Say the input to the non-inverting terminal to this op-amp is 5 V, what is the voltage at the inverting terminal? I always assumed that the non-inverting and inverting terminals have the same voltage at all times but this is not reflected in simulation.
What is the correct answer here?
I guess the main question to ask is: why don't the terminals have the same voltage in an op-amp?
I always assumed that the non-inverting and inverting terminals have the same voltage at all times but this is not reflected in simulation.
At all times? Nope. This is true if and only if there's a negative feedback. Without feedback the op amp can't equalise its inputs.
If you want the inverting input to be equal to the non-inverting input then tie the bottom end of that 1k resistor to output instead of ground:
simulate this circuit – Schematic created using CircuitLab
NOTE: Theoretically R1 can be anything but open (infinite resistance) as an op amp's inputs draw zero current. Shorting it will create an ideal buffer (a.k.a. follower).
So if there isn't feedback, the voltage at the inverting terminal can be assumed to be 0 or very close to 0 ... Theoretically 0, as it's terminated with a 1k resistor. Practically, since almost all of the op amps have their input bias current flow from pin to outside (i.e. not into the pin) that bias current can generate microvolts, nanovolts, or even millivolts across that 1k resistor (close to 0). ... when a voltage is applied to the non-inverting terminal? regardless of the non-inv terminal voltage, we should say. Applying supply voltage only is enough.
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Commented
Dec 22, 2022 at 7:34