Are These Two BJT Circuits Equivalent?

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Discussion Overview

The discussion revolves around the equivalence of two BJT (Bipolar Junction Transistor) circuits. Participants are examining whether the circuits yield the same results and if modifications to the resistor configuration affect the outcomes. The scope includes technical explanations and conceptual clarifications regarding circuit behavior.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions if the two BJT circuits are equivalent and whether grounding a specific resistor would yield the same results as the second circuit.
  • Another participant asserts that both circuits can be drawn with the resistor going to ground, suggesting equivalence.
  • A later reply discusses the implications of removing the bottom left resistor, proposing that the emitter current would equal the sum of the base and collector currents, but this is contested.
  • Further clarification indicates that the emitter current is always the sum of the base and collector currents, and discusses how the presence or absence of the lower left resistor affects biasing and temperature sensitivity.

Areas of Agreement / Disagreement

Participants express differing views on the equivalence of the circuits and the impact of the resistor configuration. The discussion remains unresolved regarding the exact implications of these configurations on circuit behavior.

Contextual Notes

There are assumptions regarding the behavior of the circuits that are not fully explored, particularly concerning the effects of temperature on biasing and the specific roles of the resistors in the configurations.

Air
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I've got these two BJT circuits. I want to know if the result are the same. Are both circuits below same? Can the bottom left resistor be grounded and be same as the second circuit.

http://img21.imageshack.us/img21/2886/bjtt.jpg
 
Last edited by a moderator:
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Air said:
I've got these two BJT circuits. I want to know if the result are the same. Are both circuits below same? Can the bottom left resistor be grounded and be same as the second circuit.

http://img21.imageshack.us/img21/2886/bjtt.jpg
[/URL]

Yes, they both go to ground, so the circuit can be drawn either way.
 
Last edited by a moderator:
Thank you.

Also, if the bottom left resistor branch didn't exist, then the current in the emitter would just be the current in the collector plus the current in the emitter?
 
Air said:
Thank you.

Also, if the bottom left resistor branch didn't exist, then the current in the emitter would just be the current in the collector plus the current in the emitter?

Not exactly, and I think you made s small typo anyway. The emitter current in this configuration is always the base+collector currents.

If the lower left resistor is not there, then the base bias current is set by the upper left resistor and the supply voltage (Vs-Vb/R = Ibias). With the lower left resistor there, the bias is set more by a resistor divider effect, and will be a bit less temperature sensitive in terms of the base bias current versus temperature (Quiz Question -- whi?).
 

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