Hey I stumbled upon this problem while trying to teach about operational amplifiers and more specifically the input impedance/resistance of an inverting opamp. The textbooks will all say that the input impedance is equal to the input resistor. The problem I am having with this is that a virtual ground does not have the properties of a real ground and since the input of the inverting input is very high, all current must flow through both resistors and then into the opamp. Which makes the input resistance R1+R2.
Now because I wanted some proof of my theory I tried simulating it. And I got 1 result that confirms my theory, and another that debunks it. The first result is attached as input impedance 1. The spice simulation tells us that indeed the input resistance is R1+R2 like I thought it was. Then in the second attachment I replaced the ground of the opamp with a supply of -10V. Now the textbook explanation applies, input resistance = input resistor.
I'm starting to understand that the virtual ground kind of forces a current to flow from the input voltage source, through the input resistor to the virtual ground, but because the current isnt allowed to travel into the input, it's forced to go through the input resistor, kinda like a current source. But that still leaves me questioning the results of the spice simulations.