EE 238 Module 4 (1)

EE238 Module 4: Bipolar Junction Transistor (BJT)

Introduction to BJTs

• Definition: BJTs are three-terminal semiconductor devices used for signal amplification.

• Bipolar Term: Refers to the participation of both holes and electrons in the conduction process.

• Invention: Developed on December 23, 1947, by Walter H. Brattain and John Bardeen at Bell Telephone Laboratories.

• Advantages:

  • Smaller and lighter than vacuum tubes.

  • No heater requirements, making them more power-efficient.

  • More robust construction with lower damage susceptibility.

  • Operates at lower voltages and costs due to abundant materials.

Types of BJTs

• Types: NPN and PNP transistors.

• Materials: Can be made from Silicon, Germanium, or Gallium Arsenide (GaAs).

• Structure: Comprised of three layers:

  • Emitter (highly doped)

  • Base (lightly doped)

  • Collector (lightly doped)

Operating Principles

• NPN vs PNP Transistors:

  • NPN: Majority carriers are electrons in n-type material.

  • PNP: Majority carriers are holes in p-type material.

• Biasing: Input voltages can make junctions forward or reverse biased depending on application requirements.

BJT Current Flow

• In NPN, majority electrons flow from emitter to collector; in PNP, majority holes flow from emitter to collector.

• Current Relationships:

  • Emitter current (IE) = Base current (IB) + Collector current (IC)

  • Typical currents: Base current in microamperes, Emitter and Collector currents in milliamperes.

Transistor Configurations

• Common Configurations:

  • Common Base

  • Common Emitter

  • Common Collector

• Each configuration has distinct traits and applications in circuits.

Amplifiers

• Definition: Devices or circuits that increase signal amplitude (voltage, current, or power).

• Gain Formulas:

  • Voltage Gain (Av) = Vout/Vin

  • Current Gain (Ai) = Iout/Iin

  • Power Gain = (Vout * Iout)/(Vin * Iin)

Common Emitter Configuration

• Characteristics:

  • High current and voltage gain.

  • Phase inversion of output signal (180 degrees out of phase with input).

• Electrode Connections:

  • Emitter is common to output and input.

  • Input is between base and emitter, output is between collector and emitter.

Important Parameters

• Alpha (α): Reflects the ratio of collector current to emitter current in different operational conditions.

• Beta (β): Ratio of collector current to base current; indicates the DC current gain of the transistor.

Biasing Techniques

• Fixed Bias: Simple biasing method with minimal temperature stability.

• Voltage Divider Bias: More stable to temperature variations and minimizes distortion, using two resistors to set base voltage.

Transistor as a Switch

• Switching Conditions:

  • Cutoff Region: Transistor is "off"; negligible current flows.

  • Saturation Region: Transistor is "on"; maximum current flows.

  • Real vs Ideal Conditions: Realistically, transistors can experience leakage currents even when "off".

Testing and Specifications

• Testing: Using tools like ohmmeters and curve tracers to evaluate transistor characteristics.

• Specifications Includes:

  • Max collector current (ICmax), voltages, and thermal parameters.

  • Current gain specifications (hFE).

Summary

• BJTs serve as fundamental components in electronics, utilized for amplification and switching within circuits. Proper biasing, configuration selection, and understanding current flow in transistors are crucial for their intended applications.