Kirchoff voltage law to transistor circuits?

In summary, the conversation discusses how to arrive at the equation for the base emitter side and collector emitter side of a transistor using Kirchhoff voltage law. The speaker mentions their attempt at a solution and suggests looking for a simple equivalent circuit model to replace the transistor. They also mention the importance of keeping track of the direction and sign of the current.
  • #1
logearav
338
0

Homework Statement


I want to know how to arrive at the equation for the base emitter side and collector emitter side of a transistor using kirchhoff voltage law.


Homework Equations


Kirchoff voltage law is [itex]\sum[/itex]V = [itex]\sum[/itex] IR.
Please refer my attachment in which i have to arrive an equation for base emitter side. So starting from the point O i proceed in the direction given in red arrows which gives
IERE+I1R2 = -VBE
Am i correct? Similarly help is needed to arrive the same to the collector emitter side.


The Attempt at a Solution




 

Attachments

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  • #2
Since the base is drawing some (small) current, the voltage at the base will not be precisely the value given by a simple voltage divider action of the the base bias resistors R1 and R2. If R1 and R2 happen to be relatively small in value and they carry a current between Vcc and ground that is much greater than the base current, then the approximation may be adequate. Usually this is not the case (in a practical circuit it would be a wast of power).

You might want to look for a simple equivalent circuit model with which to replace the transistor in the circuit so that you can write the loop equations. Maybe something like this:

attachment.php?attachmentid=37993&stc=1&d=1313333193.gif


[Image source: http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_models/bjt_models.html ]
 

Attachments

  • Fig1.gif
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  • #3
From the diagram it appears I1 is defined as flowing downward, yet your (red arrows) loop defines it flowing upward. So you need to change its sign.

Secondly, you did not make clear what values are given and what are unknown. From your text it is clear that Vbe is unknown, but what about the other symbols on the diagram? Are we to assume all other symbols on the diagram are given?
 
  • #4
Mr. fleem, the arrow mark in the red is the direction i proceed in a closed loop for arriving the kirchhoff voltage law.
This is a voltage divider bias used in npn transistor.
 
  • #5
logearav said:
Mr. fleem, the arrow mark in the red is the direction i proceed in a closed loop for arriving the kirchhoff voltage law.
This is a voltage divider bias used in npn transistor.

Yes, but I'm just saying be careful to keep track of the sign of the current. Since the red arrows point opposite the direction of the current defined in the diagram, so (assuming) you've been given the value for that current in the diagram, you must change its sign to match the definition used by the red arrows.
 

1. What is Kirchoff's Voltage Law (KVL)?

Kirchoff's Voltage Law is a fundamental principle in circuit analysis that states that the algebraic sum of voltages around a closed loop in a circuit must equal zero. This law is based on the principle of conservation of energy, and can be applied to any type of circuit, including transistor circuits.

2. How is KVL applied to transistor circuits?

In transistor circuits, KVL can be used to analyze the voltage drops across the individual components in the circuit. This can help determine the overall behavior of the circuit and ensure that all the voltages are balanced and consistent with the law.

3. What is the significance of KVL in transistor circuit analysis?

KVL is an essential tool in the analysis of transistor circuits, as it allows for the determination of unknown voltages and the prediction of circuit behavior. It also helps in troubleshooting and identifying any potential issues in the circuit.

4. Can KVL be applied to non-linear transistor circuits?

Yes, KVL can be applied to both linear and non-linear transistor circuits. However, in non-linear circuits, the voltage drops may not be constant and may vary based on the operating conditions of the circuit.

5. Are there any limitations to KVL in transistor circuit analysis?

While KVL is a powerful tool in analyzing transistor circuits, it does have some limitations. It assumes ideal conditions and does not take into account factors such as parasitic capacitance and inductance, which may affect the accuracy of the analysis.

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