This shows a simple non-inverting Op-Amp circuit within which you can change:-

• the circuit gain via the resistor values
• the input voltage
• negative or positive feedback
• the presence or absence of circuit noise on the feedback node
• to reset all voltages to sensible values at the start of a simulation

With negative feedback, provided that the output voltage is between the power supplies, you will see the output "wobble" around the correct value (input x gain). With positive feedback, it will wobble a little, then head for one of the the power supplies. Even without noise added, positive feedback will still have this effect as rounding errors Excel's calculations will act as a form of noise! Once the circuit is initialised using the reset button, you should then set it to "simulate", click anywhere on the white screen and then press and hold function key F9 to simulate.

• Run simulation here