Calculating R2 for 5W Output in Op-Amp Circuit - Homework Help

[hogrampage]

Homework Statement

Calculate the value of R2 required to deliver 5W to RL if Vin equals 5V.

Homework Equations

Vout=iRL
P=Ri^2

The Attempt at a Solution

I started by using KVL for the input loop, noting that current is going counter-clockwise:

Input loop: iR2-Vin=0→R2=Vin/i

I found i by using P=Ri^2:

i=sqrt(5/50)
=0.316A

Then, I used i from above to solve for R2:

R2=Vin/i
=5/0.316
=15.8Ω

The answer should be 23.1Ω according to the book (Engineering Circuit Analysis 8ed). Where did I go wrong? I have tried so many different approaches, but I can never get to the answer of 23.1Ω.

 

[Redbelly98]

The currents through R2 and RL are not the same.

Question for you: what must Vout be, to get the necessary power in RL?

 

[hogrampage]

Wow, how did I not see the currents were not the same?

Anyway, I figured it out after finding Vout (15.81V). It makes complete sense to me now, thank you so much for your help!

 

[Redbelly98]

You're welcome, glad it worked out.

Understanding op-amps can take some getting used to, but they are incredibly useful once you get the hang of them.

 

[rezeew]

Please help.

There are a few possible reasons why you are not getting the correct answer of 23.1Ω. Here are some things to consider:

1. Make sure you are using the correct value for RL. The problem states that RL should be delivering 5W, so make sure you are using the correct value for power (P=5W) and not resistance.

2. Check your units. Make sure all of your values are in the correct units (e.g. voltage in volts, current in amps, power in watts).

3. Double check your calculations. It's always a good idea to go back and check your work to make sure there are no errors in your calculations.

4. Consider other factors that may affect the circuit. The problem may not be as simple as it seems and there may be other factors (such as internal resistance) that need to be taken into account.

5. Consult with your instructor or classmates. It's always helpful to bounce ideas off of others and get their input on the problem.

Remember, problem solving is a key skill in science and engineering and it's important to approach problems with a critical and analytical mindset. Keep trying and don't be afraid to ask for help when needed. Good luck!