When a bipolar transistor is driven into saturation, it builds up charge which slows the recovery time when it is switched off. What I would like to know is, how well is this behaviour captured in spice models? Does spice model this? If I pick any given transistor from the library, how accurate is the recovery behaviour likely to be?

I am using LTSpice. I ran a quick sim (results below) and I can see some difference in recovery time for saturated vs unsaturated. But the difference doesn't seem large, and I'm not sure if it is due to other factors such as different collector currents.

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  • \$\begingroup\$ It'll depend on the model rather than Spice itself. And it looks like you're seeing about 0.5 us ... what were you expecting? \$\endgroup\$ – Brian Drummond Jul 24 at 14:19
  • \$\begingroup\$ Try comparing that with a VCSW. If there's no difference then the BJT is a virtual switched resistance, but if there is, it's likely some attempt at modeling the real device, somehow. \$\endgroup\$ – a concerned citizen Jul 24 at 16:49

FOR excitement, add 100pF in parallel with each of the base resistors.

This will dump extra charge into the base, and is a standard speedup method for saturated bipolars.

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