How Do You Calculate Thevenin Equivalent Circuits?

In summary, the Thevenin equivalent with respect to the terminals a,b can be determined by using the equation Rth=Voc/Isc. To find the short circuit current, the given values for the circuit can be used to calculate the current in V2, which is then multiplied by 80 to find Isc. To find the open circuit voltage, the feedback fraction of V2 is substituted in the equation given in the solution. The feedback also affects the output impedance, input impedance, and gain. The voltage across the 80ib dependent current source is used to calculate Vth or Voc, and the 50kohm resistor remains a part of the circuit.
  • #1
orangeincup
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Homework Statement


Determine the Thevenin equivalent with respect to the terminals a,b

Homework Equations


Rth=Voc/Isc

The Attempt at a Solution


I'm doing this practice problem and I calculated the the short circuit current below:

500uA*100ohm/(100ohm+1310ohm)= 35.460 uA

since 80*ib=current in v2, 35.460 A * 80=2836.87 uA

So Isc=2836.87

Now for the open circuit voltage..
80ib* 50kohm= 40.0*10^5 ib
Now to solve for ib is where I'm lost.
In the solution they have 4*10^-5v2=-160ib, but I have no idea where it came from?
 

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  • #2
A fraction of v2 is fed back to influence the input. That fraction is given as 4x10-5v2, so substitute in this your value for v2 (which you have in terms of ib).
 
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  • #3
Thank you. I have a question about calcuating Isc vs. Voc, why isn't Voc simply Isc/50kohms?

I know the circuit is suppose to be "open" but isn't it still the same thing?
 
  • #4
orangeincup said:
Thank you. I have a question about calcuating Isc vs. Voc, why isn't Voc simply Isc/50kohms?
It would be if the Thèvenin resistance were 50kΩ, but it isn't. That's what you are trying to find! The feedback changes the output impedance. And it changes the input impedance, and it changes the gain.
 
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  • #5
So is the Vth or Voc I'm calculating simply the voltage across the 80ib dependent current source?

In the open circuit voltage I thought the 50kohm resistor would still be part of the circuit, and the voltage across that would equal the voltage across a-b?
 
  • #6
orangeincup said:
So is the Vth or Voc I'm calculating simply the voltage across the 80ib dependent current source?
Yes, but the 50k remains in place, it's part of the model. (Without some resistive load, the voltage across a current source would be infinite.)

In the open circuit voltage I thought the 50kohm resistor would still be part of the circuit, and the voltage across that would equal the voltage across a-b?
Yes, and yes.
 
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Related to How Do You Calculate Thevenin Equivalent Circuits?

1. What is Thevenin's theorem and how does it relate to Thevenin equivalent circuits?

Thevenin's theorem states that any linear, two-terminal circuit can be represented by an equivalent circuit consisting of a voltage source in series with a resistor. This equivalent circuit is known as the Thevenin equivalent circuit, and it simplifies the analysis of complex circuits by reducing them to a single voltage source and resistor.

2. How is Thevenin equivalent resistance calculated?

The Thevenin equivalent resistance is calculated by removing all voltage sources and short-circuiting all current sources in the original circuit, and then finding the resistance between the two terminals of interest. This resistance value is then used in the Thevenin equivalent circuit.

3. Can Thevenin's theorem be applied to non-linear circuits?

No, Thevenin's theorem only applies to linear circuits, meaning that the current through any component is directly proportional to the voltage across it. In non-linear circuits, this relationship does not hold and Thevenin's theorem cannot be applied.

4. How does Thevenin's theorem simplify circuit analysis?

Thevenin's theorem simplifies circuit analysis by reducing a complex circuit to a single voltage source and resistor. This allows for easier calculation of circuit parameters such as voltage, current, and power. It also simplifies the process of solving circuit equations and determining the behavior of the circuit.

5. What are the limitations of Thevenin's theorem?

Thevenin's theorem is only applicable to linear circuits and cannot be used for non-linear circuits. Additionally, it only applies to circuits with two terminals and cannot be used for circuits with more than two terminals. Lastly, Thevenin's theorem assumes that the circuit is operating at a steady state, meaning that all transient effects have dissipated.

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