transistor saturation

Discussion in 'Electronic Basics' started by [email protected], Apr 15, 2008.

1. Guest

So I'm reading up on transistor basics:

http://www.kpsec.freeuk.com/trancirc.htm

"A transistor that is full on (with R_CE = 0) is said to be
'saturated'."

Is saturated R_CE really ever 0, though? I mean, 0.000 ohms?

My DMM routinely gives 0.1 ohm readings on bare wire, for instance.

What are typical REAL values for saturated R_CE? Say, for a TIP31A or
a 2N3055?

The datasheets don't seem to give these saturated R_CE values,
interestingly enough.

Thanks,

Michael

2. EeyoreGuest

NO.

For starters there's the intrinsic resistance of the semiconductor
material. You can NEVER get below that. That's a big issue for low noise
too.

Graham

3. Guest

I had a feeling that's why they can get hot... I^2 R heat losses...

Why are R_CE values not standard on datasheets though?

Michael

4. Guest

<maximally_pedantic_mode_on>
meant to say I^2 R power (not heat) losses.

take care of your units, and your units will take care of you...
<maximally_pedantic_mode_off>

Michael

5. EeyoreGuest

For sure.

Because most bjts are used in the linear region I suppose ?

Graham

6. JamieGuest

Look harder on the data sheet.
Saturation means it's at the lowest R level it can reach per that
component.
The spec's will tell you the voltage across the CE at that point.
from there you can use ohms law to find the actual R and what ever
else you need.

http://webpages.charter.net/jamie_5"

7. EeyoreGuest

NO. That will make NO DIFFERENCE.

Graham

8. EeyoreGuest

At what ratio of Ib/Ic ?

You're talking out of your bottom as usual. There is no simple definition of
saturation, it has to be specified by Vce and Ic and Ib/Ic. All THREE need
to be specified.

Graham

9. JamieGuest

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
as usually, you're doing your norm. Looking at guys asses!
Just don't get me going!.
And I'll say it again for the other reader, "LOOK HARDER" the
spec's are in the data sheet that will yield maximum saturation.

10. EeyoreGuest

At WHAT RATIO of Ib/Ic ???????

You DO NOT know what you're talking about as usual.

Graham

11. JamieGuest

Stop wasting my time ignoramus, you're way out of

http://webpages.charter.net/jamie_5"

A bipolar transistor doesn't have a fixed resistance any more than a
diode does.
current, the voltage across it changes much less than does
"resistance," defined as voltage divided by current. So with a diode
you graph voltage versus current. Nobody talks about resistance.
A bjt has diode junctions in it.
If you're looking for a measurement that will stay fairly steady for a
bjt, you are better off looking at the collector-emitter voltage
drop. Even that will vary as you change the current. But get some
numbers from a datasheet and do a little arithmetic and decide for
yourself whether it makes more sense to talk about "resistance" or
voltage across a bjt.
If you want a semiconductor that has an operating region where it acts
like a resistor, check out a mosfet. But be aware that for any given
gate voltage, you have to keep the drain current below a certain
maximum value or the device will stop acting like a resistor and start
acting like a constant-current source.
It's all very interesting, you could study this stuff for years.
Now get started, ya bum!

13. Bob MonsenGuest

In response to mdarrett, the way to ensure that a bjt takes a particular
current is to use a resistor from emitter to rail. So, say you want about
10mA. you do this:

I |
V
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V = 0.7 + I*R |/
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( ) .-.
\-/ | | R
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=== ===
GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The thing you can depend on is that the NPN transistor has a voltage drop of

So, you use that fact to make the current be a (relatively) linear function
of base voltage.

You never try to figure out what the actual current is going to be, give a
base voltage or current, since it depends on too many factors, like
manufacturing variance, temperature, etc. Also, the current is an
exponential function of voltage, so it is very hard to get that voltage just
right so the transistor passes the right current.

Now, there are exceptions to every rule except one (this one).

Regards,
Bob Monsen