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

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    Circuit Op-amp
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Discussion Overview

The discussion revolves around calculating the value of resistor R2 in an op-amp circuit to deliver 5W to a load resistor (RL) given an input voltage (Vin) of 5V. The focus is on the application of circuit analysis principles and the relationships between voltage, current, and power in the context of op-amp circuits.

Discussion Character

  • Homework-related
  • Technical explanation

Main Points Raised

  • The initial attempt at a solution involved using Kirchhoff's Voltage Law (KVL) to relate R2, Vin, and current (i), leading to the equation R2 = Vin/i.
  • The participant calculated the current (i) using the power formula P = Ri^2, resulting in i = 0.316A.
  • Using the calculated current, R2 was determined to be 15.8Ω, which differed from the expected answer of 23.1Ω from the textbook.
  • Another participant pointed out that the currents through R2 and RL are not the same, prompting a reevaluation of the output voltage (Vout) needed to achieve the required power in RL.
  • A later reply indicated that the participant realized the importance of Vout and calculated it to be 15.81V, which resolved their confusion regarding the circuit.

Areas of Agreement / Disagreement

Participants initially disagreed on the correct approach to calculating R2, particularly regarding the relationship between the currents through R2 and RL. However, the later posts indicate a resolution for the original poster's confusion without establishing a consensus on the initial calculations.

Contextual Notes

The discussion highlights potential misunderstandings regarding current flow in op-amp circuits and the application of circuit laws, but does not resolve the discrepancies in the calculated values or the assumptions made in the initial approach.

Who May Find This Useful

Students and practitioners interested in op-amp circuit analysis, particularly those seeking assistance with homework problems related to power delivery in electrical circuits.

hogrampage
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Homework Statement


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

oacircuit-1.jpg


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.
 
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The currents through R2 and RL are not the same.

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

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