Op-Amp Voltage Follower Bug Report

[mityeltu-2]

I have the following circuit in Multisim14.0. Simple voltage follower using 741 Op Amp. 5VAC on non inverting input with electromechanical SPST switch. Inverting input tied to output. ±12VDC for V+ and V-. I am using the oscilloscope to monitor input (5VAC) and output (should be 5VAC). That's all there is.

When I simulate the circuit with the switch in the closed position, it works as expected. However, when I open the switch, I am still getting an output from the op amp. Now, call me crazy, but how is that even possible?

The circuit is attached. If you can tell me what I have done wrong, I would be delighted to know as this is really puzzling to me.

[jg_geadinc]

There are a couple of problems. First of all you should never leave an input to float like that. That is actually a clue as to what is wrong because the floating input should generate an error. If you tie the input to ground through a resistor the behavour changes dramatically. 

So anyway, the reason for what you are seeing is obivous in the spice netlist. A switch is modeled as 2 different resistor values. 0.0001 for 'on' and 100000000 for 'off'. So it is not a perfect short and open as you are thinking it is. Therefore it is not truely ever 'disconnected'. 

Here is the line from the spice netlist for the switch: 

" xS1 11 2   SwitchS1 Params: Ron=0.0001 Roff=100000000 "