Nodal Analysis- direction of currents flow

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  • #1
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The figure shows a transistor with a bias circuit. If Ic=50IB and if VBE=0.7V, find VCE.
b4sra8.jpg

My question is:
The current through 700Ω resistor is H=[0.7+(51IB(250))]/700,
then when KCL is applied at the left hand node, the current flowing through the 3kΩ resistor suppose to be (according to hint),H+ IB. However isn't it suppose to be minus(H+IB)? (since the currents drop through resistors equals minus IB, it flows out of the node to the source)?

Thank you for any help you can provide.
 
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Answers and Replies

  • #2
625
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Well the correct answer is that the current flowing through the 3kΩ resistor is equal to H +Ib.
Why you think that H - Ib is correct answer ?
 
  • #3
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-For the nodal analysis (at the left-hand node), does the part towards the current IB flows regarded as resistor? (By that I mean shouldn't the transistor part be regarded as a current source?)

- Does the direction of flow through 3kΩ and 700Ω resistors is downwards due to the polarity of the voltage source 9V?

Thanks in advance.
 
  • #4
sophiecentaur
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With the battery 'that way up', current will be flowing from top (+) to bottom (-) of the circuit. Thus, the current through the 3k will split through the lower resistor and the base.
The base of a transistor does not appear as a constant current - just a high resistance, when an emitter (voltage feedback) resistor is in place; it is the collector which approximates to a constant current source.
I think I would re-write your equation
H=[0.7+(51IB(250))]/700
as H=[0.7+(IE(250))]/700, initially, and develop it from there (if you haven't already done so).
 
  • #5
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The base current in NPN transistor always flow into the base. Flow in direction show by the arrow in your diagram.

- Does the direction of flow through 3kΩ and 700Ω resistors is downwards due to the polarity of the voltage source 9V?
Yes, simply current flow from "+" to "-". And we have "+" at top and "-" at the bottom.
So when looking at schematics, most people get comfortable with the general notion that things flow top to bottom and left to right.
 
  • #6
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sophiecentaur : Thank you very much!

Jony130: Thank you very much!
 

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