Bipolar Junction Transistor Basics

• Introduction to the transistor
• Structure of a bipolar junction transistor (BJT)
• Types of bipolar junction transistors (NPN and PNP)
• Symbol and terminals of a transistor
• Basic working principle of a transistor

Structure of a Bipolar Junction Transistor

• Three layers: emitter, base, and collector
• Doped with specific impurities (N-type or P-type)
• Sandwiched together to form the transistor
• Emitter region is heavily doped
• Base region is lightly doped
• Collector region is moderately doped

Types of Bipolar Junction Transistors

• NPN Transistor:

  • N-doped emitter and collector regions
  • P-doped base region

• PNP Transistor:

  • P-doped emitter and collector regions
  • N-doped base region

Symbol and Terminals of a Bipolar Junction Transistor

• Symbol:
  • Arrow points in the direction of conventional current flow from emitter to base
• Terminals:
  • Emitter (E)
  • Base (B)
  • Collector (C)
• Transistor packages may have different pin configurations, but the terminal names remain the same.

Basic Working Principle of a Transistor

• Bipolar junction transistors are current-controlled devices.
• The base-emitter junction is forward biased.
• Electrons from the emitter region diffuse into the base region.
• A small base current controls a much larger collector current.
• The collector-emitter junction is reverse biased.
• Transistor amplifies current or acts as a switch.

Transistor in Active Region

• Base-emitter junction forward bias voltage (Vbe) is applied.
• Emitter current (Ie) flows into the base region, controlled by base current (Ib).
• Majority charge carriers (electrons or holes) cross the base region.
• Part of the majority carriers recombine with minority carriers.
• Majority carriers emerge in the collector region and form the collector current (Ic).

Transistor Current Amplification

• Transistor can amplify the base current signal to a much larger collector current.
• Current Gain (β):
  • Ratio of collector current (Ic) to base current (Ib)
  • Typically ranges from 20 to 1000 for common BJTs
• Formulas:
  • Ic = β × Ib
  • Ie = Ic + Ib

Transistor as an Amplifier

• Small input signal applied to the base, resulting in a small change in Ib.
• Large output signal obtained at the collector, resulting in a large change in Ic.
• Amplification is achieved due to the current gain (β) of the transistor.

Transistor as a Switch

• Transistor can be used as an electronic switch.
• In the cutoff region, both junctions are reverse biased, and no current flows.
• In the saturation region, both junctions are forward biased, allowing maximum current flow.
• Switching behavior depends on the base-emitter voltage (Vbe).

Example of Transistor as a Switch

• Light switch:
  • When the switch is closed, the base-emitter junction is forward biased.
  • Current flows through the collector-emitter path and the light bulb turns on.
  • When the switch is open, the base-emitter junction is reverse biased.
  • No current flows, and the light bulb turns off.

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Common Emitter Amplifier Circuit

• The common emitter amplifier is the most commonly used configuration of a transistor amplifier.
• It provides high voltage gain and current gain.
• The input is applied to the base-emitter junction, and the output is taken from the collector-emitter junction.
• Characteristics of a common emitter amplifier:
  • Inverting amplifier
  • High voltage gain
  • Medium input impedance
  • Medium output impedance

Common Collector Amplifier Circuit

• The common collector amplifier, also known as an emitter follower, is another configuration of a transistor amplifier.
• It provides high current gain but no voltage gain.
• The input is applied to the base-emitter junction, and the output is taken from the emitter to ground.
• Characteristics of a common collector amplifier:
  • Non-inverting amplifier
  • Unity voltage gain
  • High input impedance
  • Low output impedance

Common Base Amplifier Circuit

• The common base amplifier is the least commonly used configuration of a transistor amplifier.
• It provides high current gain and low voltage gain.
• The input is applied to the emitter-base junction, and the output is taken from the collector to ground.
• Characteristics of a common base amplifier:
  • Non-inverting amplifier
  • Unity current gain
  • Low input impedance
  • High output impedance

Applications of Bipolar Junction Transistors

• Amplification: Transistors are extensively used in audio and radio frequency amplifiers.
• Switching: They are used in digital logic gates and as switching devices in various electronic circuits.
• Oscillation: Transistors can be used to generate continuous waveforms, such as in radio frequency oscillators.
• Voltage regulation: They can be used in voltage regulator circuits to maintain a constant output voltage.

Transistor Biasing

• Biasing is the process of establishing a DC operating point for the transistor.
• It ensures that the transistor operates in the active region for amplification and avoids cutoff or saturation.
• Different biasing methods can be used: fixed bias, emitter bias, and collector feedback bias.

Fixed Bias Configuration

• The fixed bias configuration uses a voltage divider network of resistors to set the base-emitter voltage.
• This establishes the DC operating point (Q point) of the transistor.
• The voltage divider provides negative feedback and stabilizes the DC operating conditions.

Emitter Bias Configuration

• The emitter bias configuration includes a resistor connected in series with the emitter.
• This resistor provides negative feedback and helps stabilize the operating conditions.
• The base-emitter junction is forward biased by the voltage drop across the emitter resistor.
• Emitter bias configuration provides improved stability compared to the fixed bias configuration.

Collector Feedback Bias Configuration

• The collector feedback bias configuration utilizes a combination of fixed bias and emitter bias techniques.
• This configuration provides better stability and avoids variations due to changes in β (current gain).
• A resistive network connects the collector to the base, providing feedback and stabilization.

Transistor Input and Output Characteristics

• Input Characteristics: Shows the relationship between base-emitter voltage (Vbe) and base current (Ib) for a given collector-emitter voltage (Vce).
• Output Characteristics: Shows the relationship between collector-emitter voltage (Vce) and collector current (Ic) for different base currents (Ib).
• These characteristics help in analyzing the transistor’s behavior and selecting suitable operating conditions.

Summary

• Bipolar junction transistors are three-layered semiconductor devices used for amplification, switching, and other applications.
• The main types of BJTs are NPN and PNP transistors.
• Transistors work on the principle of minority carrier injection and transistor action.
• They can be used as amplifiers or switches depending on the circuit configuration.
• Transistors require proper biasing to ensure stable operation in the active region.
• Different biasing techniques include fixed bias, emitter bias, and collector feedback bias.
• Transistor input and output characteristics help analyze their behavior.
• BJTs have a wide range of applications in various electronic devices and circuits.