I'm making a simple amp > voltage doubler rectifier circuit to drive an lm3916 meter circuit and when simming to choose between JFET, BJT or OpAmp (just for my learning, I'm looking to a Jfet in the end I think) I have this weird behaviour I don't understand.
I go to add a blocking cap for real world use
And the input goes low to -7v or so, rendering the output unuseable. N001 is OpAmp Non-Inv input
But add a resistor to the input stage
And it all works again.. What is causing this? I don't know why the coupling cap causes the circuit to 'not work' without an input impedance of some sort, separate to the OpAmps near infinite impedance. I simmed many different values of R3 and as long as there's 'some resistance' it works, but given what I know about OpAmps, and the fact that it works without a coupling cap I have no idea what causes the fail when I add the cap..
Any advice well appreciated!