Nodal Analysis- direction of currents flow

In summary, the conversation discusses the current flow through a transistor with a bias circuit and the calculation of VCE. The correct answer for the current through the 3kΩ resistor is H + Ib, not H - Ib. The direction of current flow is downwards due to the polarity of the voltage source. Additionally, the base current in an NPN transistor flows into the base and the direction of current flow is typically top to bottom and left to right when looking at schematics.
  • #1
user5
38
0
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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  • #2
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
-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
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
The base current in NPN transistor always flow into the base. Flow in direction show by the arrow in your diagram.

user5 said:
- 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
sophiecentaur : Thank you very much!

Jony130: Thank you very much!
 

1. What is Nodal Analysis and why is it important in circuit analysis?

Nodal Analysis is a method used to analyze the behavior and characteristics of electrical circuits. It is important because it allows us to determine the voltage and current at certain points in the circuit, which is crucial in understanding and troubleshooting circuit problems.

2. How do you determine the direction of current flow in Nodal Analysis?

The direction of current flow in Nodal Analysis is determined based on the polarity of the voltage sources in the circuit. If the positive terminal of a voltage source is connected to the node, then the current flows into the node. If the negative terminal is connected, then the current flows out of the node.

3. Can the direction of current flow change in Nodal Analysis?

Yes, the direction of current flow can change in Nodal Analysis. This can happen when the voltage sources in the circuit have different polarities or when the circuit has multiple loops. In these cases, the direction of current flow at a particular node will depend on the specific path and polarity of the voltage sources connected to that node.

4. What happens if the direction of current flow is incorrect in Nodal Analysis?

If the direction of current flow is incorrect in Nodal Analysis, it will result in incorrect calculations and analysis of the circuit. This can lead to errors in determining the voltage and current values at different points in the circuit, which can affect the overall understanding of the circuit's behavior and performance.

5. Are there any tools or techniques to help determine the direction of current flow in Nodal Analysis?

Yes, there are several tools and techniques that can help determine the direction of current flow in Nodal Analysis. These include using arrows to indicate the direction of current flow, using Kirchhoff's Current Law to analyze the flow of current at different nodes, and using simulation software to visualize the behavior of the circuit and its currents.

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