a common NPN or PNP transistor emitter to determine the current and voltage gain values.

Material Required

●     An n-p-n transistor.

●     A 3-volt battery.

●     A 30-volt battery.

●     Two high-resistance rheostats.

●     One 0–3-volt voltmeter.

●     One 0–30-volt voltmeter.

●     One 0-50 μA micro-ammeter

●     One 0-50 mA milli-ammeter.

●     Two one-way keys.

●     Connecting wires.

Theory

Before performing the experiment, the students must know the definitions of the following terms:

1. Input Resistance: The resistance offered by the collector junction and due to forward biasing, has a very small resistance, which is called the Input Resistance.
2. Output Resistance: The resistance is high value due to reverse biasing and is offered by the collector junction. It is called Output Resistance.
3. Current Gain: The ratio of change of collector current and the base current is called Current Gain.

Procedure

• Make sure that all the connections are tight, neat, and clean.
• Note down the least count and zero errors of ammeters and voltmeters.
• Make sure that the readings of V₁ and V₂ are zero before inserting the keys.

For Input Characteristics

1. A forward bias voltage must be applied on the base junction to read the base current and voltage from µA.
2. The base voltage should be increased until the base current increases suddenly. The base voltage should be noted for every value of the base current.
3. Repeat the above steps five to six times, making the collector voltage = 10 V
4. With the collector voltage = 20 V and 30 V, repeat step 3 again.
5. After noting down all the values, make the readings zero.

For Output Characteristics

1. By adjusting the collector voltage to zero, note the collector voltage, and by adjusting the base voltage, note down the base current to 10 µA.
2. Make the collector voltage 10 V, 20 V, and 30 V and note down the collector current.
3. Repeat the above steps with base currents 20 µA, 30 µA, and 40 µA.
4. Record all the observations correctly.

Observations

Through this experiment, the readings about base voltage, base current, collector voltage, and collector current can be noted down. Create a separate table for base current, base voltage, and collector current. Also, note down the following observations:

1. Least count of the voltmeter.
2. Zero error of voltmeter.
3. Least count of the voltmeter.
4. Zero error of voltmeter.
5. Least count of milliammeter.
6. Zero error of milliammeter.
7. Least count of microammeter.
8. Zero error of microammeter.

Calculations

Plot a graph between base current and base voltage. The graph obtained is therefore known as the input characteristics of a transistor.

Plot another graph between collector voltage and collector current. The graph thus obtained is known as the output characteristics of a transistor.

Plot the third graph between base current and collector current. The graph will depict a straight line called the current gain characteristic of a common emitter transistor.

Result

The current and voltage gain values can be obtained from the graphs plotted.

Precautions

• Make sure that all connections should be properly connected.
• The keys should be used only when the circuit is used.
• Forward and reverse bias voltage should not be applied beyond breakdown.

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