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| Floating kilofarad capacitor causes noise in LTspice BJTs |
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| magic:
Posting in "beginners" because I'm new to spice and not sure if it's something considered "normal" in this software :-// Anyway, I was simulating some discrete amplifier and added a 1k capacitor from feedback input to ground to test its AC open loop behavior. When I was done, I disconnected it from ground but left it there in case it would be useful again. After some further changes, the circuit developed weird noise issue and I spent a whole hour trying to nail it down. It looked like there is some serious current noise or oscillation in the input BJT, sometimes reaching into GHz frequencies :scared: It made completely no sense and, ultimately, removing the capacitor fixed it |O I presumed that leaving dangling passives like that makes them effectively disabled. But some people on the Internet say that floating nodes may cause simulation errors unless a high value resistor is added to ground. Not sure why would it be, but I added 1Meg and it's still the same. 1k solves it though, as does direct short of course. I looked closer at the currents and it's evident that spice doesn't even respect Kirchhoff's law here :scared: Red is feedback resistor current, it follows IN- to OUT differential voltage quite well. Green is current though that damn capacitor, almost zero almost all time, but not always. Blue is transistor base current and it goes into weird oscillation every now and then, mysteriously pulling voltage on IN- down. Not sure what to think of that. :D |
| exe:
Did you try to reduce the value of capacitor? To, say, 1 farad or even less. May be there are rounding errors or something. |
| T3sl4co1l:
Try adjusting RELTOL, VTOL, CHGTOL or whatever TOLs LTSpice has. Also try TRAP integration, the default hacked integration is notoriously inaccurate. Tim |
| alanb:
Do you really mean Kilo farad i.e. 10^3 farad? that is exceptionally large. |
| T3sl4co1l:
Yup! It's a common strategy for testing the AC response of a circuit, without having to worry about DC bias (.op solve it for you) or LF cutoff. Tim |
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