Transistor analysis: find currents and voltages

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Discussion Overview

The discussion revolves around analyzing a transistor circuit to determine various voltages and currents, specifically VC, VB, VE, IC, IB, and IE. Participants are attempting to apply equations from a previous example to this problem, which involves understanding the relationships between these parameters in the context of transistor operation.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about calculating VE, noting that there is no resistor on the emitter side to determine IE.
  • Another participant suggests that VE should be zero since it is connected directly to ground, and encourages calculating the base current instead.
  • Some participants clarify the equations being used, questioning the appropriateness of symbols and the presence of an emitter resistor (RE).
  • A later reply emphasizes the importance of understanding the equations rather than relying solely on a previous example, which may be misleading.
  • One participant calculates IB, IE, IC, VE, VC, and VB, but another questions the interpretation of VB and its relationship to VBE.
  • There is a clarification that VB should be expressed as the sum of VBE and VE.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct approach to the problem, with multiple viewpoints on how to interpret the circuit and the equations involved. Some participants challenge each other's calculations and assumptions, indicating ongoing debate.

Contextual Notes

There are unresolved issues regarding the definitions of certain parameters, particularly the role of the emitter resistor (RE) and the interpretation of voltages like VB. Participants express uncertainty about the implications of these definitions on their calculations.

Who May Find This Useful

This discussion may be useful for students learning about transistor circuits, particularly those struggling with circuit analysis and the application of relevant equations in practical scenarios.

Steve Collins
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Homework Statement



I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

Homework Equations



These equations are taken from a previous example

VE = VBB – VBE
IE = (VBB – VBE)/RE
IB = IE/β+1
IC = β/(β+1) x IE
VC = VCC – ICRC

β=100
VBE ≈ 0.7V


The Attempt at a Solution



VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out IE. I'm sure that I am missing something obvious...
 

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Steve Collins said:
VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out IE. I'm sure that I am missing something obvious...

VE will be zero since it is connected directly to ground :)

As you've mentioned, VBE ≈ 0.7 which means you should be able to calculate the base current.
 
Steve Collins said:

Homework Statement



I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.

Homework Equations



These equations are taken from a previous example

VE = VBB – VBE
this is the voltage across the 100K, so IB = VE/RB. "VE" is not a good symbol for this voltage. [/color]
IE = (VBB – VBE)/RE
what's RE?
IB = IE/(β+1)[/color]
right ...
IC = β/(β+1) x IE
right ...

VC = VCC – ICRC
right
β=100
VBE ≈ 0.7V right

The Attempt at a Solution



VE = 5 - 0.7 = 4.3V

My attempt falls down at this stage as there is no resistor on the emmiter side to work out IE. I'm sure that I am missing something obvious...

See above comments
 
I took the relevant equations from a previous example where there was a resistor on the emitter side, RE.

The previous example was from my first and only lecture on this, so far!, and I am trying to use this previous example to work through the above problem. I have attached the previous example in this thread so that you guys can see the extent of my knowledge.

The help is much appreciated as I would like to get my head around this before the subject becomes confusing.
 

Attachments

Last edited:
Steve Collins said:
I took the relevant equations from a previous example where there was a resistor on the emitter side, RE.

The previous example was from my first and only lecture on this, so far!, and I am trying to use this previous example to work through the above problem. I have attached the previous example in this thread so that you guys can see the extent of my knowledge.

The help is much appreciated as I would like to get my head around this before the subject becomes confusing.

OK, but here RE = 0 so you don't want to divide by zero ..

You need to understand the equations rather than just go by a previous example. The equations I gave you, plus the ones you got right, should be fully understood, and they are all you need. Then you can tackle more complex circuits too.
 
Steve Collins said:

Homework Statement



I’m having trouble with analysing the transistor circuit attached to find VC, VB, VE, IC, IB and IE.
Don't concern yourself with finding any of these, first off, except for IB.

Set aside your previous example, it is misleading you. The first thing you need to find here is IB. You don't need to know any other parameters before determining IB. (Though you can assume you are dealing with a typical Si junction at B-E.)
 
Is this correct?

IB = (VBB - VBE)/RB = (5 - 0.7)/100000 = 43μA

IE = IB(β+ 1) = 43x10-6 x 101 = 4.343mA

IC = β/(β + 1) x IE = 100/101 x 4.343x10-3 = 4.3mA

VE = 0V

VC = VCC - ICRC = 10 - 4.3x10-3 x 2000 = 1.4V

VB = VBB - VBE = 5 - 0.7 = 4.3V
 
Steve Collins said:
Is this correct?

IB = (VBB - VBE)/RB = (5 - 0.7)/100000 = 43μA

IE = IB(β+ 1) = 43x10-6 x 101 = 4.343mA

IC = β/(β + 1) x IE = 100/101 x 4.343x10-3 = 4.3mA

VE = 0V

VC = VCC - ICRC = 10 - 4.3x10-3 x 2000 = 1.4V

VB = VBB - VBE = 5 - 0.7 = 4.3V
It's looking better, :smile: though I won't check that your calculator is correctly performing its arithmetic.

But I don't follow this line: VB = VBB - VBE
What is VB if it isn't VBE? VB is usually the voltage difference between base and earth, and here it's 0.7V.
 
So VB = VBE + VE

Makes sense, cheers.
 

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