Circuit Analysis Homework Help: Parts A & C

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Discussion Overview

The discussion revolves around a circuit analysis homework problem, specifically focusing on parts A and C. Participants explore concepts related to operational amplifiers, voltage drops, current calculations, and the application of Ohm's law and nodal analysis.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant states that if Va = 4V, then pins 2 and 3 also have 4V, questioning if this implies Vin = 4V.
  • Another participant suggests that the argument should be based on the characteristics of an op-amp with negative feedback and mentions the "golden rules" of ideal op-amp analysis.
  • There is confusion about how to find I1, with a suggestion to apply Ohm's law given the known voltage across R1.
  • Participants discuss the voltage at Vb being 6V and the drop to Va being 4V, leading to a voltage drop across the resistor of 2V.
  • One participant calculates the current as 2mA using V=IR, while questioning whether to use nodal analysis.
  • Another participant confirms the current direction based on the potential difference, indicating it flows from high to low potential.

Areas of Agreement / Disagreement

Participants generally agree on the application of Ohm's law and the behavior of the op-amp in the circuit, but there are uncertainties regarding the approach to finding I1 and the use of nodal analysis. The discussion remains unresolved on some aspects, particularly in part C.

Contextual Notes

There are limitations in the discussion regarding assumptions about the op-amp behavior and the specific conditions under which the calculations are made. The application of nodal analysis is debated without a clear resolution on its necessity.

Who May Find This Useful

Students or individuals studying circuit analysis, particularly those interested in operational amplifiers and related calculations.

KTiaam
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Homework Statement


4gL67ax.png

Need help on part a.
and c.

The Attempt at a Solution


  • Part A
Since Va = 4V then pin 2 and 3 also have 4v. Would that mean that Vin is = 4v as well?
Im just confused how to find I1.
Also I am not sure how to find Vc.
  • Part C
As for part c, I am having trouble with it entirely.

Any help is appreciated!
 
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KTiaam said:
Since Va = 4V then pin 2 and 3 also have 4v. Would that mean that Vin is = 4v as well?
Well, yes, but you'll have to argue that based on the characteristics of an opamp with negative feedback. If you think in terms of the two golden rules of ideal opamp analysis..

KTiaam said:
Im just confused how to find I1.
You know what the voltage is across R1. What about Ohm's law?

KTiaam said:
As for part c, I am having trouble with it entirely.
Consider again the behavior of an ideal opamp with the feedback shown. It desperately wants to turn on that BJT hard, so it can reduce the voltage across its terminals to zero, but that really only requires the BJT to just barely move out of its cutoff region. You usually assume something about VBE then.
 
Last edited:
milesyoung said:
You know what the voltage is across R1. What about Ohm's law?

The thing that threw me off, is what voltage to use with r1,
But based off of the orientation of the current source, I use Vb?
 
KTiaam said:
The thing that threw me off, is what voltage to use with r1,
But based off of the orientation of the current source, I use Vb?
The voltage (with respect to ground) at Vb is 6 V, and you drop down to 4 V as you cross the resistor to Va.

Does that help?
 
milesyoung said:
The voltage (with respect to ground) at Vb is 6 V, and you drop down to 4 V as you cross the resistor to Va.

Does that help?
so that means the voltage drop across the resistor is 2v, then using V=IR
i get 2mA?

or do i use Nodal Analysis?
 
KTiaam said:
so that means the voltage drop across the resistor is 2v, then using V=IR
i get 2mA?
Yes. :smile:

Since Vb is at a higher potential than Va, what does that tell you about the direction of the current?

KTiaam said:
or do i use Nodal Analysis?
Nodal analysis is useful when you need to solve for unknown node voltages, but you're already given the node voltages on either side of the resistor, so all there's left to do is just apply Ohm's law.
 
milesyoung said:
Yes. :smile:

Since Vb is at a higher potential than Va, what does that tell you about the direction of the current?

Current goes from a high to low potential so it goes from right to left!

Thank you for all the help Miles! I might have more questions incoming lol.
 

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