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he never said that it proves the transistor is voltage controlled
I have always thought a BJT was voltage controlled [...] but is still the voltage that is controlling the current.
he never said that it proves the transistor is voltage controlled
I have always thought a BJT was voltage controlled [...] but is still the voltage that is controlling the current.
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.
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.
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.No, you never think of current control.
But then you don't think of voltage control either in that example.
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For which purpose ? Beta does not help finding gm, does it?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.
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
View attachment 12450
Plot collector current versus applied voltage ramp from 0.1V to 5V.
View attachment 12451
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
+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.
* 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).
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 .
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
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 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
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