# Op Amp KCL loop question

#### Nathen Leung

Joined Feb 20, 2016
7
I asked the professor and he hinted that I should use a KCL loop at the inputs with virtual ground. But I still don't fully understand it myself.

#### shteii01

Joined Feb 19, 2010
4,644
Is that negative resistance?

#### Nathen Leung

Joined Feb 20, 2016
7
No it's just a dash. Values are 91k and 51k

#### shteii01

Joined Feb 19, 2010
4,644
Ok.
Can you follow the current from V1 and to the ground? Call this current A. Can you write out which components it goes through? What the voltage would be across that component?

#### Nathen Leung

Joined Feb 20, 2016
7
Yep
Ok.
Can you follow the current from V1 and to the ground? Call this current A. Can you write out which components it goes through? What the voltage would be across that component?
Yep

#### shteii01

Joined Feb 19, 2010
4,644

#### Nathen Leung

Joined Feb 20, 2016
7
Would current A be going through R1 and R2. I don't know what the voltage that would be

#### shteii01

Joined Feb 19, 2010
4,644
Would current A be going through R1 and R2. I don't know what the voltage that would be
Almost correct. It will go through R1, R2 and Rgen1.

So you are looking at something like this:
A*R1+A*R2+A*Rgen1-V1=0
A*R1+A*R2+A*Rgen1=V1
A*(R1+R2+Rgen1)=V1
A*(91+91+51)=V1
233A=V1

Now do the same for the V2. Some current is leaving V2, call it current B, follow its path. Also you might want to redraw the "V2 circuit". Instead of Vout being a dot in space, replace it with a circle like the other independent voltage sources.

#### Nathen Leung

Joined Feb 20, 2016
7
So then BR4+BR3+BRgen2=V2 and since its a difference amplifier do I subtract the voltages?

#### shteii01

Joined Feb 19, 2010
4,644
So then BR4+BR3+BRgen2=V2 and since its a difference amplifier do I subtract the voltages?
Vout is connected to R4... It is part of this circuit.

#### Nathen Leung

Joined Feb 20, 2016
7
Oh woops! so BR4+BR3+BRgen2+V2=vout

#### WBahn

Joined Mar 31, 2012
26,068
What assumptions are you making about the op-amp? Are you assuming it is ideal? Even though it is stated that it's a 741?

Part (b) talks about "both circuits" -- what "both"?

#### shteii01

Joined Feb 19, 2010
4,644
Oh woops! so BR4+BR3+BRgen2+V2=vout
That is good. So.
Vout=B*R4+B*R3+B*Rgen2+V2
Vout=B*(R4+R3+Rgen2)+V2
Vout=B*(91+91+51)+V2
Vout=233B+V2

Now. Tell me about the "virtual ground" junctions. What do we know about those junctions? The junctions that I am talking about are:
- Junctions of R1 and R2 and OA1 positive input
- Junction of R3 and R4 and OA1 negative input
What do we know about these junctions?

#### Nathen Leung

Joined Feb 20, 2016
7
Well it's asking for vout in term of V1 and V2 and I'm assuming that its a difference amplifier. I'm guessing there was 2 circuits to the problem but he removed one

#### WBahn

Joined Mar 31, 2012
26,068
But what you get for Vout in terms of V1 and V2 depends on what you assume about the opamp. In particular for this circuit, are you taking into account the input bias current or not?