It sounds like you are trying to drive the transistor from an RS232 source. Typical "RTL(family) logic" levels don't swing +/- 12 volts, unless you're trying to interface to it. A few tens of mA forward bias is
not going to do the transistor any good. A current limiting resistor small enough to turn the transistor on is all that's needed. Keep the base from going negative with a diode to ground, anode to the ground
side. In order to answer your question in context, more info is needed.
Rick

Omega Software wrote:

> Hello people,
>
> what happens to small signal NPN bipolar transistors (BC548C) if I apply to the base -12V and +12V, when the collector is always at +5V respect to the (grounded)emitter? Currents are small, few tens mA max.
>
> Should I fear damage or bad behaviour? I need to use the transistor as a RTL (Resistor Transistor Logic) simple switch.
>
> Thanks!
> Andrea
>
> --

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