Is the Claim that Ic = Ie and Veb = 0.7 Volts Accurate for this Circuit?

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Discussion Overview

The discussion revolves around the accuracy of the claims that the collector current (Ic) equals the emitter current (Ie) and that the base-emitter voltage (Veb) is 0.7 volts in a specific transistor circuit. Participants explore the implications of these assumptions in the context of circuit analysis and design.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • Some participants note that while Ic is assumed to equal Ie, they also emphasize that these currents are not exactly the same due to the presence of base current (Ib), which is often neglected.
  • One participant mentions that the base current is typically small enough to be ignored, suggesting that the base bias circuit is designed to ensure that the base current is significantly less than the currents through R1 and R2.
  • Another participant refers to the amplification factor (β) of a BJT transistor, stating that it is usually around 100, which implies that the base current is about 100 times smaller than the emitter current.
  • There is a suggestion that the assumption of neglecting the base current can simplify calculations, but it is also noted that this assumption should be verified against the results of the analysis.
  • One participant provides specific calculated values for the currents and voltages in the circuit, indicating that their results align with the assumptions made earlier in the discussion.
  • Another participant suggests that while the discharge time through a resistor is negligible in this example, it may not always be the case in other scenarios.

Areas of Agreement / Disagreement

Participants generally agree on the assumption that Ic = Ie for simplification purposes, but there is acknowledgment of the nuances involved, particularly regarding the base current. The discussion reflects multiple viewpoints on the relevance of the base current and its impact on the overall analysis.

Contextual Notes

Participants express uncertainty about the exact values of the base current and the implications of neglecting it. The discussion also highlights the dependence on specific circuit conditions and assumptions made during analysis.

Femme_physics
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The current through the base of a transistor is always pretty small.

Often it is neglected to make the calculations easier.

I see you also have Ic=Ie.
These 2 currents are also not exactly the same.
It should be Ie=Ic+Ib.
So here the base current is also neglected.

How big do you think Ib is (approximately)?
 
I see you also have Ic=Ie.
These 2 currents are also not exactly the same.
It should be Ie=Ic+Ib.
So here the base current is also neglected.

The exercise tells me to assume Ic = Ie , that's why I wrote it :)

How big do you think Ib is (approximately)?

I'm told that the base current is negligible
 
Femme_physics said:
The exercise tells me to assume Ic = Ie , that's why I wrote it :)

I'm told that the base current is negligible

So if we treat Ib as being zero, then I1 is the same as I2...Btw, I believe you have a formula ##I_e = β I_b##, with β the amplification factor.
For a BJT transistor, β usually has a value of about 100.

In other words the base current is about 100 times as small as the emitter current.
 
What he said...

It's common for a circuit designer to choose the current flowing in the base bias circuit R1 and R2 to be at least 10 times the expected base current so that the base current can be ignored (engineers 1/10th rule). Sometimes worth assuming that they have done this, solving the rest of the problem to work out the collector current and then go back and check your assumption was correct.

In this case if we assume the base current can be ignored then IR1 is about 0.1mA

So the base is at about 8V and the emitter 8.7V

So Ie is about 12-8.7/R3 = 1.1mA

If the transistor is half decent it will have a gain of 100 so the base current about 1.1mA/100 = 0.011mA which is indeed ten times less than 0.1mA.
 
Ahh...yes... since IB is irrelevant the same current flows... true...

Thanks :) I'll solve it tomorrow and see if I get the same results.
 
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Looks ok. For completeness you should probably comment on the discharge time through R4. Negligible in this example but might not allways be.
 
Hey!
There is a little squirrel in there!
That looks alright. :)
 
  • #10
Thanks you two :)

And yea, everybody loves the Ice Age squirrel!^^
 

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