So I'm reading up on transistor basics:
"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
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
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
meant to say I^2 R power (not heat) losses.
take care of your units, and your units will take care of you...
Michael
For sure.
Because most bjts are used in the linear region I suppose ?
Graham
--
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
JF
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.
NO. That will make NO DIFFERENCE.
Graham
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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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.
-- http://webpages.charter.net/jamie_5"
At WHAT RATIO of Ib/Ic ???????
You DO NOT know what you're talking about as usual.
Graham
Stop wasting my time ignoramus, you're way out of your league.
A bipolar transistor doesn't have a fixed resistance any more than a diode does. Under ordinary operating conditions, as you gradually adjust the diode 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!
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 | | | | | V = 0.7 + I*R |/ .----------| | |>
| | | | | | /+\\ | ( ) .-. \\-/ | | R | | | | '-' | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05
The thing you can depend on is that the NPN transistor has a voltage drop of about 0.7V from base to emitter under most load situations.
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
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.
MOSFETs act a lot like resistors when they're conducting, so you _can_ specify an R_DS without lying.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
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.
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
Which gets interesting when Ic = 0
John
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