Vbi is the barrier voltage and is constant, whether a source is connected or not. The source will not reduce the barrier voltage, it will reduce the junction voltage Vj = Vbi -Vbe. Vj = Vbi only when Vbe = 0. I don't have the same Sedra book you have.
Barrier or built-in voltage is fine. The junction voltage is the result of subtracting Vbe from Vbi. We appear to agree on that.
MrAl,
Hot flash! Increasing the collector current too much is what brings a transistor into saturation. Saturation occurs when Ib no longer controls Ic. A zero Ic is the cutoff point. How come no one but I pointed that out to you?
Were you trying to infer something about causal control of a BJT when it is not in the active region?
Ratch
We are definitely not on the same page here. I made it clear that i was talking about increasing the collector current to force the transistor OUT of saturation. I made it clear with the example of the oscillators that come out of sat due to increased collector current when the magnetics saturate and draw a much greater current. The transistor pulls abruptly out of sat and the collector voltage rises sharply. That is coming OUT OF SATURATION, certainly not going into saturation, which you for some reason want to talk about even though i made it clear how the operation was happening. No one else point this out because it doesnt happen that way. I cant imagine what the heck you are talking about there. Perhaps a typo or something?
Well now we come to another interface: Active or not active? Perhaps you would like to define that too and want to restrict our discussion further to only part of the transistor operating area where it's only considered in the 'active' region. Let me guess, you want to define this area at the place where the transistor looks like it is Vbe controlled?
Heck, from now on i say that we no longer allow the transistor to operate over the full range of possibilities. From now on we have to restrict the base emitter operation to 10ma to 10.1ma and the collector current from 100ma to 101ma. So you see everyone here and everyone in the past is totally wrong about the transistor...it's really just a big fat resistor! That's it
The point is, if we restrict the operation to what we want it to be we can make it look like whatever we want it to look like. Ii think it's a big zener diode, because i can force it to operate with a fixed collector voltage over a reasonable temperature rangeYeah that's it, the transistor is really a big fat zener diode in disguise And to think we all thought that the transistor could amplify, ha.
BTW, if i remember right when we increase Ic the Vbe does increase, in the active mode. Can we really say that Vbe is controlling anything then? If i remember right, this component is kT/q*ln(Ic/Ic0) where Ic0 is collector current at T0 and Vbe0, and k is the Boltzmann constant, q is the charge on one electron, T is the temperature in K, Ic is collector current.
One more little question. What would happen if we used the photoelectric effect to generate a current to control the transistor. That would mean pushing electrons into a higher energy band using photons. Thus, a current would flow without the need for a potential voltage difference.
Still Ratchet, i will certainly consider what you are pushing if only you could provide some experiment that will conclusively show that what you say is true. Relax and have a cup of coffee, take a step back, sit down, design a little experiment...that's all it takes. When a scientist wants to prove something he says is true he designs an experiment that proves it one way or the other. We cant do this with everything in the universe, but we should certainly be able to do this with something we can put our hands on like a transistor.
MrAl,
The fact that a transistor comes closer to saturation when its collector current increases is not in doubt. The only way to bring it out of saturation is to lower its current, or increase the reverse bias of the base-collector. This link directly contradicts what you say. **broken link removed**
From your comment, I can tell that you are not in possession of a good basic transistor textbook. This is basic stuff. The following link lists the 5 regions of BJT operation. Look in the "Regions of Operation" section of the link. Notice that they say that saturation facilitates high current conduction. You can expand your knowledge from there. Yes, I do want to confine this discussion to the active region. Can you think of why? Ans: Because you cannot control the Ic with Ib in the saturation region of operation.
<snip>
Ratch
I'm sorry but that link you provided does not talk about the conditions i had set up for that experiment where the collector current was doing all the controlling. That link is out of context for that discussion because it talks about changing Vbe again and that is NOT what the experiment is made to do. For that experiment we DO NOT CHANGE Vbe.
It appears that your hands on experience may be a little limited here since you didnt understand how that oscillator works. If you understood you would have known immediately what i was talking about as others here knew right away.
If you understood you would have known immediately what i was talking about as others here knew right away.
Using that circuit as an illustration, perhaps you can tell me how Q2 turns off, that is, how does Q2 begin to turn off, what makes it start to turn off.
Ratchit, nothing in the references you cite even remotely state that Vbe is "causal". Repeating it an infinitum does not make it true. Ic is simply Ie - Ib, but Vbe has a functional relation w/ all 3. You've been presented w/ the fact that Ib/Ie as well as Ic change before Vbe does. Your response is that it does not matter that Vbe lags the currents, it is still the cause variable. In all scientitfic as well as non-scientific debates, as soon a variable is shown to lag behind another, it is automatically ruled out as "causal". If you refuse to acknowledge that a cause must always precede its effect, then what is the point of continuing? Obviously if the timing of events does not dissuade you, then nothing well.
I stated earlier that your assertion of Vbe being causal is "dogma". Your response was to invoke the "Galileo persecution complex". I hate to inform you, butI, as well as Brownout & Mr. Al are the "Galileo's" in this issue, & you are not accepting logic because it weakens your strongly held convictions. Every person on both sides of any issue can claim the galileo complex. Because Galileo was opposed by the majority, & you are opposed by the majority, does it logically follow that since Galileo was right, you must be as well? Galileo? You take yourself way too seriously.
In Galileo's day, the majority was farmers & tradesmen. Little industry existed & lay people were not schooled in science as they are today. Galileo's concepts were counter-intuitive to farmers, coachmen, milkmaid, blacksmmiths, actors, musicians, doctors, midwives, etc. The industrial revolution has taken place since then. The people whom you are at odds with are degreed, & working scientists/engineers. I have but 3 patents, w/ 2 pending. I have 2 published papers. That in & of itself does not make me right by any stretch. It is just being mentioned to remind you, that unlike Galileo's critics, I am not a milkmaid, coachmen, actor, etc.
You are opposing highly skilled people from National Semiconductor, Texas Instruments, Fairchild, On Semiconductor, etc. THese people are much more qualified than Galileo's critics. Equating yourself w/ Galileo is absurd & ridiculous to say the least. If I asserted that the moon is made of green cheese, & you call me crazy, then I respond with "Galileo was called crazy as well", does that vindicate me?! Invoking Galileo is a joke.
Regarding what voltage is, it is work per unit charge transporting said charge from point a to b. It is impossible to move charges through a p-n jcn w/o affecting both current & voltage. As I've stated before, both are involved & important to know. But both cannot be the control quantity. One is forced or controlled, the other is consequential, albeit equally important.
The Baker clamp illustrates that a clamped & unclamped bjt are driven to saturation. Then when the base bias is forced to zero, there is a delay in turn off. The unclamped bjt takes longer to cut off due to stored charge. Charge control details this property, but VC does not, nor does CC. Since voltage & current are both related to charge, you can force a relationship based on I or V. But the quantity of Q determines the recovery time. This is equally true w/ diodes. QC is the best model for this internal action. It hasn't changes in more than half a century. My most recent product design involves a 10 hp 3-phase brushless dc motor controller, using bjt, FET, & IGBT as well as diodes. The design of the hardware cannot be done w/o the QC info from the OEM data sheet. Neither CC nor VC is adequate to develop the hardware.
Hopefully no soon to be qualified Electrical Engineers are following this thread.
Ratchit is simply posting to try and confuse the known facts about Semi Conductor junctions which have worked for yonks for qualified Electrical Engineers who have gone on to design literally Millions of successful products.
Ignore Ratchit. He is simply baiting. Let him be.
He has his own world to live in afterall.
MrAl,
Yes, the link does talk about the conditions from the transistors point of view. Your last answer to me was the first time you published the circuit. Vbe does if fact change and direct the Ic before it goes into saturation. It also cuts off the collector current when in saturation. You are changing Vbe indirectly.
How would you know what my experience is, and whether I understand how it works?
And if you published the circuit the first time, I would have known what you were talking about. Just because many did not comment on it does not mean they understood it.
Sure, easily. Q2 switches back and forth from saturation to cutoff. When in saturation, it shunts the current from the LED, turning it off. When Q2 is cutoff, the current passes through the LED and it turns on. Q2 is biased to saturate by the collector current from Q1 passing through Q2's base circuit. While the current in Q2 is building up toward saturation, the current in the coil L2 is also increasing and building up a magnetic field. When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1. This turns off Q1 and thereby Q2, because Q2 doesn't have any more base current. This continues until the coil's magnetic field finishes collapsing, and cannot sustain any more positive voltage to the base of Q1, at which time the cycle repeats itself. So you see, the transistor Q2 comes out of saturation because its collector current is cut off by its base current being cut off. Not because its collector current is increasing.
Ratch
How would you know what my experience is, and whether I understand how it works?
I always like a drawing too, which is why i asked you for one. Now you see why i wanted a drawing so bad, because it sometimes conveys information that is very hard to get across by simply chatting. Still, it may not help that much anyway but if you like you can do a drawing at some point.
I also have to comment on your patience again, because i can see you have quite a bit there, and i am happy about that so we can continue our discussion without getting all upset about some theory or another. So i have to thank you again for your patience, and you have to realize that im a bit older now and not as sharp as when i was back in school so it may take more to show me what you mean then it did say when i was back then.
In the case of this very circuit however, which i posted as an example where Vbe control would be VERY hard to claim, i think you need to take another look at the action of the transistor when it starts to turn off. This action is paramount to my illustration, to this experiment and what exactly it was meant to convey.
Quoting you from above if you dont mind, just to get to the heart of the matter:
"When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1."
Well, that would be nice i guess if it happened that way, but i am sorry to say that that's not the way it happens. If the saturation current 'stabilized' the magnetic field would never change, as in an electromagnet. So what would make you think the field would collapse all by itself, ie what is the mechanism that forces the field to collapse especially if the current somehow stabilized? The coil is not AC coupled BTW, but is solidly DC coupled as the schematic shows.
Well I think that it is another way around. When Q2 is first "ON" is in saturation and with time during the increase of current in Q2. Q2 "switch" from saturation to active region. And thanks to C1 an this "switch" to active region Q1 and Q2 start to cut-off.After saturation is reached, the current increase stops, and the coil stops opposing the B+ voltage. So the saturation current is constant, the voltage across the coil is zero
MrAl,
A circuit diagram is a specialized drawing that would not help in describing my viewpoint.
I think you get rusty on things you have not looked at for quite a while, but your cognitive skills should not deteriorate until you get very old. Also you now have more experience.
You are right, I got ahead of myself, so let me clarify. The coil is not DC coupled to Q1 due to the capacitor linking the two.
During the increase of current in Q2, the magnetic field builds up, and the coil opposes the B+ voltage. After saturation is reached, the current increase stops, and the coil stops opposing the B+ voltage. So the saturation current is constant, the voltage across the coil is zero, the magnetic field is present, and the base of Q1 receives a positive voltage, which cuts off Q1 and Q2. This stops the existence of the current in Q2 and L1, and that will cause the magnetic field to collapse and sustain a positive voltage on Q1 for a time.
Ratch
Well, I dont know what makes you think that the saturation current has to stop rising. With the transistor CE shorted out and Resr of the inductor zero, the current would rise to an infinite level.
BTW the circuits that Jony posted illustrate the same basic principle of more or less Vbe control vs Ic control.
vbi is not the barrier voltage. The source reduces the barrier voltage, just as Sedra and Smith says. It's in the book, you only have to read it. Also see Gray and Mayer.
Also, the reduction in barrier voltage is equal to vbe, This might seem trival, but the ramification is if you control the barrier voltage, then you control vbe. And Sedra and Smith shows that current controls barrier voltage by neutralizing charge in the depletion region. If you need an equation, vbe = vt*ln(Ie/Is), also given in S&S
It becomes apparent after reading that lame circuit description. You got almost nothing right.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?