Dave,
Good question. Voltage. Even if you switched on a constant current source, the voltage would have to be there. Even if you modulated the light with a current, the voltage would still have to be there.
Majority carriers come from the emitter. That is why it is called the emitter. The base current is not involved in this process. The base current is a sideshow caused by Vbe. Vbe is needed to change the wide of the depletion region, which in turn controls how many carriers get to the collector.
My beliefs are based on facts.
I have published my insights on this. In this thread. Ratch
Yes, I have many BJTs around. Why do you ask such a bad question?Originally Posted by Ratch
You can talk all you want about how a BJT is designed and manufactured, but after it is made, Vbe controls its collector current.
Answered by Dave
Have you ever used a BJT in a real application? Suggesting such a proposition, I can only suggest that you have not.
Let me ask a simple question: voltage source connected to a light - is the light controlled (controlled being the act of being turned on) by the voltage of the source or current through the bulb?
Good question. Voltage. Even if you switched on a constant current source, the voltage would have to be there. Even if you modulated the light with a current, the voltage would still have to be there.
No, not directly. The final control will be Vbe. That is what will determine the collector current.So if the base current is modified by a current source (i.e. a current source to base), then a current will be controlling Vbe? That'll be current-control then won't it.
Many people are beguiled by the fact that Ic is proportional to Ib, and it is true that you are changing Vbe by changing Ib. But it is the Vbe that is controlling the collector current. In an automotive braking system, the driver controls the pressure on the brake piston, the piston controls the force on the brake shoes, and the brake shoes press against the rotor to produce friction. So who is controlling the braking? Some people say it is the driver, but I would say it is the brake shoes.Consider a BJT with the base driven by a switch thrown resistor, R, tied to Vdd. Emitter to ground and collector driving a load tied to Vdd (which for arguments sake is greater than twice the forward voltage of the b-e junction). For a given value of R Ib will be determined by the resistor. Charge flow to the base (as you would prefer it be called) will then force Vbe to ~0.6V to forward bias the b-e junction (in fact the potential divider created by throwing the switch causes charge to flow into the base which in turn creates the potential divider across the b-e junction). Currents in the circuit, including Ic, will change accordingly. If R is changed to a different value, the charge flow to the base will increase/decrease accordingly and will force Vbe to ~0.6V - by virtue of the dependence of Ib on Vbe and vice versa it will force Vbe to a slightly different value as changes on Ib are reflected on Vbe. The fundamental point is that the control mechanism is provided by the current to the base which sets the circuits behaviour including, Vbe and Ic. No-one denies the fact that the BJT is voltage-controlled, implicated by the characteristic of Vbe, but it is equally current controlled. Mathematics and intuition support support this idea, as does the theoretical and practical analysis of many experts in this field.
I don't deny that Ib is a function of Vbe. I am saying that Ib is unwanted and unnecessary. Notice how Ib would go to zero if you could make a transistor with infinite beta.Originally Posted by Ratch
If somehow the base current did not exist, as it does not in low, low, low Vbe values, then the emitter still provides current to the collector. I keep trying to say that the base current is a side effect and not necessary to the control of the collector current. If the width of the base could be modulated physically, there would still be a waste base current because of the Vbe present. The presence of base current does not prove or indicate base current control.
Answered by Dave
I'm sorry but you are wrong. The base-current is a function of Vbe:
Ib is a function of Vbe: no Vbe - no Ib, no Ib where there is no Vbe (looking at this from a purely control perspective, i.e. ignoring thermal effects and so forth).
If you ignore the need to replenish majority-carriers in the base, then you can deduce that the base-current is superfluous. However, you cannot ignore this function of the base-current.
Majority carriers come from the emitter. That is why it is called the emitter. The base current is not involved in this process. The base current is a sideshow caused by Vbe. Vbe is needed to change the wide of the depletion region, which in turn controls how many carriers get to the collector.
Current control using Ib occurs because Vbe is present. Vbe is the dog that wags the tail. Ib is always present in a BJT, but it does not directly control the collector current even if it is proportional to it. I never said that a BJT cannot be controlled by current. I acknowedged that the Ib-Ic proportionality is useful for design. I only said that the real final response of the collector current is from Vbe. Perhaps I should have said the BJT is intrinsically voltage responsive.You clearly don't want to accept the point that voltage-control through Vbe and current control through Ib are one and the same thing; how they differ is in their implementation. This is a notion supported by all the leading academics and practitioners on this subject. If you don't want to "believe" the BJT can be current-controlled (and I say "believe", because of in the face of overwhelming evidence, theory, and experimental citations to suggest to the contrary, and in the absence of any forthcoming experimental evidence on your part to disprove these, it necessitates a belief-system), then it is not my place to try and convince you differently.
My beliefs are based on facts.
I have published my insights on this. In this thread. Ratch