Transistors / hfe / Active region conflicts

Why does it seem the active region has a constant collector current for all base currents? What is it about the characteristic curves at university that is different from the ones used at BTEC? Please post the images.

 

You want to know why the collector of a BJT in the active region acts like a current generator? That is an easy question to answer. The Vbe brings the charge carriers into the base region where they are swept away into the collector circuit by the collector voltage. A small percentage of the charge carriers from the emitter go into the base circuit as waste current, but the majority of the charges form the collector current. So it is the Vbe voltage that controls the availability of the of the charge carriers that go into the collector circuit. That is why cranking up the collector voltage does not increase the collector circuit. If there are no more charges in the base for the increased collector voltage to attract, then you have a change in voltage causing no change in current. That is the definition of a current source--constant current despite the voltage change. It is all a matter of transistor physics.

Ratch

 

You have to understand that a BJT operating in the active region without any external base, emitter, or collector resistance is a transconductance amplifying device. That is, a change in Vbe voltage controls a change in Ic collector current. In other words, a BJT by itself is a voltage controlled current source. You can get philosophical and poetical about how a current source changes its resistance or distributes its voltage to keep its current output constant, but a current source just is what it is.

Ratch

 

Of course they are the same graph, but of different components. The first graph gives the characteristic curves of an idealized transistor whereas the second graph gives the characteristic curves of a real transistor, where the current gain h.FE varies with the collector-emitter voltage and h.FE varies with the collector current. One can draw a load line on the second graph just as on the first.

Also note that while the BJT operates as a transconductance device (collector current controlled by base voltage), it is not characterized as a transconductance device. That is because the transconductance effect is highly non-linear and quite temperature sensitive thereby making Vbe a rather useless quantity to consider except as a diode forward voltage drop (~0.6V). Real circuits are designed to generate the bias point Vbe by forcing a controlled current to flow through the base-emitter junction. Perhaps this is why transistors used in circuits are thought of as being current-controlled even though a transconductance transistor model may be be used for small-signal analysis. In that case it is delta-Vbe and not Vbe that is the relevant quantity.

 

Neither of the above graphs are really graphs of the device alone. Whenever you drive a transistor with a current source, you are inserting an external resistor from the current source into the base circuit. This external resistor is not part of the device. What you now have is a current amplifier or current controlled current source (CCCS) circuit, not a device. That is fine if you know what is going on, namely that a transconductance device and been converted into a current amplifier by the addition of lots of resistance to the input.

Ratch

 

Whenever you drive the base of a transistor with a voltage source, you are inserting an external resistor from the voltage source into the base circuit. The better the fidelity of the voltage source, the closer this external resistor (i.e., source internal resistance) approaches zero. As this external resistor approaches zero it becomes indistinguishable from a short circuit.

Whenever you drive the base of a transistor with a current source, you are inserting an external resistor from a voltage source into the base circuit. The better the fidelity of the current source, the closer this external resistor approaches infinity. As this external resistor approaches infinity it becomes indistinguishable from an open circuit. By definition an open circuit cannot be part of the transistor device. For an ideal current source, it is as though base current magically appears from nowhere.

So consider which scenario becomes a part of the transistor device, a short-circuit (voltage source) or an open-circuit (current source).

 

A transistor always functions "normally", unless you apply too much voltage or do not limit its current and burn it out. Even within a circuit, it functions as a transconductance amplifier, just as an op-amp functions as a dual high amplification voltage amplifier even when it is externally wired to be a current amplification circuit.

Your impression is wrong. Ib is an indicator of Ic, not a control of Ic. It is a wastage current that cannot be avoided. You need to read my explanation of what Vbe does in post #75 of this link. https://www.electronicspoint.com/th...stor-works-base-current-version.269901/page-4 , It is hard to manufacture a BJT that "leaks" only a minimal amount base current. Both the base current and collector are exponentially related to Vbe, so Ic and Ib are somewhat linearly related to each other within a range. This relationship is called BETA.

Fabrication specialists have tried to manufacture transistors with extreme low base currents (very high betas), but found it to be impractical due to low lot yields and numerous out of specification parameters that occur due to the design methods needed to keep the current out of the base circuit..

Ratch

 

By definition, a voltage source has no internal resistance. In practice, it can be modeled as a ideal voltage source in series with a small resistor.

Not quite so. A short circuit differs in that it does not have a voltage source in series with it. Most voltage sources have an internal impedance that is insignificant compared with the bulk resistance of the transistor. For practical purposes, the internal resistance of the voltage source can be ignored.

Not quite so. An open circuit differs in that it does not have a current source in series with it. Most current sources have an internal impedance that overwhelms the impedance of the transistor. For practical purposes, the internal resistance of the current source can be considered infinite.

Not quite so. A current source is not the same as a open circuit unless the current is turned off. Base current does not appear by magic, it appears because the current source supplies the current.

Neither one does.

Ratch

 

The back voltage is caused by uncovered charges due to the diffusion process. If Vbe cancels some of this back voltage, the diffusion will increase and the depletion region will become narrower because a smaller depletion region corresponds to a lessor back voltage. I am referring to the emitter-base junction, not the base-collector junction.

Vbe does not affect the C-B depletion region. The Vcb does.

The Vbe and Vcb each have their own independent depletion regions.

Ratch

 

Yes, you are describing base width modulation or the Early effect. It is what makes the Vc-Ic curve have a slant instead of being horizontal. You can read all about it here and elsewhere. https://en.wikipedia.org/wiki/Early_effect

Ratch

 

So now let us return to your original statement:

A current source has an infinite resistance; when anything is connected to a device through an infinite resistance, the device is unable to discern that anything has been connected.

And this is quite true (from an external standpoint; while from the viewpoint of the transistor seeing an infinite resistance at its base terminal, it's still magic) but the point is that the base current is controlled while Vbe is not directly controlled by an applied voltage source. Vbe is free to do what it will do.

This would be true because the base-emitter junction is a forward biased diode where the I-V relationship obeys the ideal diode equation which gives an exponential relationship between Vbe and Ib. It is not possible to develop Vbe without a corresponding base current. Even though it is forbidden to apply a voltage source directly to the base-emitter junction, doing so would create a corresponding base current; it is the base current which controls Vbe because base-emitter voltage is never directly applied from a voltage source.

So there is the full story: base current controls Vbe, and Vbe controls collector current.

 

An infinite resistance source does not mean that the device cannot receive current. It means than the device will receive the same set amount of current no matter if the device changes its resistance, unless it changes its resistance to compare to the current source, like turning itself off. If the device were connected to a voltage source, it would be able to control its current by changing its resistance.

No magic involved. I already explained what seeing an infinite resistance by a device means to it. Vbe can be directly controlled by applying a voltage to the base-emitter terminals. However, that is not a good idea because of the nonlinear characteristics of the transistor's transconductance.

That is true. The waste current that comes out of the base circuit is impossible to stop. Too bad it cannot be shunted to the collector where it would do some good.

You can apply a voltage directly to the base-emitter junction even though it is not a good idea due to its nonlinear voltage-current relationship.

You got that backwards. The physics of the diode shows that voltage controls the diode current, not the other way around. The voltage lowers the back-voltage caused by the depletion region and allows more current to be present. Current does not control the voltage across the depletion region. Diode current is an indicator of the applied voltage, not its control. Only voltage can lower the back-voltage of a diode allowing more charge carriers to flow. Therefore voltage controls current in a junction diode, not vice versa.

No, Vbe controls both the base current and the collector current in an exponential relationship according to the physics of the transistor. That is why the collector and base currents are related to each other by BETA.

Ratch

 

Exactly! No matter what the device does, nothing changes. It's as though the device were connected to nothing.

Damnable Physics!

One can do many things which are forbidden by common sense, but that leads to perdition.

When the diode is driven by a current source the current is set by the current source, and Vbe becomes whatever value it must to accommodate that current because the current source controls the base current.

The value of Vbe is totally irrelevant to the design process of setting the transistor bias current, which is why Vbe is assumed to be a standard value. If Vbe controlled the base current, it would lead to perdition.