# Inverting Op Amps - Nodal Analysis

#### okiedokiedude

Joined Mar 18, 2020
1

Hey all,

I'm having trouble figuring out how my professor did nodal analysis in part b - I keep getting -0.417 mA so I think I'm just overlooking something simple (probably with polarity). I would really appreciate some help with this problem - thanks in advance!

Edit: Just realized I titled this "inverting" when I meant "non-inverting" - sorry about that!

#### crutschow

Joined Mar 14, 2008
30,463
The polarity at the (+) input needs to be the same as the (-) input since the op amp always tries to keep those two voltages essentially equal (if there is a negative feedback loop).

#### WBahn

Joined Mar 31, 2012
26,933
Hey all,

I'm having trouble figuring out how my professor did nodal analysis in part b - I keep getting -0.417 mA so I think I'm just overlooking something simple (probably with polarity). I would really appreciate some help with this problem - thanks in advance!

Edit: Just realized I titled this "inverting" when I meant "non-inverting" - sorry about that!
So let's ask which of those two answers makes the most sense.

You can tell by inspection that V+ is higher than 0 V.
You know that, if the opamp is operating in it's active region, that V- is then higher than 0 V.
What does that tell you about the direction of the current in R1?
What does that tell you about the direction of the current in RF2?

There's now way for us to even guess how you are getting -0.417 mA for I since you don't show us YOUR work.

You seem to be saying that you have no problems with part (a), so if Vout is 7.5 V and V- is 5 V, what does the sign of Ia have to be (assuming you are using conventional current)?

The instructors work in part (b) is not a nodal analysis (which is the systematic application of KCL), but rather just applying Ohm's Law to RF2.