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Undestanding BJT datasheet.

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alphacat

New Member
Hey guys,

I'm trying to understand the terms specified in a BJT's datasheet, and to link it with the what i was taught in university.

- Base-emitter saturation voltage - VBE(sat).
What does that mean?

- Base-emitter voltage - VBE(on).
What is the difference between VBE(on) and VBE(sat)?

Thank you.
 
Last edited:

mneary

New Member
- Base-emitter saturation voltage - VBE(sat).
What does that mean?
This is the base voltage when the BJT is saturated (Vce < Vbe).

- Base-emitter voltage - VBE(on).
What is the difference between VBE(on) and VBE(sat)?
This is the base voltage when the BJT is not saturated (Vce > Vbe). Note that a Vce is specified.
 

alphacat

New Member
Thank you.

Why do you define saturation as VCE<VBE?
Isnt the BJT saturated when VCE<VCE_SAT?

for VCE_SAT=1V & VBE=0.7V,
VCE could be larger than VBE - for example, VCE=0.8V - and the BJT would still be saturated.
 

audioguru

Well-Known Member
Most Helpful Member
Look at the datasheet of any transistor like the 2N3904 for example.
With a collector current of 10mA and a base current of 1mA then its typical (as shown on the graph) collector saturation voltage is less than 0.05V and its base-emitter voltage is typically 0.77V. So the collector voltage is much less than its base voltage.

The definition of a saturated transistor is when the base-collector diode is forward-biased so the base voltage must be one diode drop higher than the collector voltage.
 

alphacat

New Member
Yeah, well:
VCE = VBE - VBC.

for VBC = 0.1V, the Base-collector junction is still not forward-biased, therefore the transistor isnt saturated.
But you still get
VCE = VBE - 0.1V < VBE
 

audioguru

Well-Known Member
Most Helpful Member
The collector voltage of a saturated transistor is a diode voltage drop less than its base voltage.
 

alphacat

New Member
Oh sorry, i got you now.

I still think that you're missing a detail here.
You say that a saturated BJT has VBC ~ 0.7V.

Well, a BJT can also be reversed-biased - meaning:
VBC ~ 0.7V
VBE ~ 0V or < 0V

But according to your definition a reversed biased BJT is saturated.
 

Karkas

Member
Vbc ~ 0,7V, means collector junction forward biased, and Vbe ~ 0V means that the emitter junction is reverse biased, that region is named Reverse Active.

-If collector junction & emitter junction are both forward biased, then it is saturated.
-If collector junction is reverse biased and emitter junction is forward biased, the transistor is active (you can see it when you assume in a circuit that the transistor is not cut-off, you assume Vbe=0,7 for an NPN).
-If collector junction & emitter junction are both reverse biased then the transistor is cut-off (Ie=Ib=Ic=0A).

Now what you mentoned Vbe ~ 0V means that emitter junction is not forward biased yet, then: If emitter junction is reverse biased and collector junction is forward biased, then it is Reverse Active. It is not used (at least that's what I've been taught) because in that region hfe is way smaller than in forward active or just active.
 

mneary

New Member
Sometimes I use a transistor in Reverse Active mode for audio muting. The HFE doesn't matter much because I can just increase the base current.
 

Karkas

Member
Hi mneary I would like to see how do you use it. Do you have some schematic of that? ro can you just tell me how do you do it?

Thanks.
 

mneary

New Member
It was for a very crude automatic muting circuit, a click eliminator on the 'insert' jack of an audio mixer. The audio input was about 100mV, and I applied it through a 1k resistor to the collector of a 2N3906. I then hooked the collector to my output terminal.

When I wanted to kill the audio, I applied about 5mA to the base.

It's much more effective to use a small MOSFET (2N7000 etc.) but I had to come up with this overnight.
 

Karkas

Member
Nice idea mneary, thanks for the explanation. Maybe someday I use it.
 

alphacat

New Member
Thank you very much fellas.

You know, the datasheet sometimes only gives you VBE(sat) and VCE(sat), meaning they dont give any details about VBC(sat).

Therefore, the rule i set to myself was that a transistor is saturated when:
VBE is in the VBE(sat) range.
VCE < VCE(sat).

Is it correct?
 

MrAl

Well-Known Member
Most Helpful Member
Hi there,

The exact point where the transistor enters saturation is a little fuzzy.
You have to realize that what we are attempting to do as humans is
determine something about a physical device from it's electrical
characteristics and so there is a tiny bit of abstraction there and
that may cause a problem in determining when it actually enters a
different mode of operation.
The way i like to think about it is that saturation starts when the gain
starts to drop significantly as the base current increases and causes
more collector current and lower Vce, but that's just an intuitive
way of looking at it.
The way some spice programs work is they calculate two quantities
from the forward and reverse emission coefficients and the thermal
temperature and then compare these two quantities to both the
base emitter voltage Vbe and the base collector voltage Vbc in
order to determine if the transistor is in the active region, saturation
region, or neither.
To get a better idea about this you should probably look into the
calculation of bipolar transistors in spice programs.

This reminds me of looking at the current through an inductor with an
iron based core and trying to determine when the inductor can be called
'saturated'. Then we end up talking with phrases like when it actually
"enters saturation" and when it is "fully saturated" and stuff like that.
What happens is that it sometimes partly depends on the application
it is being used in. For example, if 10 amps is the 'normal' current
and we see 11 amps, do we call that saturated? I doubt it, but we may
say that it is "starting to enter saturation", and if let go like that it may
cause a problem. On the other hand, if we see a huge spike of current
(which brings to mind a derivative) we would probably say that it became
saturated and something has to be done about this design to prevent this
or else the driver transistor might overheat and burn up.

If you look at the transistor base current and collector to emitter voltage
you will see a point where increasing base current starts to have much less
effect than it did before that point, and that might be said that that is where the
transistor begins to enter saturation.
 

alphacat

New Member
Thank you very much for that awesome explanation, it really helps me understanding the saturation resion intuitively. :)
 

audioguru

Well-Known Member
Most Helpful Member
There is no reason to look at Vbc if you know the Vbe and the Vce which are on the datasheet.
Vbc is simply the Vbe minus the Vce.
 

MrAl

Well-Known Member
Most Helpful Member
There is no reason to look at Vbc if you know the Vbe and the Vce which are on the datasheet.
Vbc is simply the Vbe minus the Vce.

Hi audio,


Yes, and when i open a window for some fresh air ventilation in the summer
months i always calculate how far it has opened by subtracting the part that
is still closed from the height of the whole window :)

Seriously though, programmatically it is either:

"if Q1>Vbc then"

or:

"if Q1>(Vbe-Vce)"


But then again sometimes Vbc is a directly measured quantity.
 
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