Transistor conduction

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
I agree Ron....I should have taken my readings with respect to ground which is the conventional way lol

i'm going to start from scratch with this and do my measurements from ground, maybe it will just click, the light will come on so to speak LOL

Neil.
 

ronv

Joined Nov 12, 2008
3,770
I agree Ron....I should have taken my readings with respect to ground which is the conventional way lol

i'm going to start from scratch with this and do my measurements from ground, maybe it will just click, the light will come on so to speak LOL

Neil.
Transistors (BJT) are current controlled devices. Look at what is happening to the base current when the resistor is changed in the chart I posted.
 

WBahn

Joined Mar 31, 2012
25,247
Usually measurements are made with respect to ground. Having them taken from the positive of the battery makes me want to stand on my head or do math in it - neither one is a good idea for me.:D
So I took the liberty of redoing your chart:
View attachment 106119
If you look at the transistor and think of the base emitter junction as a diode it will always be on if the base is more positive than the emitter by about a diode drop. So for the transistor to be off the base emitter voltage needs to be less than about .5 volts.
This refactoring of the data still doesn't make a lot of sense. If Vc is the transistor collector voltage relative to ground, then how can it be 0V (as opposed to limiting at Vcesat)?

I think we need to TS to very clearly indicate just how the measurements were made or to redo them (and specify just how the measurements were made).
 

ronv

Joined Nov 12, 2008
3,770
This refactoring of the data still doesn't make a lot of sense. If Vc is the transistor collector voltage relative to ground, then how can it be 0V (as opposed to limiting at Vcesat)?

I think we need to TS to very clearly indicate just how the measurements were made or to redo them (and specify just how the measurements were made).
It's probably like .04 volts and just doesn't show on that scale.
 

WBahn

Joined Mar 31, 2012
25,247
But why am I getting such a large change in the voltage across the collector resistor as I increase the base resistor value when nothing else is changing.....my VBE hasn't changed noticeably neither has the voltage across the base resistor.
But as you increase the value of the base resistor you get less base current for the same amount of voltage across it. Less base current means less collector current (in the active region). I suspect that part of the reason you are not seeing much change in the base voltage is because you are in a region where there isn't going to be much change in order to get the difference in collector current. For instance, in going from 680 kΩ to 820 kΩ the collector current appears to have gone from 481 uA to 440 uA. That a change of (481 uA)/(440 uA) = 1.093 which is 38.7 millidecades (where one decade is a factor of ten). At 60 mV/decade, the nominal change in Vbe would only be about 2 mV.

Another factor is that I suspect you are seeing the influence of the presence of your meter on the measurement being made. What is the input resistance of your meter on the voltage range you are using?
 

WBahn

Joined Mar 31, 2012
25,247
He can look at any number of sources to find the answers. It sounds like he is trying to determine them himself so that he learns as much as he can from actually making the measurements -- which will also teach him quite a bit about making measurements.
 

#12

Joined Nov 30, 2010
18,190
I can hope he's trying to learn how to measure, but some of his questions are easily answered by looking at my graph. Some of the questions to which I refer are in post #32. Something about needing to make the base 0.0 volts to make this unknown transistor, "fully shut off" and 0.7 volts to make it, "fully conducting". The graph shows that it's all a continuum. There is no specific voltage (Vbe) which will always cause saturation or always cause cut-off. Those voltages are always dependent on range of collector current you're working with, and that was explained 30 posts before he asked the questions.

Not reading the answers persuades me to stop providing them.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
The transistor I am using is 2n2222.......

Yes I can find all the answeres online I expect but I am having fun working with the circuit and finding out what does what.

AAC Fanatic, if you don't want to contribute anymore to my thread, I wont be offended.

Neil.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
Sure wish he would look at that blog I linked in post #2 and see that I did this experiment across 6 decades of current and the answers are all there.
The information in that link is way too scientific for me to understand.........
 
Last edited:

Sinus23

Joined Sep 7, 2013
245
Hi Neil. I took the liberty and simulated your circuit in post #8 and could not get anything close to the results that you got. And could not figure out how you could have measured those values by accident. But today I thought I'd give it another shot. After some thought I put a 1k instead of the 10k collector resistor and then I got very similar results.

Could that be the case?
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
My posts in #3 and #13 were not. They gave the simplest concept - track the currents as well as the voltages. All will be revealed.
I was actually referring to the link in post #2, I am almost fresh at this game and have no college experience, I am trying to learn some concepts from the ground up so I need simple explanations.

When I get more time I am going to redo this experiment.

thank you for all your help guys.

Neil.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
So I did the experiment again and clearly I screwed up somewhere the first time around as my results are a lot more like I was expecting and now I have some numbers to put into a chart once I figure out how to do that in the spread sheet.....

The main differences between the two experiments are I used a 5v supply instead of 9v and I took my readings referenced at ground instead of the positive supply rail.

The results in blue are my original from the first time round and the red ones are from the second time around this morning, thanks for your time on this thread everyone.

transistor fun.JPG
 

BobTPH

Joined Jun 5, 2013
2,260
Between what 2 points are you measuring Rb voltage and Rc voltage?

Are you using a little analog meter that is something like 1K Ohms / volt?

Bob
 

WBahn

Joined Mar 31, 2012
25,247
Your numbers for "Rb voltage" don't make any sense. Your Vbe numbers look fairly reasonable. But I can't figure out any place that you could be making your Rb voltage measurement that would reasonably produce those results. Here's an idea -- instead of making use guess where you are making this measurement, why don't you let us in on the secret by just telling us?

You need to make it very clear where your measurements are being made. Saying, "Rb voltage" strongly implies that it is the voltage across Rb, but you said that you took all your measurement relative to "ground" -- which you have not indicated which node is "ground" and are leaving it up to the reader to read your mind and make assumptions about what you did.

If you are going to talk about "ground" you need to indicate which node is "ground" -- the usual way to do this is to put a "ground" or better a "common" symbol on the node you are using as a reference. I've added a common symbol for you. I've also added explicit indications of what V_RB and V_RC should mean. Note that to make these measurements, you place the black lead of your meter at the '-' mark and the red lead at the '+' mark for the associated voltage. These measurements are NOT referred to ground. This is generally the better approach when you want a measurement that is the voltage across something.


Edit_2016-05-15_1.png

By Kirchhoff's Voltage Law, your measurements should satisfy the equation

V_RB + Vbe = Vcc.

Also, as I and others have asked, it would sure be nice to know what meter you are using. At least give us a make and model. Also, tell us what range you have it set to.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
WBahn I disagree with your theory that Vcc should be made from Vbe + Vrb since Rc comes into play somewhere I would think? although I could be wrong....I didn't measure Vrc on my second test but maybe that voltage added to the Vbe and Vrb voltage = the Vcc supply?

I am using a craftsman digital multimeter 82082....love this little meter!

Yes the negative is what I am referring to as ground, I am using a bench power supply, not a battery.
 
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