Common-Base Transistor Configuration

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Questions and Answers

What is the main characteristic of the active region in a common-base configuration?

  • Both the base-emitter junction and collector-base junction are forward-biased.
  • The base-emitter junction is reverse-biased.
  • The base-emitter junction is forward-biased while the collector-base junction is reverse-biased. (correct)
  • The collector-base junction is forward-biased.

Which current is defined as the collector-to-base current with the emitter leg open?

  • Emitter current ($I_E$)
  • Collector current ($I_C$)
  • Reverse saturation current ($I_{CBO}$) (correct)
  • Input current ($I_E$)

What happens to the collector current ($I_C$) when the current $I_E$ is equal to zero?

  • $I_C$ becomes infinitely large.
  • $I_C$ equals the reverse saturation current ($I_{CO}$). (correct)
  • $I_C$ equals the sum of the emitter current and collector current.
  • $I_C$ equals the input current ($I_E$).

What does the symbol with an arrow represent in the context of transistor operation?

<p>Direction of emitter current. (A)</p> Signup and view all the answers

In which region is the current $I_{CO}$ negligible due to improved construction techniques in modern transistors?

<p>Active region. (D)</p> Signup and view all the answers

Why does the cutoff region appear horizontal in the $I_C$ versus $V_{CB}$ graph?

<p>The collector current ($I_C$) is effectively zero. (A)</p> Signup and view all the answers

What is represented on the output characteristic curve for a common-base amplifier?

<p>The relationship between collector current and output voltage. (A)</p> Signup and view all the answers

Which two sets of characteristics are necessary to fully describe a three-terminal device like a common-base amplifier?

<p>Input and output characteristics. (D)</p> Signup and view all the answers

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Study Notes

Common-Base Configuration

  • The common-base configuration in transistors has the base terminal connected to both the input and output.
  • Current flow in the text is referred to as conventional (hole) flow, where arrows indicate the direction of current flow.
  • Emitter current (IEI_EIE​) is defined by the arrow in the graphic symbol and represents the direction of conventional current through the device.
  • The relationship between currents is: IE=IC+IBI_E = I_C + I_BIE​=IC​+IB​.
  • Biasing voltage sources are oriented in a specific direction for each branch.

Common-Base Transistor Characteristics

  • Input Characteristics: This set relates input current (IEI_EIE​) to input voltage (VBEV_{BE}VBE​) for various levels of output voltage (VCBV_{CB}VCB​).
    • The relationship between input current (IEI_EIE​) and input voltage (VBEV_{BE}VBE​) is depicted in the input characteristics curve.
    • The curve changes based on different output voltage (VCBV_{CB}VCB​) levels.
  • Output Characteristics: This set relates output current (ICI_CIC​) to output voltage (VCBV_{CB}VCB​) for various levels of input current (IEI_EIE​).
    • The output characteristics curve shows the relationship between output current (ICI_CIC​) and output voltage (VCBV_{CB}VCB​) for various input current (IEI_EIE​) levels.
    • The curve has three distinct regions: active, cutoff, and saturation.

Transistor Regions of Operation

  • Active Region: The base-emitter junction is forward-biased, while the collector-base junction is reverse-biased. This region is characterized by a linear relationship between input and output current.
    • When IEI_EIE​ is zero, ICI_CIC​ is solely due to reverse saturation current (ICOI_{CO}ICO​).
  • Cutoff Region: In this region, both junctions are reverse-biased, leading to minimal current flow, essentially zero collector current (ICI_CIC​).
  • Saturation Region: The collector-base junction is forward-biased leading to a higher current flow compared to the active region and very low collector-emitter voltage.

Reverse Saturation Current (ICBOI_{CBO}ICBO​)

  • ICBOI_{CBO}ICBO​ represents the current flow from the collector to the base while the emitter is open circuited.
  • ICBOI_{CBO}ICBO​ is typically very small compared to ICI_CIC​ (microamperes vs. milliamperes) and can be neglected for most low- and mid-power applications.
  • ICBOI_{CBO}ICBO​ is a significant factor to consider in high-power transistors.
  • When IE=0I_E=0IE​=0, the collector current is equal to ICOI_{CO}ICO​.

In Summary

  • The common-base configuration is one way to utilize a transistor.
  • Common-base amplifier characteristics are complex and depend on operation region.
  • Understanding the characteristics is crucial to choosing the right transistor and configuration for a specific application.

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