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Level Shifting

Raven Luni

Oct 15, 2011
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Greetings,

Just wanted to be sure about this: Is it ok to have an NPN transistor's base voltage higher then the collector? It's for a simple logic level shift (9V - 5V). And the inversion doesnt matter (so no need for a second transistor)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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The answer is: it depends.

The important voltages are:

1) Vbe (forward biased BE junction). For this to increase you have a high forward current -- bad juju.

2) Veb (reverse biased BE junction) keep it small -- under 5V is safe.

3) Vce, Vcb -- normal operation. Keep the voltage below the max specification

And you're asking about

4) Vbc (forward biased BC junction). I've never seen a specification for this, but it's petty rare to have the transistor operating in this regime.

For typical operation the BC junction is never forward biased. In any case where it is, the base resistance which limits Ibe should also protect Ibc.

If you keep all of the other voltages within range using the normal methods, then any forward biasing of the BC junction should be likewise protected.

What sort of circuit are you using for this to be a possibility? (does it have no base resistor?)
 

Harald Kapp

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4) Vbc (forward biased BC junction). I've never seen a specification for this, but it's petty rare to have the transistor operating in this regime.
Steve: it depends :)
In saturation Vce < Vbe, therefore Vbc>0V, albeit only by a few 100 mV
 

kpatz

Feb 24, 2014
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Bipolar transistors are controlled by current, not voltage. In a typical circuit as you describe, you'd have a base resistor to limit base current to a few mA (depending on the circuit, your circuit's needs, and the gain of the transistor).

With the resistor in place, you could feed 9V into the base resistor, a current would flow through the resistor and base, switching on the transistor. The actual voltage at the base would be the Vbe voltage drop, usually <1V (similar to a diode's voltage drop). The rest of the voltage would be across the resistor.
 

Raven Luni

Oct 15, 2011
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The actual voltage at the base would be the Vbe voltage drop, usually <1V (similar to a diode's voltage drop). The rest of the voltage would be across the resistor.

RIIIIGHT! Thanks! :D It's a headache trying to visualise this stuff sometimes.
 

duke37

Jan 9, 2011
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I thought that schottky digital circuits have a diode to stop the collector going well below the base and keep the transistor out of saturation to get faster switching.
 

Arouse1973

Adam
Dec 18, 2013
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Greetings,

Just wanted to be sure about this: Is it ok to have an NPN transistor's base voltage higher then the collector? It's for a simple logic level shift (9V - 5V). And the inversion doesnt matter (so no need for a second transistor)

Do you not just need a transistor switch?. See attached.

Thanks Adam
 

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