Slide 1

  • Topic: Bipolar Junction Transistor Basics
  • Objective: To understand the fundamentals of bipolar junction transistors
  • Agenda:
    • Introduction to transistors
    • Types of transistors
    • Bipolar junction transistor structure
    • NPN and PNP transistors
    • Working principle of a transistor

Introduction to Transistors

  • Transistors are semiconductor devices that amplify or switch electronic signals
  • They are widely used in electronic devices like amplifiers and computers
  • Transistors replaced vacuum tubes due to their smaller size, lower power consumption, and reliability

Types of Transistors

  1. Bipolar Junction Transistor (BJT)
    • NPN (Negative-Positive-Negative)
    • PNP (Positive-Negative-Positive)
  1. Field Effect Transistor (FET)
    • Junction Field Effect Transistor (JFET)
    • Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
  1. Darlington Transistor
  1. Schottky Transistor
  1. Thyristor
  1. Power Transistor
  1. Photo Transistor

Bipolar Junction Transistor Structure

  • BJT consists of three layers of semiconductor material—two layers of one type (either P or N) sandwiched between a layer of the opposite type
  • These layers are called the emitter, base, and collector regions
  • The two types of BJT structures are NPN and PNP, based on the doping of the layers BJT Structure

NPN Transistor

  • NPN transistor has N-type material as the emitter and collector regions, with a P-type material sandwiched in between as the base
  • Electrons are the majority carriers in N-type material and flow from the emitter to the base and then to the collector
  • NPN transistors are commonly used in electronic circuits

PNP Transistor

  • PNP transistor has P-type material as the emitter and collector regions, with an N-type material in between as the base
  • Holes are the majority carriers in P-type material and flow from the emitter to the base and then to the collector
  • PNP transistors are also widely used in electronic circuits

Working Principle of a Transistor

  • Transistors work on the principle of transistor action, due to the flow of majority charge carriers (electrons or holes) across the junctions
  • In an NPN transistor, a small current flowing into the base terminal controls a larger current flowing between the collector and emitter terminals
  • The collector current (IC) is Beta (β) times the base current (IB), where β is the current gain of the transistor Example: If β = 100 and IB = 10μA, then IC = 100 x 10μA = 1mA

Slide 8

  • The transistor can be used as an amplifier or a switch
  • In the amplifier mode, a small input signal controls a larger output signal by amplifying it
  • In the switch mode, the transistor acts as either an open circuit or a closed circuit, depending on the current flow into the base terminal Example: A small input current can control the flow of a larger current for various applications like digital logic gates

Advantages of Bipolar Junction Transistors

  • High current gain (β) allows small base currents to control larger collector currents
  • Fast switching speeds
  • High power gain
  • Low voltage drop between collector and emitter terminals
  • Compact size and low cost

Summary

  • Bipolar junction transistors (BJTs) are important semiconductor devices used in various electronic circuits
  • NPN and PNP are the two types of BJT structures based on the arrangement of the emitter, base, and collector regions
  • BJTs work on the principle of transistor action, where a small input signal controls a larger output signal
  • They can be used as amplifiers or switches, depending on the application and mode of operation

NPN Transistor Characteristics

  • NPN transistors have the following characteristics:
    • The base-emitter junction is forward biased.
    • The base-collector junction is reverse biased.
    • Current flows from the emitter to the base and then to the collector.
    • The emitter current is much larger than the collector current.

PNP Transistor Characteristics

  • PNP transistors have the following characteristics:
    • The base-emitter junction is forward biased.
    • The base-collector junction is reverse biased.
    • Current flows from the base to the emitter and then to the collector.
    • The emitter current is much larger than the collector current.

Transistor Configurations

  • Common emitter configuration:

    • The emitter is common to both input and output.
    • Provides high voltage gain and high current gain.
    • Used in amplifiers.

  • Common base configuration:

    • The base is common to both input and output.
    • Provides high current gain and low voltage gain.
    • Used in current amplification applications.

Transistor Biasing

  • Biasing a transistor refers to applying the appropriate DC voltages to create the desired operating point or Q-point.
  • Two common transistor biasing methods are:
    • Fixed bias: Uses a voltage divider circuit to establish the base voltage.
    • Collector-emitter bias: Uses a resistor in series with the collector and a voltage source to establish the collector current.

Transistor Amplifier Gain

  • The voltage or current gain of an amplifier can be calculated using the following formulas:
    • Voltage gain (Av) = Vout / Vin
    • Current gain (Ai) = Iout / Iin
  • The gain of a transistor amplifier depends on its configuration, biasing, and load resistance.

Transistor as a Switch

  • Transistors can also be used as electronic switches.
  • When used as a switch, the transistor operates in cutoff or saturation regions.
  • In cutoff, the transistor is off and there is no current flow.
  • In saturation, the transistor is on and allows current flow.

Common Transistor Applications

  • Audio amplifiers in sound systems
  • Radio frequency amplifiers
  • Digital logic circuits
  • Power control circuits
  • Oscillators
  • Voltage regulators

Transistor Heat Dissipation

  • Transistors can generate heat during operation.
  • Heat sinks are used to dissipate the heat efficiently.
  • Heat sinks increase the surface area and provide better heat dissipation.
  • Cooling fans may also be used in some cases.

Transistor Reliability

  • Transistors are known for their reliability.
  • However, they can fail due to various factors such as excessive heat, voltage surges, or improper handling.
  • Manufacturers provide specifications and guidelines for operating and handling transistors to ensure their reliability.

Summary

  • NPN and PNP transistors have different characteristics but operate on similar principles.
  • Transistors can be used in various configurations and applications.
  • Biasing, gain calculations, and transistor as a switch were discussed.
  • Heat dissipation and reliability considerations were highlighted as well.

Applications of Transistors

  • Audio amplifiers in sound systems
  • Radio frequency amplifiers
  • Digital logic circuits
  • Power control circuits
  • Oscillators
  • Voltage regulators

Transistor Heat Dissipation

  • Transistors can generate heat during operation.
  • Heat sinks are used to dissipate the heat efficiently.
  • Heat sinks increase the surface area and provide better heat dissipation.
  • Cooling fans may also be used in some cases.

Transistor Reliability

  • Transistors are known for their reliability.
  • However, they can fail due to various factors such as excessive heat, voltage surges, or improper handling.
  • Manufacturers provide specifications and guidelines for operating and handling transistors to ensure their reliability.

Summary

  • NPN and PNP transistors have different characteristics but operate on similar principles.
  • Transistors can be used in various configurations and applications.
  • Biasing, gain calculations, and transistor as a switch were discussed.
  • Heat dissipation and reliability considerations were highlighted as well.

Summary of Key Points

  • Bipolar junction transistors (BJTs) are semiconductor devices used for signal amplification and switching.
  • BJTs can be of NPN or PNP type based on the arrangement of their layers.
  • The NPN and PNP transistors have different polarities of the majority charge carriers.
  • Transistors can be used in various configurations such as common emitter, common base, and common collector.
  • Transistors can operate as amplifiers or switches, depending on their mode of operation.
  • Biasing is essential to establish the operating point of the transistor.
  • Voltage gain and current gain are important parameters for transistor amplifiers.
  • Heat dissipation and reliability considerations are important for the proper functioning of transistors.

Review Questions

  1. What are the two types of bipolar junction transistors?
  1. Explain the working principle of a transistor.
  1. What is biasing in a transistor? Why is it important?
  1. Differentiate between NPN and PNP transistors.
  1. What are the applications of transistors?
  1. How does a transistor operate as an amplifier?
  1. Define voltage gain and current gain in the context of transistor amplifiers.
  1. How is heat dissipation managed in transistors?
  1. What factors can affect the reliability of transistors?
  1. Why are transistors preferred over vacuum tubes in electronic devices?

Example Problems

  1. Calculate the collector current (IC) in an NPN transistor with a base current (IB) of 20μA and a current gain (β) of 100.
  1. A common emitter amplifier has a voltage gain (Av) of 50. If the input signal has an amplitude of 2V, calculate the output voltage.
  1. In a PNP transistor with a collector current (IC) of 1mA and a current gain (β) of 80, calculate the base current (IB).
  1. Design a biasing circuit for an NPN transistor with a collector current (IC) of 2mA and a voltage drop (VBE) of 0.7V.
  1. Explain the working of a transistor as a switch in both the cutoff and saturation regions.

Equations

  1. Current gain of a transistor:
    • NPN transistor: IC = β * IB
    • PNP transistor: IC = -β * IB
  1. Voltage gain of a transistor amplifier:
    • Av = Vout / Vin
  1. Biasing circuit equation:
    • VBB = VBE + IC * RC
  1. Calculation of base current:
    • IB = IC / β

Summary

  • Bipolar junction transistors (BJTs) are important semiconductor devices used in various electronic circuits.
  • NPN and PNP are the two types of BJT structures based on the arrangement of the emitter, base, and collector regions.
  • BJTs work on the principle of transistor action, where a small input signal controls a larger output signal.
  • They can be used as amplifiers or switches, depending on the application and mode of operation.
  • Biasing, gain calculations, and transistor as a switch were discussed.
  • Heat dissipation and reliability considerations were highlighted as well.

Questions for Discussion

  1. What is the significance of transistor action in electronic devices?
  1. Compare and contrast NPN and PNP transistors.
  1. What are the advantages of using transistors in electronic circuits?
  1. Explain the different modes of operation in a transistor.
  1. How does biasing affect the performance of a transistor?
  1. Describe the working of a common emitter amplifier.
  1. How can transistors be used as electronic switches?
  1. Discuss the factors that can affect the reliability of transistors.
  1. Are there any limitations or drawbacks of using transistors?
  1. Can you think of any other applications where transistors are commonly used?