DC equivalent circuit of CB transistor?

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SUMMARY

The discussion centers on the DC equivalent circuit of a common base (CB) transistor, specifically addressing why the collector diode is modeled as a current source. It is established that the collector is treated as a current source due to the presence of an emitter resistor, which provides negative feedback. This feedback mechanism stabilizes the transistor's operation by reducing the base-emitter voltage when excessive emitter current flows. The output signal at the collector node is represented as the collector current (Ic) in the small-signal equivalent model.

PREREQUISITES
  • Understanding of common base (CB) transistor configuration
  • Knowledge of transistor operation and characteristics
  • Familiarity with small-signal equivalent models
  • Concept of negative feedback in electronic circuits
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  • Study the role of emitter resistors in transistor circuits
  • Learn about small-signal analysis in transistor amplifiers
  • Explore the impact of negative feedback on circuit stability
  • Investigate different transistor configurations and their equivalent circuits
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Electrical engineers, electronics students, and anyone involved in designing or analyzing transistor circuits will benefit from this discussion.

abhijath
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figure 1 shows dc model of CB transistor, why is it that collector diode is taken as current source in equivalent circuit, why not emitter is also taken as current source ?
 

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Generally the collector in any configuration is considered a current source provided there is an emitter resistor and a stable voltage on the base. The emitter resistor provides negative feedback. If too much emitter current is flowing the voltage across the emitter resistor rises which in turn reduces the base-emitter voltage and of course this moves the transistor slightly more towards off.
 
I rather think that the answer has nothing to do with an emitter resistor, which might be there or not.
In CB configuration the OUTPUT is at the collector node - and the small-signal equivalent model shows the source that delivers an output "signal" at this node.
And the "signal" is identical to a current (Ic).
 

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