transistor saturation

Main Page | Report this Page

Science Forum Index » Electronics - Basics Forum » transistor saturation

Page 2 of 2 Goto page Previous 1, 2

Author

Message

John Popelish

Posted: Wed Apr 23, 2008 8:18 am

Guest

Tim Wescott wrote:

Quote:

mrdarrett@gmail.com wrote:
On Apr 15, 10:41 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com
wrote:
(snip)
Why are R_CE values not standard on datasheets though?

Probably because a bipolar junction transistor in saturation doesn't
really act like a resistor, so characterizing it with an R_CE would be
misleading.

In reality, I think bipolar junction transistors do act
quite a bit like resistors, collector to emitter (with a
small DC offset in series, perhaps) when the collector to
emitter voltage drop is below the base to emitter voltage in
magnitude. And more base current lowers the value of that
resistance and the resistance holds fairly constant even as
the collector to emitter voltage passes through zero.

Quote:

MOSFETs act a lot like resistors when they're conducting, so you _can_
specify an R_DS without lying.

MOSFETS also show a resistive effect whenever the drain to
source voltage is less than the gate to source voltage in
magnitude. But since the gate to source voltage tends to be
a lot larger than the base to emitter voltage where the
resistive effect occurs, the resistive mode of a MOSFET is
usable over a larger voltage range than the resistive mode
of the bipolar transistor.

--
Regards,

John Popelish

John Larkin

Posted: Wed Apr 23, 2008 8:49 am

Guest

On Tue, 15 Apr 2008 15:35:02 -0500, John Fields
<jfields@austininstruments.com> wrote:

Quote:

On Tue, 15 Apr 2008 10:06:22 -0700 (PDT), mrdarrett@gmail.com wrote:

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.

---
They give you Vce(sat) at one, or several currents and from that you
can calculate the resistance at those currents:

Vce(sat)
Rce(sat) = ---------
Ic

Which gets interesting when Ic = 0

John

Page 2 of 2 Goto page Previous 1, 2 All times are GMT - 5 Hours The time now is Sun Jul 27, 2008 3:26 am Science Forum Index