At 01:30 PM 2/21/2004 +0100, you 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.

Beta degradation, even with limited base current. Use a diode from emitter
to ground (and a series base resistor). Of course you need a collector
load resistor too (so when the transistor is on, the collector will no
longer be at +5V). The base will actually go +/- 0.7V under these
conditions.

>Should I fear damage or bad behaviour? I need to use the transistor as a
>RTL (Resistor Transistor Logic) simple switch.

Yes.

http://www.fairchildsemi.com/ds/BC/BC548.pdf
See the Vebo specification (5V abs max)


                                       +5
                                        o
                                        |
                                       .-.
                                       | |
                                       | |
                                       '-'
                                        |
                                        +------o
                                        |
                              ___     |/
                  +/- 12V  o-|___|--+-|
                                    | |>
                                    -   |
                                    ^   |
                                    |  ===
                                   === GND
                                   GND

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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