# Determine unknown voltages in an ideal op amp circuit

Discussion in 'Homework Help' started by Bangersandmash, Jun 10, 2016.

1. ### Bangersandmash Thread Starter New Member

Jun 10, 2016
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0
Hi Everyone,
I am trying to determine the unknown voltages in an ideal op amp and I am really struggling to find a worked example or a formula on how to do it. I have loads of literature on op amps and I have become overwhelmed with the amount of info, yet can't seem to find the information I am seeking.
Does anybody have a direct example of how to do this? Ideally I need an op amp circuit diagram with a solution on how to find the unknowns. I have attached an image of the circuit in question. I am not looking for somebody to solve it for me but if anybody has a similar example so I can reference it to solve myself that would be great.

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3. ### WBahn Moderator

Mar 31, 2012
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4,699
Since it is an ideal opamp, what is the voltage at the right side of the input resistor?

What is the voltage V1 at the left side of the input resistor (you are given that 10 uA of current is flowing in it)?

Where does that 10 uA of current HAVE to go?

What is the voltage at the output given the voltage at the left side of the feedback resistor and the current in the feedback resistor?

What is the current in the opamp's output terminal (magnitude and direction)?

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4. ### dannyf Well-Known Member

Sep 13, 2015
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Two principles for opamp:

1) the input terminals have infinite impedance -> ie no current flows in / out of them;
2) the voltage gain is infinite -> ie when configured as negative feedback, the two input terminals are at the same potential.

the first principle means that the current flowing through the 10K resistor will flow through the 100K resistor; the 2nd principle means that the negative inverting input's potential is at ground, because that's the potential of the non-inverting input.

Anything after that requires common sense and elementary school level math.

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5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
Formula:
$
V_{output}=-\frac{R_{feedback}}{R_{input}}*V_{input}
V_2=-\frac{100k}{10k}*V_1=-10*V_1
$

Now, like others already told you, examine V1 and 10k resistor.

What is V1?
V1 is voltage from one end of resistor to the ground.

What is 10k resistor?
10k resistor has V1 on one end, the other end is at ground. Which means that you have V1 across 10k resistor.

You know the value of 10k resistor. You know the current that passes through 10k resistor. Apply Ohm's Law to find V1.

Plug V1 into the formula to find V2.

Can you do all 'dat?

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6. ### ham3388 Member

Jul 3, 2012
82
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If still you can not make out, tell us we will help you further.

7. ### Bangersandmash Thread Starter New Member

Jun 10, 2016
26
0
Thanks for the replies.

This is what I have come up with:

V1= 10uA*10k = 100mV = +0.1V
V2 = 0-(100k*10uA) = -1000mV = -1.0V
Does that look right?