I seem to have trouble with simulating op amps, odd results

Hi there.

So when building circuits with Op Amps and simulating I often get odd results. When I do then rolling the circuit back to something really simple still seems to present the odd results. I'd like to know if it's something I'm doing or if it's just a known issue with circuit lab.

For example with the simple op amp as a buffer circuit attached I get DC 50v appearing on the feedback loop. I have had an op amp circuit work correctly before suddenly showing 50kV somewhere. The signal seems to disappear randomly sometimes, am I the only one having this problem with op amps? I can't imagine this is something that everyone is experiencing as it seems so fundamental! In most of these cases I sim on LTSpice with the same circuit and see no errors in the circuit.

I AM using safari, but I experience the same results with chrome too.

Edit, I rechecked just now with a 9v opamp buffer circuit and it's giving me -22Megavolts on the non-inverting in of the buffer, as well as no signal passing.. https://www.circuitlab.com/circuit/r963h497a74y/simple-op-amp-balancing-circuit/

and here is the circuit posted above slimmed down to just the buffer, no signal passing and -50kv on the feedback and non-inv-input. would be keen to see if others see it working as expected... https://www.circuitlab.com/circuit/867jq87e5s9q/bug-check/

Can anyone help with what I've done wrong or illuminate me to known bugs etc?

by SpiceBiscuit
6 days, 16 hours ago

From your first circuit, it looks like you have capacitively-coupled inputs to the non-inverting (+) terminal of OA1 and OA2, without any DC path to ground.

Can you try adding a high-value resistor (perhaps 1M or 10M) from the + input to ground (or some mid-level offset)? This will provide a DC path to center the non-inverting input's DC level within a workable range for the op-amp. Let me know if that fixes things.

by mrobbins
6 days, 16 hours ago

Thanks very much for your reply!

Yes, that does help get rid of the huge voltages! The trouble is that it also removes the 4.5v bias voltage from the OpAmps + pin. I'm attempting to build a kind of virtual ground to power an opamp off of a 9v adapter so everything post OA6's output should be floating... At least, I've made circuits like that that work, but not exactly this one and theres a good chance that I'm missing something crucial to be honest.

The fact it removes the huge voltages is encouraging. The output signal now appears, too, but only the positive phase! I assume because the op-amps bias is removed by the resistor to ground...

by SpiceBiscuit
6 days, 15 hours ago

OK, in that case, instead of connecting your large $1 \text{M} \Omega$ resistor from the non-inverting input to ground, try connecting it from the non-inverting input to your virtual ground.

by mrobbins
6 days, 13 hours ago

That works, thanks! And then I shifted it around so the input is isolated with a cap from the bias/virtual voltage. It works ok, the only thing now is the output cap, C6 doesn't seem to block DC, I have the bias voltage on node PIN2L despite it's presence.. but anyway, it works and the 1M resistors make sense! Thanks!


by SpiceBiscuit
6 days, 13 hours ago

Great! Looks like PIN2L is also floating at DC. You'll want to add a resistor to it as well to bias it where you want at DC.

For future reference: at DC, pretend capacitors didn't exist (were open circuit). Is there something anchoring the DC voltage of every node? If not, you'll need to do something like this.

by mrobbins
6 days, 9 hours ago

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