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what happens if emitter-base & base-collector junctions are forward biased with 0.7V?

dexterdev

Feb 6, 2012
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I have a doubt regarding BJT. what happens if both emitter-base & base-collector junctions are forward biased with 0.7V ? will BJT be in saturation region? What will be the voltage across the collector and emitter and Ic ?

-Devanand T
 

jackorocko

Apr 4, 2010
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Didn't you read that thread I linked you to? Seeing as how you have two PN junctions together but in opposite directions I don't think both junctions will be forward biased.

In saturation mode, the voltage drop across the C-E junction is the resistance * current. The current will be the Hfe * B-E current @ a specific voltage. If you look at the datasheet they graph this because it varies.
 
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BobK

Jan 5, 2010
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Well, you could do it. Just connect the base of and NPN to positive supply and both the emitter and collector to ground. In this case you would be shorting the supply with two parallel diodes, and your transistor will not last long.

In a normal configuration, the voltage on the collector is going to be a little higher than that of the emitter even at saturation (Vcesat) and they will not both be forward biased by 0.7V. I guess the is why Vcesat is never zero, get it?

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Vce(sat) is 0 for Ic = 0mA ;) (well kinda)
 

Merlin3189

Aug 4, 2011
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I tend to agree with Dexterdev, I too have a doubt regarding what happens if both emitter-base & base-collector junctions are forward biased with 0.7V
I remember with early transistors (OC70 type) it was ok to reverse the emitter and collecter. The gain was usually lower, but they worked. This is still keeping one fwd biased & one reverse.
But say you grounded the base and led both the E and C to the + power rail via suitable resistors. Would variation of the emitter current have any effect on the collecter current / Voltge (or vice versa)? Certainly there should be some effect due to the common base resistance simply being a resistance. But would there be any other effect? It seems unlikely, but maybe not impossible?
 

Resqueline

Jul 31, 2009
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Reversing c & e is a different matter, and can be done on silicon transistors too - with a lower gain as a result.
Shorting c & e could lead to a more or less good current tracking, and could have a theoretical interest.
Shorting c & b is done on one of the (matched) transistors in a current mirror setup.
 

jackorocko

Apr 4, 2010
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I tend to agree with Dexterdev, I too have a doubt regarding what happens if both emitter-base & base-collector junctions are forward biased with 0.7V
I remember with early transistors (OC70 type) it was ok to reverse the emitter and collecter. The gain was usually lower, but they worked. This is still keeping one fwd biased & one reverse.
But say you grounded the base and led both the E and C to the + power rail via suitable resistors. Would variation of the emitter current have any effect on the collecter current / Voltge (or vice versa)? Certainly there should be some effect due to the common base resistance simply being a resistance. But would there be any other effect? It seems unlikely, but maybe not impossible?

From his other thread I do not think he was asking the question as you have understood it. Can both junctions be forward biased, possibly but will it work like a typical transistor, I doubt it. Far as I know he was looking to understand how a transistor is supposed to work. Not how you can improperly use one. But whatever...
 
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