op amp summing question

[BigMan52]

Hi,
I have a problem that I am trying to solve.
I have an op amp with an output voltage of -2 V and one input value of 1V the other is unknown. The resistance of the unknown voltage input is half of the other resistance. How do I solve the missing voltage?
Is it simply:
Vout = - (V1-V2)? this gives 1V......

I have attached a circuit diagram.

Op amp problem.docx

circuit diagram

(33.89 KiB) Downloaded 392 times

Thank you!

[Pauldf]

Remember that in that configuration the output will be trying to make the -ve input of the chip 0v. If the resistors were all the same size you'd have the nice simple equation where the two input voltages will equate to the inverse of the output voltage.

I think as you have half the resistance on one leg it will act differently i.e. this input will have double the effect hence for it to equal zero at the -ve input I think you'll only need 0.5v on the bottom input but don't quote me on it!

[BigMan52]

Thank you so much for the reply. I think I now realise that would be the effect of the input resistance being halve of the other one.
Thanks again!

[Pauldf]

In essence they are the input voltages multiplied by their relevant gains being added, if we call the top input resistor R1, the bottom R2 and the feedback resistor Rf, substituting them for say 1K and 500R resistors and x for the unknown input voltage we end up with:

(1v * (Rf/R1)) + (x * (Rf/R2)) = 2v

(1v * (1K/1K)) + (x * (1K/500)) = 2v

1v + (x * (1K/500)) = 2v

x * (1K/500) = 1v

x * 2 = 1v

x = 1v/2

x = 0.5v

It is more usual to use same value resistors for all the inputs so we are just adding the result of voltage multiplied by gains of 1.