Differentiating Op-Amp basic exercise

[Femme_physics]

When the voltages are worked out, the resulting formula is:
$$V_{out} = -{R_f \over R_{in}}V_{in}$$

Note the minus sign?
The minus sign implies it is inverting - it inverts the sign of the voltage.

Alright, perfectly clear. I guess the "ground" sign confused me. I actually have this circuit in an exercise, I will try to solve it and see if i struggle before I turn to the "sages"...spasiva bolshoy, you two :)

 

[Femme_physics]

this seems about right to me, yes? (ignore the part of the sketch where it says 18)

http://img847.imageshack.us/img847/8926/scan750.jpg

 

[technician]

FP
I see your working and it is correct. Looks neat and well organised

 

[Femme_physics]

FP
I see your working and it is correct. Looks neat and well organised

Really? I thought so too! I don't know why my teacher told me I was mistaken. I showed him the exact page yesterday. Are you 100% sure? I'll be on his case if you are.

 

[technician]

Well... here is the way I approach all of these problems
1) Voltage at the + input = 2V (that battery)
2) Voltage at the - terminal must also be 2V
3) R1 is 2KΩ and has 2V across it. The LH end of R1 is connected to 0volts... ground or earth
4) Current through R1 = 2V/2kΩ = 1mA
5) This current must come from Vout through R2...therefore Voltage across R2 = 1mA x 10kΩ
=10V
6) The LH end of R2 (-input) is at +2V therefore Vout must be 2V + 10V =12V
7) V across LED = 1.4V therefore V across 2kΩ = 12 - 1.4 = 10.6V
8) Current through 2kΩ and the LED = 10.6/2kΩ = 5.3V

 

[Femme_physics]

Right! That's what I thought!

So, I'll be on his case, then! Just you wait. I'm not giving up that easily now. I'll make him write to me his solution.

 

[technician]

I will be interested to see it.
Make certain you post it for me.

 

[technician]

The only possible criticism I can see is that you have the gain of the inverting amplifier as (R1+R2)/R1.
Normally you would expect to see just R2/R1 but with that battery connected to the +input your equation is correct.