Or the lowered voltage allows more charge to flow because there is less voltage to oppose the diffusion voltage.
Thanks for correcting my spelling.crutschow,
Compliment is the correct word. But I took your remark in the best possible light.
Ratch
A good example of a functional current-operated device is a bipolar junction transistor (BJT). The solid-state physics theory is that the base-emitter voltage is what determines the collector-emitter current. But since the input impedance of a BJT looks like a forward biased diode and thus has a very low impedance, the BJT acts much like a current operated device with the collector-emitter current related to the base-emitter current by the current gain (beta or Hfe). Thus functionally the device acts like a current-operated device with a relatively low input impedance, at least for large-signal operation (switching or bias-point calculations). Thus you must always have a resistor in series with the base to limit the base current with a BJT (for a common-emitter circuit).Kavan here. Do you mind if I ask you what it means "functionally"?
I am still trying to get the distinction between voltage and current operated devices. Both types seem to work if a voltage is applied to them. I also understand the difference between linear and nonlinear devices, where the current and voltage are not proportional: a change in voltage causes a nonlinear change in current...
still I don't get the difference between voltage and current operated devices. What do you mean that some devices are "functionally" current operated?
Hi Claude,
I think you may be talking more fundamentally then we want to get into for this discussion, if i understand you right. In which case we seem to need a push before we can get anything to move. Nothing moves on it's own unless it is first pushed from some external force or had been pushed on in the past and is now in motion because of that past (perhaps very distant past ) push.
It's true that when we push something it usually moves at least a tiny bit, so we could say that the push and the move have to be simultaneous. But theoretically it is possible to push something and not move it at all, if it is considered entirely rigid. So it either moves or it doesnt, but if it doesnt, then we can definitely say that the push came before the move because we've observed things being pushed that do not move, and it's entirely feasible that someone else was pushing the other way and when they let go it moves. So the original push came first, then the movement. So do we think of the case where the push and the movement occur at the same time as a separate case? That's possible i guess, but it seems more reasonable to think that in order to move something it requires a push first even if there is no delay. Because there is no delay it's still arguable i guess, but to me it seems more reasonable to look at it as push comes first.
If we want to make charge move, we've got to provide a push. We've at least got to provide something else that had a different charge distribution if we are not to push directly. That means we had to 'push' something else first even if it wasnt the charge itself, but the charge holder.
Ultimately, the field has to be set up first before anything moves, and the only way to set up the field is to push the charge into place or allow the charge holder's field influence to reach the target.
Ratch right here is the source of your confusion. Vbe is NOT the external voltage "applied". First of all we never apply Vbe directly to a constant voltage source, CVS! Vbe does not make charges move. We need an example. Sue, a singer, uses a mic, connected to an amp. The 1st bjt in the preamp stage will serve as our discussion example device. Sue imparts acoustic energy from her lips. This energy impinges on the mic diaphragm resulting in generated I & V. The signal propagates through the mic cable at finite speed. When it reaches the b-e junction, what happens? The charges are already flowing, Ib is already established. The Vbe at the junction is there because the bjt device is biased from a dc network to establish a good quiescent point for the bjt.
The dc Ib/Ie consists of charges transiting through the junction, forming a depletion zone, DZ, resulting in a barrier, Vbe. Does Vbe "control" Ib? We must examine the actions at the junction to determine what controls what, knowing that it's possible neither Ib nor Vbe nor Ie is the quantity ultimately in control. Sue's mic adds small signal charge carriers, the ac current will be denoted in lower case, "ib/ie", with voltage signal as "vbe".
At the b-e junction we have a fixed dc bias Ie/Vbe/Ib. When the signal carriers transit through the mic cable & arrive at junction, what happens? Since the charge carriers moving through said junction increase, as we now have Ib+ib, Ie+ie, the no. of accumulated charge carriers in the DZ also increase. With more carriers present & finite lifetime, there is an increase in excess stored charge in the DZ. Hence the barrier potential increases. Thus vbe is a direct consequence of ib/ie, that is the increase in ib/ie forced vbe to increase. Similarly, a reduction in ib/ie produces a reduction in vbe.
Of course, it is true that to a very limited degree, vbe has a 2nd order influence on ib/ie. If the external source providing the drive for the network is constant voltage, such as a generator, Hall sensor terminated in a high impedance, the current is determined by (Vg-vbe)/R. Let's say a generator outputs 1.00 volt, the junction drops 0.650 volt, with a resistance of 1.00 kohm. Current is 0.350V/1.00kohm = 0.350 mA. If the generator output increases to 1.10 volts, the initial current is (1.10V-0.650V)/1.00kohm = 0.45 mA. But when the charges arrive at the junction, the DZ changes, & the forward voltage drop increases to 0.655 V. So when equilibrium is reached, the current is now (1.10V-0,655V)/1.00kohm = 0.445 mA. So Vd(Vbe, whatever), does have a slight 2nd order influence on the current.
But in this example, the generator output voltage was of the same order of magnitude as the junction forward voltage drop. If the generator outputs 10 volts, the junction drop has less influence. If the generator was constant current source, then Vd/Vbe has no influence at all.
When a junction diode or bjt b-e is driven by a voltage source through a resistance, then the current is related to Vg-Vbe. But an effective topology reduces errors due to Vbe as well as temp, etc. An effective biasing scheme uses either current sources/sinks so that Vbe has little influence, or a voltage source plus resistor scheme where the voltage supply rails are way larger than Vbe. If the supply rails are tens of volts, and Vbe incremental changes are tens of millivolts, then Vbe has only a slight influence on current.
So, Ratch, regarding Vbe having an influence on Ib/Ie (& ultimately Ic as well), via (Vg - Vbe)/R, I concede on that point w/o an argument. Take an LED powered from a +5V source & 330 ohm resistor, with a forward drop, Vd, of 1.80V. Id = (5.00V - 1.80V)/330 ohm = 9.70 mA. Say an ambient temp increase occurs, & Vd drops to 1.70V, then Id = (5.00V - 1.70V)/330 ohm = 10.0 mA. A 5.56% drop in Vd resulted in a 3.0% increase in Id. But if the source is +12V instead of +5V, with R=1.0 kohm, we get:
Id = (12.0V - 1.8V)/1.0kohm = 10.2 mA. Then temp increase forces Vd to 1.7V: Id = (12.0V - 1.7V)/1.0kohm = 10.3 mA. So the same 5.56% decrease in Vd resulted in only a 1.0% increase in Id. No doubt, when a p-n junction is driven by a voltage source plus series resistor, the forward drop Vd (or Vbe for a bjt) definitely has a SLIGHT influence over I. But well designed circuits minimize the influence of Vd over Id. A constant current drive is immune to changes in Vd as long as Vd does not reach values too large for the current source to comply with.
On that 1 point Ratch, we agree. But do the numbers & you will see that V exerts a very minimal influence on I. BR to all.
Hi Ratch,
what do you think about this explanation by J. Beaty? does it make sense?
http://amasci.com/amateur/transis.html
thanks
kavan
Just remember, models tell you what the device will do, but it does not tell you how or why it works the way it does. For that, you need to dig into the physics of the device.
You keep repeating that in every debate you are involved with.. and mostly everybody agrees with that. Physics will explain in detail how the device works.
But why can't you agree that the detailed physics are not important in practical use. You keep pushing the physics over the models.
And you keep arguing against the models also. The models tell an engineer how to use the device, not the physics. If I'm using a microcontroller, I need to know the architecture; the memories and instruction set etc. I don't need to know how the silicon chip was made and how a single transistor or memory cell works in physical level.
If you try to increase the collector current by applying a 'voltage' to the base, you find that the diode equation says that I and V are so inextricably related that you can't just 'apply' a voltage because the diode, in order to do this, requires current as well. In point of fact, you can do it the other way around, you can apply a current and not yet see the voltage rise until the input capacitance has been also satisfied. Once the voltage does rise, then we see a change in collector current, but the voltage didnt get there by magic...it took significant current to get there, and the current has to be maintained in order to keep the collector current going.
So the basic junction requires both current and voltage.
Another way to think about it...how can we accomplish 'anything' without at least some energy (power)? Voltage or current alone do not constitute energy, it takes both. We can set up a magnetic field with current, but if that magnetic field accomplishes anything physical it's going to take voltage too.
Yes you can say that, but then when you say "physics" you're talking about the PN junction itself, not about the diode as a whole. So you're still applying some physics yet ignoring other physics.
A better illustration maybe...
You have a diode with a PN junction aboard a spacecraft 1 light year away from Earth. Assuming a radio signal travels at the speed of light, how long does it take to forward bias a PN junction from the ground base on Earth (by pressing a button which energizes a circuit which sends out the signal that eventually reaches the spacecraft). The answer is about 1 year. Now why didnt the voltage of the button affect the PN junction immediately? Because we had other physics to consider. And this 'other' physics we had to consider was not too trivial to the application, it was paramount. It took a whole year vs microseconds on Earth.
So it appears that you want to isolate the discussion to focus on the PN junction alone, the theory behind it. That's ok i guess, but it is a little removed from reality because we always have other things to consider when we talk about an *actual* physical device. As im sure you know, we have inductance and capacitance across and though all distances, no matter how short. And we also have resistance too. So we've always got time constants to think about.
Also, is there any proof that voltage alone can perform some function in a solid state device like a PN junction?
Once the voltage gets there it's too late, because then we've already seen the power dissipated and that event is over.
Once the event is over we cant say that the voltage did it because that would be leaving out what happened just before that, which is also part of the picture. The 'theory' will ignore this physical aspect because theories are there to do just that, ignore some things and concentrate on other things so that our understanding can come more immediately. If we had to concentrate on voltage and current at the same time it would be a more difficult learning process, and would also impede some design procedures.
I tried to give another example removed from the present discussion, based on magnetics. If we generate a magnetic field we need current but not voltage, but as soon as that magnetic field starts to do anything we can call physical (like pick something up or push something) then some energy has to be expended. We can say that the current caused the magnetic field and maintains the magnetic field, but if the magnetic field operated on something it would require at least some voltage otherwise we'd have an efficiency of over 100 percent. For example, say we have a magnetic field generated by a current and that field squeezes a conducting strip made of semi conductive foam tighter and tighter. As the foam compresses the conductance goes up more and more. Thus, a current flows through the strip from end to end that increases with the magnetic field....
So it appears that you are talking about PN junction theory, removed from an actual device, but the actual device doesnt behave that way. And even with the PN junction alone some energy has to be applied to get anything to happen therefore it can not be solely controlled by voltage. The only thing controlled solely by voltage is the theory
BTW, if anything was truly controlled by voltage alone, then that same voltage would be able to control an infinite number of said devices.
MrAl,
Yes, I am concentrating on the junction itself, not the lead connections or the bulk resistance.
That sounds like sophistry to me.
These "other" things are irrelevant or insignificant with respect to what I am I am saying with respect to voltage control.
Yes, obviously voltage causes current to exist in a junction diode.
Who cares if energy is used or dissipated? The voltage caused the event to happen.
That sounds more like a philosophical argument than a scientific one.
I am not going to expand the discussion into the physics of semiconductor foam and superconductors, where I would have to study the phyics of each of those materials.
I beg to differ. Any breadboard will show that a junction device does behave in accordance with Schlockey's equation and semiconductor theory. The fact that energy is used does not abrogate the fact of voltage control.
Yes, and the point is?
Ratch
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