A doubt regarding saturation mode of BJT

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Discussion Overview

The discussion revolves around the saturation mode of bipolar junction transistors (BJTs), specifically addressing the conditions under which both the emitter-base and base-collector junctions are forward biased. Participants explore the implications of voltage levels across these junctions and the resulting current flow.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Devanand T questions why the voltage across the base-collector junction is less than the typical 0.7V required for forward biasing, given that the collector-emitter voltage (Vce) is 0.2V in saturation.
  • Another participant suggests that 0.7V is an approximation and provides a more precise equation for the base-emitter voltage (V_{BE}), indicating that the base-collector junction also follows a similar relationship.
  • Devanand T reiterates the initial question about the base-collector junction voltage and notes that it is not reverse biased, but does not imply significant current flow through the junction.

Areas of Agreement / Disagreement

Participants express differing views on the voltage levels required for forward biasing the base-collector junction, and there is no consensus on the implications of these voltage levels for current flow.

Contextual Notes

The discussion includes assumptions about the behavior of silicon p-n junctions and the application of Kirchhoff's voltage laws, but does not resolve the underlying questions regarding current flow and voltage levels.

dexterdev
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Hi all,
I have a doubt regarding saturation mode of BJT. Saturation occurs when both emitter-base junction and base-collector junction gets forward biased. For forward biasing a silicon p-n junction only 0.7V is needed. So is for emitter-base junction. But not for base-collector junction. Because we know at saturation voltage across collector and emitter Vce is 0.2V. Using Kirchoff's voltage laws the voltage at base-collector p-n junction is less than 0.7V. Why is it lesser?

-Devanand T
 
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0.7V is only an approximation.The more exact equation is:

V_{BE}=V_T\; \ln [\frac {I_c}{I_s}]

The Vbc diode obey the same equation. Actually I am working with transistors of Ic at 50uA range and the measured Vbe is about 0.5V!
 
Last edited:
dexterdev said:
Hi all,
I have a doubt regarding saturation mode of BJT. Saturation occurs when both emitter-base junction and base-collector junction gets forward biased. For forward biasing a silicon p-n junction only 0.7V is needed. So is for emitter-base junction. But not for base-collector junction. Because we know at saturation voltage across collector and emitter Vce is 0.2V. Using Kirchoff's voltage laws the voltage at base-collector p-n junction is less than 0.7V. Why is it lesser?

-Devanand T
No one said there is heavy current flow from B-C. It's just not reverse biased. For an NPN, VBC = +0.5v is definitely a bias in the forward direction. But does not correspond to significant current flow through a silicon diode.
 
thankyou for your replies
 

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