# Find unknown voltage of a Op amp

#### dieggo3

Joined Jul 6, 2016
8
Hi,
Hoping for a prompt in the right direction regarding a unkown voltage on a Non Inverting Op Amp.
Any help would be greatly appreciated.

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#### Papabravo

Joined Feb 24, 2006
19,250
How much current flows through R1 in to the non-inverting input? Answer this question and you'll have your answer.

#### WBahn

Joined Mar 31, 2012
27,384
Hi,
Hoping for a prompt in the right direction regarding a unkown voltage on a Non Inverting Op Amp.
Any help would be greatly appreciated.
Reduce the problem to smaller problems that, if solved, would let you solve the bigger problem.

For instance, IF you knew the voltage at the junction of R2 and R3, combined with knowledge of how much current flows into or out of an ideal opamp's input terminals, could you find the voltage at the output of the opamp?

If so, then you are now faced with the simpler problem of finding the voltage at the junction of R2 and R3. Do you see a way to get at that?

#### dieggo3

Joined Jul 6, 2016
8
I do know that unknown voltage is 100 mV as op amp is ideal. V+=V-
I believe to show just answer it's not enough.

#### WBahn

Joined Mar 31, 2012
27,384
I do know that unknown voltage is 100 mV as op amp is ideal. V+=V-
I believe to show just answer it's not enough.
The step you are missing is establishing that V+ is equal to 100 mV. Once you do that, then you can use the property of an opamp that states that, in the active region, V+ is equal to V-. But it's a different properly of an ideal opamp that allows you to claim that V+ is equal to 100 mV. What property is that?

#### dieggo3

Joined Jul 6, 2016
8
Is it a closed loop ?
The formula for circuit is:
V=(R2)/(R2+R3)*Vout
Am I getting on the right track, or just confusing my self.

#### shteii01

Joined Feb 19, 2010
4,644
Ok. This actually a much simpler problem. But, reading of textbook is required.

Assuming ideal op amp.
Ip is current entering none inverting input.
In is current entering inverting input.
Ip=In=0 This is fundamental property of ideal op amp.
This means that there is 0 current through R1. This means that there is no voltage drop across R1. Therefore none inverting input of the op amp has 100 mV.

Now we apply other property of ideal op amp. The Virtual Short between the inputs of ideal op amp. We now know that input to none inverting input is 100 mV. Therefore the inverting input will also have 100 mV. V is the voltage at the inverting input, therefore V is 100 mV.

#### Papabravo

Joined Feb 24, 2006
19,250
Ok. This actually a much simpler problem. But, reading of textbook is required.

Assuming ideal op amp.
Ip is current entering none inverting input.
In is current entering inverting input.
Ip=In=0 This is fundamental property of ideal op amp.
This means that there is 0 current through R1. This means that there is no voltage drop across R1. Therefore none inverting input of the op amp has 100 mV.

Now we apply other property of ideal op amp. The Virtual Short between the inputs of ideal op amp. We now know that input to none inverting input is 100 mV. Therefore the inverting input will also have 100 mV. V is the voltage at the inverting input, therefore V is 100 mV.
Good explanation. I know you know the electronics part, but why do you keep using none for non? As in non inverting input. Do you really not know the correct spelling?