# Basic Op-Amp question

• posted
Assuming I have an 'ideal' op-amp.
If I have an Rf and Rin with a voltage into the inverting input and the
non-inverting is grounded: my basic formulas is (-Rf/Rin) * Vin
What if I put a voltage directly into the non-inverting input with the
above conditions?
I understand everything except getting my signs correct. To use real
numbers:
Rf 10k
Rin 5k
Vin1 2 volts
Vin2 1 volts (this is the voltage on the non-inverting input).
My calculations will be:
The current across Rin will be the difference between 2 volts and 1 volt
because the op-amp will make the inverting equal to the non-inverting input
(in this case 1 volt).
This difference in voltage is divided by Rin to get the current
The current is then multiplied by Rf to get the voltage drop across Rf.
Now my problem is: I have all the correct voltages and currents, however,
my signs get confusing and I'm not sure what to get for the op-amp's outptu
voltage especially when I start changing the input voltages (i.e. making
one higher than the other, making one negative, and one positive, vice
versa, etc..)
Is there is an easier way to do this, a formula, or am I not thinking about
it correctly?
• posted
On Sun, 11 Feb 2007 12:55:56 -0600, Peter Gave us: