Calculating Emitter Current - Voltage-Divider Bias

In summary: The two methods are interchangeable for the purposes of this problem. However, the Thevenins method is for loading effects, while the voltage-divider method is for unloaded effects.
  • #1
?n0t_A_nUmb3R?
12
0
I am aware that there are two methods of working out IE for a Voltage-Divider circuit.

The first is:
https://www.google.co.za/search?q=V...redoc.org%2Fdocs%2Findex-114597.html;2048;730
And then work out VE = VB - VBE

And the second is:
https://www.google.co.za/search?q=v...rcuits.com%2Fvol_3%2Fchpt_4%2F10.html;480;257
(VTH and RTH)

I then tried to calculate a specific problem using both methods. The results were not consistent and varied.
My question is can we use both methods (first and second above) interchangeably?
Are there any differences between the two methods?
 
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  • #2
?n0t_A_nUmb3R? said:
I then tried to calculate a specific problem using both methods. The results were not consistent and varied.
Then please show what you did and which setup you mean. Otherwise it is hard to tell what went wrong.
 
  • #3
There is a difference of 0.31mA. Is this still acceptable?
Can I also use these methods interchangeably? (Voltage Divider Method and VTH/RTH Method)
 

Attachments

  • Question + Voltage Divider Answer (Collector Current).pdf
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  • My Answer (Collector Current).pdf
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  • #4
I don't understand your equation for IE. A difference of 6% is a bit large for rounding errors.
 
  • #5
As I know it to be, it is basically simplifying the circuit using Thevenins Theorem.
Problem is I don't know where to use it.
From the examples I've worked it seems as if I can use this method OR the voltage divider method.
But in some instances, there is a considerate difference in my calculations.
My lecturer is out of office at the moment. So unfortunately I cannot email him.
 

Attachments

  • Thevenins Equivalent.pdf
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  • #6
I do realize that the Thevenins method is for loading effects and the voltage-divider method is for unloaded effects. But I have come across questions where they do not mention whether the circuit is loaded or not.
 
  • #7
You must inevitably arrive at the same answer, unless you make different or wrong approximations along the way, or through oversight or blunder.
 

1. How do I calculate the emitter current in a voltage-divider bias circuit?

To calculate the emitter current, you will need to know the values of the resistors in the voltage divider circuit and the base current. The emitter current is equal to the base current multiplied by the current gain of the transistor. You can also use Ohm's law to calculate the emitter current, which is the voltage across the emitter resistor divided by the resistance of the emitter resistor.

2. What is the purpose of a voltage-divider bias circuit?

The purpose of a voltage-divider bias circuit is to provide a stable and adjustable bias voltage for the base of a transistor. This helps to ensure that the transistor operates within its linear region and allows for a more accurate control of the output current.

3. How does the choice of resistors in a voltage-divider bias circuit affect the emitter current?

The value of the resistors in the voltage-divider circuit will affect the voltage at the base of the transistor, which in turn affects the base current and therefore the emitter current. Higher resistor values will result in a lower base current and emitter current, while lower resistor values will result in a higher emitter current.

4. Can the emitter current be controlled in a voltage-divider bias circuit?

Yes, the emitter current can be controlled by adjusting the values of the resistors in the voltage-divider circuit. By changing the voltage at the base of the transistor, the base current and emitter current can be altered, providing control over the output current.

5. What are the potential drawbacks of using a voltage-divider bias circuit?

One potential drawback of using a voltage-divider bias circuit is that the bias voltage may not be as stable as other biasing methods. This can lead to variations in the emitter current, which can affect the performance of the circuit. Additionally, the choice of resistor values must be carefully selected to ensure that the transistor operates within its linear region.

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