Andrew Beckett
Senior Fellow
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Life, don't talk to me about Life...
Posts: 1734
Bracknell, UK

The other way of thinking about this is that you have a negative time constant  this means that because you have a response which is of the form V(t)=V_{0}(1e^{t/Τ}) the exponent ends up being positive rather than negative, and so explodes. If the time constant is only a small negative number (because your resistance is small), it will blow up slowly, and the simulator will end up having to take very small time steps to follow the exponentially increasing signal.
So it's hardly surprising behaviour! I've managed to do this by accident in the past where I unwittingly made the output impedance of a block negative in a behavioural model (by getting an equation back to front) and then the capacitance of the next stage caused it to blow up (in value, not physically!)
Regards,
Andrew.
