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This thread develops a twist... 
Whether to consider someting as current controlled or voltage controled for me has always bee a matter of practicality. One can (obviously) discuss this ad infinitum. Is it the current, that forces an electric field to build up across a structure (resistor, pn-junction...)? Or is it the voltage across a structure that forces a current to flow? If you can find I=f(V), you can find V=f -1 (I). Use whatever is convenient in your application.
Personally I'm more of a "Volts first" type since we can easily construct voltage sources and (almost) infinite resistances, but not so easily current sources and 0 Ω.
Whether to consider someting as current controlled or voltage controled for me has always bee a matter of practicality. One can (obviously) discuss this ad infinitum. Is it the current, that forces an electric field to build up across a structure (resistor, pn-junction...)? Or is it the voltage across a structure that forces a current to flow? If you can find I=f(V), you can find V=f -1 (I). Use whatever is convenient in your application.
Personally I'm more of a "Volts first" type since we can easily construct voltage sources and (almost) infinite resistances, but not so easily current sources and 0 Ω.
This thread develops a twist...
Whether to consider someting as current controlled or voltage controled for me has always bee a matter of practicality. One can (obviously) discuss this ad infinitum. Is it the current, that forces an electric field to build up across a structure (resistor, pn-junction...)? Or is it the voltage across a structure that forces a current to flow? If you can find I=f(V), you can find V=f -1 (I). Use whatever is convenient in your application.
Personally I'm more of a "Volts first" type since we can easily construct voltage sources and (almost) infinite resistances, but not so easily current sources and 0 Ω.
One can always invert a relationship mathematically and say that either one causes the other. But the true test of what causes what is the physics of the system. That is why I delved into the diffusion aspects of a BJT and averred that it was voltage controlled. I know of no current controlled device except a magnetic amplifier and a gas discharge tube.
[QUOTE="(*steve*), post: 1606138, member: 11517"
However there are distinct practical reasons to use a current control model for almost all BJT uses.
.[/QUOTE]
Here is - in short - the typical process of designing a common emitter amplifier:
* Choose a suitable DC operating point (Ic and Vce)
* Calculate corresponding resistors Rc and Re (negative feedback to cope for temperature and BJT uncertainties)
* Calculate two resistors for proper base DC biasing - based on a suitable voltage Vbe (0.65...0.7V).
For this calculation, of course, the DC base current Ib=Ic/beta is used (because it exists and its existence was never denied).
* For gain calculation we need the transconductance gm=Ic/Vt which is the slope of the Ic=f(Vbe) characteristic.
Question: At which step of this calculation process we have used a "current-control model"? Did we ever think about current-control?
Instead, we have used (we always must use!) the DC voltage Vbe and the slope of the Ic=f(Vbe) curve.
Additional remark: If we have a reasonable dc feedback and if we accept a small DC operating point error we even can neglect the base current during voltage divider calculation.
However there are distinct practical reasons to use a current control model for almost all BJT uses.
.[/QUOTE]
Here is - in short - the typical process of designing a common emitter amplifier:
* Choose a suitable DC operating point (Ic and Vce)
* Calculate corresponding resistors Rc and Re (negative feedback to cope for temperature and BJT uncertainties)
* Calculate two resistors for proper base DC biasing - based on a suitable voltage Vbe (0.65...0.7V).
For this calculation, of course, the DC base current Ib=Ic/beta is used (because it exists and its existence was never denied).
* For gain calculation we need the transconductance gm=Ic/Vt which is the slope of the Ic=f(Vbe) characteristic.
Question: At which step of this calculation process we have used a "current-control model"? Did we ever think about current-control?
Instead, we have used (we always must use!) the DC voltage Vbe and the slope of the Ic=f(Vbe) curve.
Additional remark: If we have a reasonable dc feedback and if we accept a small DC operating point error we even can neglect the base current during voltage divider calculation.
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No, you never think of current control.
But then you don't think of voltage control either in that example.
Sure you use the slope of the Ic=f(Vbe) curve. But most of us would simply look up the beta given in the datasheet.
What is beta again? I forget.
Oh yeah, IC/IB And that's voltage control right? Oh, no, that would require that you pull out the Eberd-Moll equations -- and even those are going to require the forward and reverse common emitter current gain.
Sure I did. I know - and I have used this knowledge - that a suitable voltage Vbe allows a current Ic that is stabilized by negative voltage feedback.
Exactly THIS is the point I do not understand: Everybody knows that he needs a voltage Vbe that allows a certain current Ic. But the cause of an Ic change should be a change of Ib ?
For which purpose ? Beta does not help finding gm, does it?
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This is my probably retarded view on all of this.
Without an electric field you can’t have a current which means you can’t get any energy. So this gives you an indication that voltage might have quite a big part to play in all this, considering the unit of volts is actually Joules/Coulomb which means to move any electrons around in an electric field requires energy.
This comes from source of energy we apply to say a PN junction diode. This energy has the quantity units Volts, so it is correct to say I have increased the voltage to a circuit which for a PN junction would result in an increase in current. So here we have I think a voltage controlling a current.
The fact that in a BJT this base current drags with it a much larger current from the collector (I know this is actually wrong) and joins up with the base current in the emitter is a function of that device.
The fact that a resistor is in the way makes no difference to me it's just allowing a more gradual increase of current through the device. I am still adjusting a voltage which is increasing the current in the transistor. And yeah thinking about it now the voltage across the PN junction must change but I suppose it's obvious isn't it. As we all know if you increase the voltage applied to a forward biased diode then the current will go up. This is also voltage control isn't it.
Example
Connect a 10K base resistor to a 2N222a
Plot collector current versus applied voltage ramp from 0.1V to 5V.
This might be wrong but I am quite comfortable with the knowledge than a transistor is a voltage controlled device. Even though I might not be able to explain it properly. It might be because I am stuck in this room all the time not meeting people, they don't let me out much.......
Adam
Without an electric field you can’t have a current which means you can’t get any energy. So this gives you an indication that voltage might have quite a big part to play in all this, considering the unit of volts is actually Joules/Coulomb which means to move any electrons around in an electric field requires energy.
This comes from source of energy we apply to say a PN junction diode. This energy has the quantity units Volts, so it is correct to say I have increased the voltage to a circuit which for a PN junction would result in an increase in current. So here we have I think a voltage controlling a current.
The fact that in a BJT this base current drags with it a much larger current from the collector (I know this is actually wrong) and joins up with the base current in the emitter is a function of that device.
The fact that a resistor is in the way makes no difference to me it's just allowing a more gradual increase of current through the device. I am still adjusting a voltage which is increasing the current in the transistor. And yeah thinking about it now the voltage across the PN junction must change but I suppose it's obvious isn't it. As we all know if you increase the voltage applied to a forward biased diode then the current will go up. This is also voltage control isn't it.
Example
Connect a 10K base resistor to a 2N222a
Plot collector current versus applied voltage ramp from 0.1V to 5V.
This might be wrong but I am quite comfortable with the knowledge than a transistor is a voltage controlled device. Even though I might not be able to explain it properly. It might be because I am stuck in this room all the time not meeting people, they don't let me out much.......
Adam
That is basically correct, but if you refer to current in a electric field, then you mean power. If you mean charge movement in an electric field, then that's energy.
Energy has units of volt-charge, not volts alone. Voltage is the energy density of the charge.
The fact that you can show a base-voltage relationship with Ic does not prove that a BJT is a voltage controlled unit. You have to also show by the physics of the device why this is so.
Everyone should read this short link featuring Winfield Hill, the man who wrote the book on electronics. http://cr4.globalspec.com/thread/68055/voltage-vs-current .
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OK - I have expressed my opinion and I do not want to repeat my arguments.
However, I have made up my mind a bit (yes - that´s possible from time to time) and I arrived at a rule that could be something like a "natural" rule:
I have the feeling (at the time being it is not more) that, in general, it will be impossible to directly control one physical quantity with another quantity of the same kind if the latter is smaller than the quantity to be controlled. I think, this could be a consequence of the law for energy conservation.
Examples: Water flow, air stream, mechanical force, air pressure, electrical current (!),....
I am very interested to hear what you forum members think of it. Perhaps you have counter-examples?
However, I have made up my mind a bit (yes - that´s possible from time to time) and I arrived at a rule that could be something like a "natural" rule:
I have the feeling (at the time being it is not more) that, in general, it will be impossible to directly control one physical quantity with another quantity of the same kind if the latter is smaller than the quantity to be controlled. I think, this could be a consequence of the law for energy conservation.
Examples: Water flow, air stream, mechanical force, air pressure, electrical current (!),....
I am very interested to hear what you forum members think of it. Perhaps you have counter-examples?
+1 on Steve's posts. I was trying to formulate how to say the same thing. Yes, the current in the base and the collector is controlled by a voltage (Vbe). But to a circuit designer this is, in most cases, useless. It is exactly the equivalent of trying to control the current through an LED by applying a fixed voltage.
When the Ib and Vbe are related by an equation, you can change either one to control the other. It turns out that changing the current to control the voltage is the much more useful model, primarily because Ic is linear WRT to Ib, but exponential WRT to Vbe.
Of course if you look at full Ebers Moll equations, the true situation is actually quite a bit more complicated. Vbc also has an effect on the relationship between Ib and Vbe.
Bob
When the Ib and Vbe are related by an equation, you can change either one to control the other. It turns out that changing the current to control the voltage is the much more useful model, primarily because Ic is linear WRT to Ib, but exponential WRT to Vbe.
Of course if you look at full Ebers Moll equations, the true situation is actually quite a bit more complicated. Vbc also has an effect on the relationship between Ib and Vbe.
Bob
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Useless?
Hello Bob - and what about the design steps I have listed in my post#65? Anything wrong?
I don't think we disagree, you just seem to want to label current control as voltage control.
From your post:
How is that not using a current control model? If you were using a voltage control model, it would read:
* Calculate two resistors for proper base DC biasing - based on a the exact voltage Vbe needed to set Ic. Remember to take into account Ib at that Vbe when calculating your voltage divider.
Bob
From your post:
How is that not using a current control model? If you were using a voltage control model, it would read:
* Calculate two resistors for proper base DC biasing - based on a the exact voltage Vbe needed to set Ic. Remember to take into account Ib at that Vbe when calculating your voltage divider.
Bob
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Hi Ratch
Yes I meant energy, you can't have power without energy either so Voltage is needed for all of them . I think I said that earlier volts is Joules/coulomb or more correctly joules per unit charge. It's interesting to note that an open circuited battery might have a rating of 9Volts (Electrostatic potential) but that's does not necessarily mean 9 Joules per coulomb does it? It might not have the ability to supply that much energy as it's internal resistance may be too high which is why voltage in this case is useless.
Adam
A joule/coulomb is not the same as a joule per unit charge. A joule per electron would be 6.24E18 joules per coulomb of electrons. Joules per unit charge is not a MKS unit.
Yes, a 9 volt battery does mean it has an energy density difference of 9 volts between its terminals. Whether it can keep up that energy density difference while under a load is a different matter.
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Hi Ratch
I thought unit charge was the coulomb. And so a joule per unit charge (Coulomb) is 1 joules for every coulomb. Is that not the same.
Thanks
Adam
I thought unit charge was the coulomb. And so a joule per unit charge (Coulomb) is 1 joules for every coulomb. Is that not the same.
Thanks
Adam
No, electrons and protons each carry a unit charge of opposite polarity. One coulomb contains 6.24E18 unit charges, and one unit charge is 1.60E-19 coulombs. http://en.wikipedia.org/wiki/Elementary_charge
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Oh ok thanks
So what is the correct term for voltage. God I thought I was reasonably competent at this, you take it to the next level. I see I have a lot more work to do on this. You see this is my hobby and also my job for the last twenty years but you just don't use this sort of thing every day.
While your here can you clear up what is the correct definition of volts per meter or V/m.
Adam
So what is the correct term for voltage. God I thought I was reasonably competent at this, you take it to the next level. I see I have a lot more work to do on this. You see this is my hobby and also my job for the last twenty years but you just don't use this sort of thing every day.
While your here can you clear up what is the correct definition of volts per meter or V/m.
Adam
Your last sentence would apply to 100% in case I could know the exact Vbe value which belongs to the desired current.. However, as we all know this is not the case and, therefore, we apply negative feedback in practice. It is easy to show that in this case, it practically does not matter if I use 0.65 volts or 0.7 volts for calculation.
Joules per coulomb is the correct MKS unit of voltage. Actually, "elementary charge" is a better term to use instead of unit charge. Someone could say "unit of charge", and you could wonder if he meant MKS units or whatever. The elementary charge unambiguously means the smallest atomic charge known, and as I said before, it is the value of charge for electrons and protons. Volts per meter is the electric field intensity. If you mathematically play with the units of volts per meter, you will also get an equivalent unit of force per elementary charge. The intensity and direction of an electric field is defined by the force and direction of a positive elementary charge. Millikan used this principle in his famous Oil Drop Experiment.
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