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
15 years ago