Op Amp : Design the circuit such that

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Discussion Overview

The discussion revolves around designing an operational amplifier (op amp) circuit to achieve a specific output voltage equation, Vo = Va + 2Vb + 3Vc. Participants explore the theoretical and practical aspects of the design, including assumptions about ideal op amps and the implications of input values.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about how to approach the circuit design and begins with a current equation based on the assumption of an ideal op amp.
  • Another participant challenges the correctness of the initial equation and suggests writing down the correct Kirchhoff's Voltage Law (KVL) equations.
  • A participant later claims to have resolved their confusion and calculates a specific output voltage, Vo = 5.5, while noting the relationship between Vo and the virtual ground voltage, Vn.
  • Another participant questions the feasibility of obtaining a numerical output when the inputs are still represented as symbols, implying a lack of clarity in the calculations.
  • A different participant provides a resource, an ebook, that may assist others in understanding op amp concepts.
  • One participant reiterates the previous concern about deriving numerical answers from symbolic inputs, referencing an earlier post for context.

Areas of Agreement / Disagreement

There is no consensus on the correctness of the initial approach or the calculations presented. Multiple viewpoints exist regarding the validity of the equations used and the feasibility of obtaining numerical results from symbolic inputs.

Contextual Notes

Participants express uncertainty regarding the assumptions made about the op amp and the implications of using ideal conditions. There are unresolved questions about the correctness of the derived equations and the calculations based on them.

ElijahRockers
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Homework Statement



JrxFC.jpg

Assume the op amp is ideal.
Design the circuit such that Vo = Va + 2Vb + 3Vc

The Attempt at a Solution



Not sure how to go about this one.

I started with

\frac{V_a}{R_a}+\frac{V_b}{15k}+\frac{V_c}{R_c} = 0 since the op amp is ideal, the sum of the currents going into the non-inverting input must be 0.

From here I haven't got any ideas how to proceed. But what is the voltage at the inputs? I'm assuming it's zero because Vn/20k = 0 : Vn = 0

EDIT: Alright I found Ra to be 30k, Rc to be 10k. Now I have to find the currents ia, ib, and ic, when Va=.5, Vb=.7, and Vc=1.2. Shouldn't that just be Va/Ra =ia? I know one of them must be negative since the total current flowing in must be 0, but how do I know which one?

I found Vo =5.5 at those values for Ra,Rb,Va,Vb,Vc.

Ok... I figured it out. I guess sometimes it just helps to type it out on a forum haha. For anyone wondering, Vo=6Vn (by KCL at the negative terminal). Since Vo = 5.5, then Vn = .9166...
 
Last edited:
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ElijahRockers said:

Homework Statement



JrxFC.jpg

Assume the op amp is ideal.
Design the circuit such that Vo = Va + 2Vb + 3Vc

The Attempt at a Solution



Not sure how to go about this one.

I started with

\frac{V_a}{R_a}+\frac{V_b}{15k}+\frac{V_c}{R_c} = 0 since the op amp is ideal, the sum of the currents going into the non-inverting input must be 0.

This equation is incorrect. Write down the correct KVL equations.
 
ElijahRockers said:
Ok... I figured it out. I guess sometimes it just helps to type it out on a forum haha. For anyone wondering, Vo=6Vn (by KCL at the negative terminal). Since Vo = 5.5, then Vn = .9166...
You really can't get "Vo = 5.5" when the op-amp has supply voltages of ±5 volts. ☹[/size][/color]
 
How can you get numerical answers of any kind when your inputs are symbols?
Are you pulling somebody's leg? :rolleyes:
 
I founded an ebook that very helpful for you: Op amp for every one

www.ti.com/lit/an/slod006b/slod006b.pdf
 
Last edited by a moderator:
rude man said:
How can you get numerical answers of any kind when your inputs are symbols?
Are you pulling somebody's leg? :rolleyes:
See post #1
 

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