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Transistor behaviour when inputing below ground level

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Murugesh_89

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Hi,
In the switching npn transistor circuit (inverter), if the input to the base resistor voltage goes below ground voltage (say -2 V), what will be its effect on output at the collector. Also will it be harmful to transistor?



Thanks,
Murugesh
 

The transistor will be off, the same as if the base-emitter voltage were zero. It's not harmful as long as the maximum reverse base-emitter voltage rating is not exceeded. This value is typically a few volts and is given in the data sheet.
 
Thanks...

What about in the case of any IC. Say if -2 volts is given as input to one channel of multiplexer IC.
 

If it is a CMOS multiplexer with the lowest supply being 0V then it is a bad idea. You will turn on the protection diodes in the multiplexer.

Keith
 

That is quite different. A single bipolar transistor has only PN junctions connected to it's base pin. An IC has lots of other components on the sharing the same silicon and unless designed to do so, a voltage below ground can make the pin draw considerable current and permanently damage it. The extent of damage depends on the type of device and how much current you allow to flow into it but the majority of ICs are only safe up to a few uA when you go beyond say 0.5V below ground. Many have protection devices at their inputs which are designed to prevent transient high voltages such as small static discharges but they are incapable of protecting against a sustained 2V input.

Brian.
 

If the input goes a diode drop below the negative supply voltage on most ICs you will forward bias the substrate and input protection diodes and large currents can flow. Worst-case this can cause a (SCR type) latchup condition which allows current to flow from the positive to negative supply, limited only by the circuit impedances, which will likely blow the IC.
 

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