Ideal Feedback Contract

Input Current Is Zero

current balances around the pin

Ideal op-amp inputs do not consume current; resistor currents balance elsewhere. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

The input pins take zero current

The resistor current is 1 ampere, but the ideal input-pin current is zero.

I+=I−=0 AI_+ = I_- = 0\ \text{A}
Zero input currentThe input-pin current markers are checked as zero.+-op amprails -12 V to 12 VVout 0 VV+ 0 VV- 0 VI+ 0 AI- 0 AIin 1 AIf 1 AV+ = V-input-current

Resistor current balances resistor current

The 1 ampere that enters through one resistor leaves through feedback.

Iin resistor=Ifeedback=1 AI_{\text{in resistor}} = I_{\text{feedback}} = 1\ \text{A}
Current balanceThe resistor currents balance without entering the input pin.+-op amprails -12 V to 12 VVout 0 VV+ 0 VV- 0 VI+ 0 AI- 0 AIin 1 AIf 1 AV+ = V-input-current