question about transistor

Thread Starter

TAKYMOUNIR

Joined Jun 23, 2008
352
i was doing troubleshooting for circuits and the transistor was bad because Vbe was about 3vd insteade of being .7v my question is why when Vbe is high or very low this mean that the transistor is bad
 

SgtWookie

Joined Jul 17, 2007
22,210
Transistors start conducting meaningful amounts of current when Vbe reaches around 0.6v, and by the time Vbe is around 1V or so, it's at around 60%-75% of it's maximum rated collector current.

Basically, the base-emitter junction acts like a forward-biased silicon diode. They usually don't start conducting a meaningful amount of current until around 0.55v or so, and they're near or at their maximum current at ~2v.

So, if a silicon transistor that is supposed to read between ~0.6v and ~2v from base to emitter reads outside of that range, it's either turned off deliberately (no current flowing through the base current limiting resistor), shorted if lower than ~0.6v, or open if higher than ~1.5v.
 

Thread Starter

TAKYMOUNIR

Joined Jun 23, 2008
352
Transistors start conducting meaningful amounts of current when Vbe reaches around 0.6v, and by the time Vbe is around 1V or so, it's at around 60%-75% of it's maximum rated collector current.

Basically, the base-emitter junction acts like a forward-biased silicon diode. They usually don't start conducting a meaningful amount of current until around 0.55v or so, and they're near or at their maximum current at ~2v.

So, if a silicon transistor that is supposed to read between ~0.6v and ~2v from base to emitter reads outside of that range, it's either turned off deliberately (no current flowing through the base current limiting resistor), shorted if lower than ~0.6v, or open if higher than ~1.5v.
please can you explain phsically why when vbe is high the transistor is open
thanks very much
 

newb7777

Joined Mar 6, 2011
2
We are talking BJT.

When Vbe is ~0.7V for a SILICON Transistor is ON state; (GERMINIUM will be lower)
Depending on the amount of current through the base usually limited by a BASE RESISTOR (Rb), the transistor could be fully turned ON thereby acting like a turned ON switch or
acting like an amplifier in the case where a controlled amount of current is flowing though it.
The relationship between base and collector current is given by the formula Ic=Ib*hfe valid for NPN BJT (note: hfe=beta from datasheet or vice versa beta=hfe)

Now if Vbe is far greater than ~0.7V then the base emitter junction is not a diode anymore!!!!!

Now if Vbe is far less than ~0.7V then transistor is shorted on the base emitter junction!!!!!

Hope that helps....... alternativly, you should try it on a breadboard take measurements with voltmeter then the concepts will be clear and you will be fully convinced. Would be good if you could jive it with some maths and make some sense of the datasheet for the specific device.
 
Last edited:

Wendy

Joined Mar 24, 2008
21,929
All good points.

Different makes of transistors act differently. If I were to see 1V or more on a 2N2222A BE (base emitter) I would declare the transistor bad. If I saw it on a 2N3055 I would take it for granted.

Both are NPN BJT transistors, but the 2N3055 is designed for much higher currents, so the excessive BE drop is normal, for it. Most NPN transistors are much lower current, so 0.7 is the norm (for a silicon diode).

As with ICs, transistors have datasheets. Datasheets are always your friend.

As described, Vbe is a diode, it acts like a diode, and it drops current like a diode. If the voltage there is high then the diode junction is damaged or open.
 
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