Over the years, I have locked horns with a lot of folks over whether a BJT is current-controlled (CC) or voltage-controlled (VC). I believe I can argumentatively pound anyone into the ground who thinks that a BJT is a current-controlled device.
Here are some of the points folks make when they aver that a BJT is current-controlled.
1) Look at this Ebers-Moll model of a BJT that uses current-dependent sources.
2) Ic changes when Ib changes in a somewhat linear fashion within a current range.
3) It depends on your point of view or how you preceive it.
4) It is simple (or easier) to explain how a BJT operates if current-control is assumed.
5) Have been designing with BJTs all my life, and current-control works fine for me.
6) It says so in the transistor manual, or textbook, or literature that the manufacturer puts out. They should know, shouldn't they?
7) Look at this circuit, it is obvious that it is a current amplifier.
There are other points that have been made and I invite you all to challenge me if you can think of any. The following are my answers to the above points.
1) Models show what a device does, not how it works. The makeup of the model can have no relationship with the physics of the device.
2) When my bedside clock shows 5:30 AM, the sun rises. Do I infer that the clock is controlling the sun, or is it celestial orbital mechanics? So why do people think that Ib controls Ic instead of considering the possibility that Ib is an indicator of Ic?
3) The way something works is never a perception issue. Arguing about whether the brake drums or the the tire friction with the pavement stops a car is not considering the system as a whole.
4) In this case, the explanation is false. Even if it appears plausible.
5) Design is one thing, knowing how a BJT really works is another. Ib is a good indicator of Ic, so it should be used in design.
6) Often they are talking about a current amplifier transistor model. They seldom go into the physics of a transistor.
7) You can make a current amplifier circuit from a voltage amplifier or vice versa. Then you are talking about the circuit and not the device.
Here are my reasons for what I say. First of all, the correct way to determine whether a device is a CC or VC to understand its physics. A BJT is a diffusion device. It does not depend directly on a voltage produced electric field to drive the charge carriers into the base of the BJT like a copper wire does. When a transistor is first manufactured, the charge carriers from the P-type and N-type material diffuse into each others material. This stops when a depletion region (charge carriers significantly reduced) forms from the holes and electrons annihilating each other. A barrier voltage forms on each side of the boundaries of the region that repels further charge carriers from entering the depletion region. All this happens without any voltage being applied across the PN junction. If a forward voltage is applied, that lowers the barrier voltage, and more charge carriers can diffuse into the base region from the depletion region. So the Vbe voltage controls the flow of charge carriers into the base. In a BJT, the forward bias of the base-collector sweeps the charge carriers from the base into the collector circuit. Therefore, the Vbe controls both the collector current and base current. The diffusion process also explains why the voltage to current function is exponential instead of linear. You cannot explain the above process by current sources. The physics will not allow it. So the BJT is a VC current source or transconductance device, not a CC current source.
Ratch
Here are some of the points folks make when they aver that a BJT is current-controlled.
1) Look at this Ebers-Moll model of a BJT that uses current-dependent sources.
2) Ic changes when Ib changes in a somewhat linear fashion within a current range.
3) It depends on your point of view or how you preceive it.
4) It is simple (or easier) to explain how a BJT operates if current-control is assumed.
5) Have been designing with BJTs all my life, and current-control works fine for me.
6) It says so in the transistor manual, or textbook, or literature that the manufacturer puts out. They should know, shouldn't they?
7) Look at this circuit, it is obvious that it is a current amplifier.
There are other points that have been made and I invite you all to challenge me if you can think of any. The following are my answers to the above points.
1) Models show what a device does, not how it works. The makeup of the model can have no relationship with the physics of the device.
2) When my bedside clock shows 5:30 AM, the sun rises. Do I infer that the clock is controlling the sun, or is it celestial orbital mechanics? So why do people think that Ib controls Ic instead of considering the possibility that Ib is an indicator of Ic?
3) The way something works is never a perception issue. Arguing about whether the brake drums or the the tire friction with the pavement stops a car is not considering the system as a whole.
4) In this case, the explanation is false. Even if it appears plausible.
5) Design is one thing, knowing how a BJT really works is another. Ib is a good indicator of Ic, so it should be used in design.
6) Often they are talking about a current amplifier transistor model. They seldom go into the physics of a transistor.
7) You can make a current amplifier circuit from a voltage amplifier or vice versa. Then you are talking about the circuit and not the device.
Here are my reasons for what I say. First of all, the correct way to determine whether a device is a CC or VC to understand its physics. A BJT is a diffusion device. It does not depend directly on a voltage produced electric field to drive the charge carriers into the base of the BJT like a copper wire does. When a transistor is first manufactured, the charge carriers from the P-type and N-type material diffuse into each others material. This stops when a depletion region (charge carriers significantly reduced) forms from the holes and electrons annihilating each other. A barrier voltage forms on each side of the boundaries of the region that repels further charge carriers from entering the depletion region. All this happens without any voltage being applied across the PN junction. If a forward voltage is applied, that lowers the barrier voltage, and more charge carriers can diffuse into the base region from the depletion region. So the Vbe voltage controls the flow of charge carriers into the base. In a BJT, the forward bias of the base-collector sweeps the charge carriers from the base into the collector circuit. Therefore, the Vbe controls both the collector current and base current. The diffusion process also explains why the voltage to current function is exponential instead of linear. You cannot explain the above process by current sources. The physics will not allow it. So the BJT is a VC current source or transconductance device, not a CC current source.
Ratch