Does Collector Current Depend on Base-Collector Voltage in Common Base BJTs?

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Discussion Overview

The discussion centers on whether the collector current in a common base bipolar junction transistor (BJT) depends on the base-collector voltage. Participants explore the mathematical relationships and physical principles governing this behavior, examining both theoretical and practical implications.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that the collector current is primarily determined by the base-emitter voltage, referencing the Shockley equation, Ic=Is[exp(Vbe/VT) -1], and suggesting that this relationship holds regardless of the transistor configuration.
  • Others acknowledge that while the collector current is mainly influenced by the base-emitter voltage, there is a small dependency on the collector-base voltage, particularly when it is backward biased, which is illustrated by the concept of Early voltage.
  • One participant expresses concern that their views may lead to disagreements, indicating that interpretations of the relationship between collector current and base-emitter voltage can vary among sources.
  • A request for a physical explanation of why the collector current does not depend on the voltage of the other junction is made, highlighting the complexity of the topic.

Areas of Agreement / Disagreement

Participants generally agree that the collector current is influenced by the base-emitter voltage, but there is disagreement regarding the extent of the influence of the base-collector voltage, with some asserting it is negligible while others argue for its significance.

Contextual Notes

The discussion reflects varying interpretations of the relationships governing BJT operation, including the implications of the Shockley equation and the Early effect, without resolving the nuances of these concepts.

Aditya Mysore
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Does the collector current in a transistor depend on the base-collector voltage in a common base configuration? If so,how does it depend mathematically?
 
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The principle behaviour of a BJT does not depend on the specific configuration (common emitter, base or collector).
In any case, the famous Shockley equation applies: Ic=Is[exp(Vbe/VT) -1].
Hence, the collector current always is determined and controlled by the base-emitter voltage only.
I am aware that in some textbooks and other publications (in particular: Internet) it is stated that Ic would be controlled by Ib because of the relation Ic=B*Ib.
This relation holds, of course, and it is often used for designing simple transistor stages - however, it does not tell anything about cause and effect.
There are many properties and effects to be observed in BJT applications that can be explained ONLY with the above mentioned equation Ic=f(Vbe).
(I would not be surprised if this reply would cause some disagreements from other forum members; I have corresponding experiences.).
 
LvW said:
The principle behaviour of a BJT does not depend on the specific configuration (common emitter, base or collector).
In any case, the famous Shockley equation applies: Ic=Is[exp(Vbe/VT) -1].
Hence, the collector current always is determined and controlled by the base-emitter voltage only.
I am aware that in some textbooks and other publications (in particular: Internet) it is stated that Ic would be controlled by Ib because of the relation Ic=B*Ib.
This relation holds, of course, and it is often used for designing simple transistor stages - however, it does not tell anything about cause and effect.
There are many properties and effects to be observed in BJT applications that can be explained ONLY with the above mentioned equation Ic=f(Vbe).
(I would not be surprised if this reply would cause some disagreements from other forum members; I have corresponding experiences.).
Thanks. Can you also explain physically why it does not depend on the voltage of the other junction?
 
Aditya Mysore said:
Thanks. Can you also explain physically why it does not depend on the voltage of the other junction?

It depends! (I assume the question is if Ic depends, correct?).
The collector current is mainly determined by the base-emitter voltage - however, there is also a small dependency on the collector-base voltage (backward biased).
This dependency is visualized by the "Early voltage" which is a measure of the BJT`s output conductance (finite slope of the Ic=f(Vce) characteristics).
 

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