Lesson 5

• Bipolar Junction Transistor (BJT)


  A three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device.
• Bipolar Junction Transistor (BJT)


  • The three terminals are the base, the collector, and the emitter
  • A signal of a small amplitude applied to the base is available in the amplified form at the collector
  • It requires an external source of DC power supply to carry out the amplification process
• Bipolar Junction Transistor (BJT) is a current controlled device
• BJT Basic Operation


  1. Emitter-base junction is forward-biased

  2. Collector-base junction is reverse-biased
• Active Region


  • Emitter-base junction is forward-biased and collector-base junction is reverse-biased
  • Transistor operates as an amplifier and Ic = β*Ib
• Saturation Region


  • Emitter-base junction is forward-biased and collector-base junction is also forward-biased
  • Transistor acts like a closed switch and Ic = I(saturation)
• Transistor


  A device with three terminals that can be used to amplify or switch electronic signals and electrical power
• Cutoff Region


  • Emitter-base junction is reverse-biased and collector-base junction is reverse-biased
  • Transistor acts like an open switch and Ic = 0
• Types of transistor configurations


  • Common-Base Configuration (CB Configuration)
  • Common-Emitter Configuration (CE Configuration)
  • Common-Collector Configuration (CC Configuration)
• Common-Base Configuration (CB Configuration)

  1. Input voltage applied between base and emitter terminal

  2. Output taken between collector and base terminal

  3. Has low input resistance but high output resistance

  4. Current gain denoted by α (alpha), defined as ratio of collector current to emitter current
• Common-Emitter Configuration (CE Configuration)


  1. Input voltage applied between base and emitter terminal

  2. Output obtained between emitter and collector terminal

  3. Has low input resistance but high output resistance

  4. Current gain denoted by ß (beta), defined as ratio of collector current to base current
• Transistor Biasing Techniques


  • Fixed Bias
  • Voltage Divider Bias
  • Emitter Bias
• Relationship between α and ß


  α = ß / (1+ß)

  ß = α / (1-α)
• Fixed Bias Circuit


  Provides a relatively straightforward and simple introduction to transistor dc bias analysis
• Common-Collector Configuration (CC Configuration)


  1. Input voltage applied between base and collector terminal

  2. Output obtained between emitter and collector terminal

  3. Has very high input resistance but low output resistance

  4. Current gain denoted by γ (gamma), defined as ratio of emitter current to base current
• Voltage Divider Bias


  Seeks to establish a stable Q point by placing a fixed voltage across an emitter resistor. This will result in a stable emitter current, and by extension, stable collector current and collector-emitter voltage.
• Determine for the voltage-divider configuration


  1. DC bias voltage VCE

  2. Current IC