Why is a BJT considered "current-controlled"?

Question

With BJTs, we can control base current using Vin (from diagram). Why do textbooks state that BJTs are current controlled when it's obvious that changing the voltage controls the current through the collector?

Answer 1

In the above circuit Vin is controlling the current going to the base, not the voltage drop across the base and emitter of the transistor itself.

The voltage drop across Vbe will always be around 0.7V for Vin > 0.7; the excess voltage will be dropped across the R1.

By changing Vin, you are actually controlling the current going to the base based on the equation:

$$I_B = (Vin-0.7V)/R1$$

Answer 2

I reorganized and expanded this answer to give three reasons why the BJT can be considered "current-controlled":

1. The input characteristic makes current the better 'control variable' in most amplifying/switching applications.
2. In the simplest, piecewise linear model of the BJT, \$V_{BE}\$ is fixed and only current can be changed.
3. In all more sophisticated models, current and voltage are inseparable so you can see the device both as current-controlled and as voltage-controlled depending on convenience (but point 1 still stands).

But first, a (double) preamble.

Preamble

Are we supplying current or voltage?
Let's start with a little digression in basic circuit theory: what makes a generator a current generator instead of a voltage generator? Look at the V-I characteristics: the one with mostly constant current (almost horizontal in the V-I plane) will be called current generator; the one with mostly constant voltage (almost horizontal in the I-V plane, and almost vertical in the V-I plane) will be called a voltage generator.

_{Current and voltage generators characteristics on the VI plane}

What does this mean, in practice? That a current generator is able to supply a given current almost independently from the load (but the voltage across it will change accordingly to the value of the load resistance), while a voltage generator is able to supply a given voltage almost independently from the load (and in this case it's the current that will adapt to the load.)

The presence of an internal resistance kind of blurs the line between current and voltage generators, since depending on the scale of the horizontal and vertical axes we can show the characteristic as almost vertical or almost horizontal. In reality a linear generator with a non-zero internal resistance can be considered either type (and converted to the conjugate type by means of Thevenin's and Norton's transformations):

_{A non-ideal linear generator and its Thevenin and Norton equivalents}

Generally speaking, a current generator has a very large internal resistance (ideally infinite), while a voltage generator has a very small internal resistance (ideally zero); when the internal resistance value is finite, 'large' and 'small' are concepts relative to the magnitude of the load and the admissible intervals of voltage and current for the particular circuit the generator is part of.

The best 'controlling' variable
In generators, the 'accent' is on the mostly constant quantity (the voltage or current supplied), while the other quantity is variable depending on the load and the compliance of the generator. In a controlled device, on the other hand, the accent is on the variable quantity.
You can see this as a direct consequence of the propagation of errors: when you have a steep function, a small error in the almost constant quantity x will turn into a much bigger error in the widely varying quantity q (and vice versa).

_{Propagation of uncertainty from the independent variable to the dependent variable}

When there is uncertainty in the variables, it is better to chose as controlling variable the variable whose uncertainty will not be 'amplified'. It is basic error propagation theory (a good first textbook on this topics is "An introduction to error analysis", by Taylor). In the case of the BJT, given the exponential input characteristic (IB vs VBE) that leaves VBE almost constant on the range of admissable currents, it is current you would like to see as the controlling variable. The bottom line is that it's easier to distinguish between 10 and 40 microamperes (1 to 4 ratio) than it is to tell 0.66 and 0.70 V apart (1 to 1.06 ratio).

Before analyzing examples that show why it's better to resort to base current as a means to control the collector current, let's see how the simplest circuital model of the BJT exclusively allows current control.

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Simplified Device Model for the BJT

This is probably the first reason why the BJT is considered a current controlled device. This is the simplest model for the BJT, as shown by Chua, Desoer and Kuh in their "Linear and Nonlinear Circuits":

in the above picture all diodes are ideal (threshold voltage is zero, and so is series resistance; these are perfectly open circuits when reverse biased and perfect shorts when forward biased). E0 adds a threshold voltage to the input characteristic, while transistor action is expressed by ic = beta * ib (in the textbook's notation). Note that current-controlled current generator. Here are the corresponding input and output characteristics

Pretty simple, right? You can compare them with actual characteristics and see that they resemble them, though. Simple as it is, this is a legit model and can be used to model circuits like a common emitter one shown later where, by changing ib (you can't change Vbe in this model, since it's fixed) you modify the value of Ic. You can see how you can make ib change by intersecting the input characteristic with the input generator's load line By changing the voltage E1 (not part of the BJT) you change ib (part of the BJT) while vbe is constant at exactly E0. Then you can find the value of ic corresponding to that value of ib, select the corresponding output characteristic and find the voltage by intersection with the output load line.

Someone will jump on their seat screaming "WHAT? You are using \$\beta\$ to design an amplifier to be put into worldwide production for mission-critical nuclear applications? Also, where do you think \$\beta\$ come from? Moreover, don't you know that \$\beta\$ can change by as much as ninetynine gazillions percent just by looking at it in a funny way?"
The point is that for a given transistor you have a reasonably defined value of beta (you can measure it beforehand, so it does not matter if the production lot shows a shameful dispersion) and if you do not wander too far, you can reasonably ignore its variation with the other electrical parameters. Note that the one I just described is a simplified model that does not model variations of beta with temperature, current, or even hair color; it's a simplified model that catches the gist of transistor action, much in the same way as the sometimes reviled "transistor man" from The Art of Electronics.

Can you find the cutoff frequency of the transistor from this model? Nope. Can you explain the Early effect with this model? Nope. Can you account for the differential resistance of the B-E junction with this model? Nope. Can it account for charge pair production due to radiation? Nope. Can you account for second field quantization and the bending of spacetime? Nope.
Does this mean that this model is completely useless? Nope.
The extremely simplified behavior of this model shows why many textbooks state that BJTs are current controlled. The actual input characteristic resemble that vertical line where you can only vary ib, and not vbe, whose value is considered fixed.

You might want to compare this model with the simplest model for a MOSFET: page 151 of Chua has that one too.

As you can see, the gate current is fixed (at zero to be pedantic), a condition dual to that shown in the BJT: the I-V input characteristic is horizontal. The only control you have here is by means of vgs. Does this mean we are negating the existence of the tunnel effect? Nope, this is just a model. A simplified model that, among other things, does not consider tunneling but still manages to show why in a MOSFET you act on the gate-source voltage.

The other current control: \$I_C = \alpha I_E\$
So far we've seen how the (simplified) relationship between ib and ic can be seen as control of ic by means of ib, through beta. But we can also use alpha, why not? Let me quote, verbatim, another textbook that consider BJTs current controlled devices: "Quantum Physics of Atom, Molecules, Solids, Nuclei and Particles 2e", by Eisberg and Resnick, p. 474 (referring to a common base configuration shown on page 475):

The basic idea of transistor action is that a current in the emitter circuit controls a current in the collector circuit. More than 90% of the current through the emitter , so that the currents are of similar magnitudes. But the voltage across the base-collector can be very much greater than that across the emitter-base connection, because the former is reverse biased, so the power output in the collector circuit can be very much larger than the power input in the emitter circuit. Hence the transistor acts as a power amplifier.

Are these two gentlemen oblivious of the role played by quantum mechanics in the band theory of solids? Have they not heard of quantum statistics? Do they even know what a hole is (not to mention the tempco)? Could they have forgotten that applying voltages could modify the energy level profiles attributed to valence and conduction bands? I don't think so. They simply chose a simpler model to explain how one can interpret the so called transistor action.
Artist Bruno Munari once said: "To complicate is simple, to simplify is complicated. ... Everybody is able to complicate. Only a few can simplify". Among others, Chua, Desoer, Kuh, Eisberg, and Resnick chose to simplify.

So, at this point we can think that current control in the BJT is a consequence of an excessively simplified model.
But wait! There is more....

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Real devices with ideal generators: pure current and pure voltage control

Real transistors have non-discontinuous characteristics, and a BJT exhibit an exponential input characteristic (Vbe, Ib) and a logarithmic output characteristic (Vce, Ic). Let's see how the propagation of uncertainties plays out in a practical (albeit exaggerated) example. Let's focus on the input characteristic (Vbe, Ib) for \$V_{CE}=10V\$ in the following images:

_{Background image of "real BJT" characteristics taken from here (the set values of current and voltage have been chosen to match the example in the text, but the conjugate variable values are a bit off w.r.t. the actual simulation).
The key takeaway here is that current control with a lousy generator makes it possible to distinguish different states, while voltage control will lead right away to a superposition in all values.}

The thing to notice here is that when ib changes greatly, by 100s of percents, vbe changes by relatively small amounts, just a handful of percents. This is because of the exponential relation ib = I(vbe), or equivalently the logarithmic relation vbe = V(ib), of the B-E junction.

Let's say you want to use this BJT to produce a collector current of 10 mA on odd days and 15 mA on even days.
You tasked a German lab with accurately measuring the beta of the particular transistor in your possession and it came out as 250 over the range of interest (at the controlled temperature you intend to run the experiment). Since you've spent all of your money in the beta certification program, let's say you only have access to current and voltage generators with ideal internal resistances but marred by an accuracy of ±10% (yes, this is the part where I exaggerate).

If you use pure current control:
You can use

$$ I_C = \beta \, I_B $$

to find the value of ib that you have to set. A simple division will show that the nominal values of 10 and 15 mA of ic require nominal values of 40 and 60 uA for ib. Given the accuracy of your current generator, you will expect to see the following current ranges in input and output:

ib = 36-44 uA --> ic = 9-11 mA
ib = 54-66 uA --> ic = 13.5-16.5 mA

_{A bjt driven by an ideal current source}

If you use pure voltage control:
You can't use \$I_B = I_C/\beta\$, so you must specify a voltage that create a VBE of... Yes, of what?

$$V_{BE} = V_{Th} log(\frac{I_C}{I_S})$$

_{A BJT driven by an ideal voltage source}

You can either compute the logarithm, or use the full Ebers-Moll model to compute the value, or maybe ask the manufacturer to provide you with a suitable chart. But let's say you've determined it's precisely 0.705 and 0.719V (these values come from a simulation.)
When you set those precise values, your low-cost ±10% accurate generator will supply a voltage with the following uncertainties

0.6345 - 0.7755 V --> back to Ebers-Moll, to compute ic, ... too bad the uncertainty will be exponentiated...

0.6471 - 0.7909 V V --> no, wait, before computing...

...it appears we already have a superposition in the voltage ranges we are supplying: we might not be able to distinguish even days from odd ones.

The figure at the beginning of this subsection summarizes how current control is the preferred way to drive your transistor. With current control, even if we allow a ±10% error on the measured value of beta, we can still (barely, but still) make out the two ranges of current (9 - 11 mA vs 13.5 - 16.5 mA) corresponding to odd and even days.
With voltage control, if we add a ±10% error on the computed (or graphically deduced) value of voltage, you are already lost in VBE uncertainty. And if that wasn't enough, that uncertainty will also be amplified to an even worse uncertainty in the values of IB and IC.

At this point we might have reached the conclusion that current control in the BJT is the best way to deal with the propagation of uncertainty with the steep exponential characteristic.
But wait! There is more...

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Real devices with real generators: slanted or bent VI chars

In the real world, your generator will have a non-ideal internal resistance. This means that a voltage generator will show a nonzero series resistance (voltage will be dropped across it as soon as you draw a current), while a current generator will show a non-infinite parallel resistance (current will flow in it, as soon as a voltage is dropped across your load.)
In general real generators behave as good current or voltage generators only within the limits of their compliance. For example, a battery with very low internal resistance behaves as a good voltage generator for wide intervals of currents (until Rs * Iout becomes appreciable wrt Vs); a battery with a very high series resistance will behave as a reasonable good current generator because Rs will dominate Rs+Rload in the determination of the supplied current (i.e. the current will be almost independent on the value of the load resistance); a solar cell operating near its short circuit point behaves as an excellent current generator.
Let's drive a BJT of known beta with one of these real generators:

Example 1: battery + resistor
In our first example we will use a 3.3V voltage (!) generator and a base resistor (actually two base resistors to be determined: one for odd days, the other for even days). A 'voltage control guy' would use this components to set the controlling voltage between emitter and base, while a 'current control guy' would use it to set the controlling current into base. Our goal is to find the two values of RB that will result in IC = 10 mA, and IC = 15 mA for a BJT of known (measured, certified) beta of 250.

_{A BJT driven by a real source with a slanted characteristic line}

Current control guy's job is easy: the controlling variable is ib, so given the values of 40 and 80 uA (with the above errors) one can compute

$$R_B = \frac{V_{Batt} - V_{BE}}{I_C / \beta}$$

even by assuming VBE constant and equal to 0.7V (uncertainties are on my side). With a know beta of 250, the results are RB1 = 65 kohm, and RB2 = 43.3 kohm. Here are the simulations IC = 10.26 mA in the first case, and IC = 15.37 mA for the second one. I'd consider this a success in current control.

_{Setting Ic = 10mA and Ic = 15mA with a real world generator.
Is the input one-port behaving more like a voltage generator, or more like a current generator?}

I am not getting into the design procedure of the 'voltage control guy' (ask them how they arrive to determine the values with real components).

Irregardless on the procedure followed to 'design' the two circuits, let's see what the BE diode of the BJT sees. It's a small battery in series with a reasonably high resistance. I say this behaves more like a current generator than a voltage generator. If we plot the VI characteristic of the one port formed by Vbatt and RB (for RB = 65 kohm and RB = 43.3 kohm) on a range of typical base-emitter voltages (0.5 to 1.2 V) and base currents (0 to 200 uA) we see this:

_{Within these ranges, it looks like the BJT is driven by a reasonable approximation of a current generator}

If we superpose the load line represented by the exponential characteristic of the diode, we obtain a figure that is essentially the same as the one drawn for pure current control. Makes sense, since the large series resistance RB serves the purpose of turning the ideal voltage generator (battery) into something that resembles a current generator.

Example 2: solar cell
Can we get a better approximation of an ideal current generator? Well, yes: solar cells are excellent approximation of a current generator, as long as they operate in the correct part of their VI characteristics. These devices have a nonlinear output characteristic that result in a kind of hybrid behaviour. For example, Have a look at the V-I characteristic of the ancient LS222 model below:

_{Solar cell characteristics - adapted from Millman, "Electronic Devices and Circuits" (1967)}

Where it crosses the I axes it is essentially a horizontal constant current line, therefore it behaves very much like an ideal current generator. On the other hand, for high resistance loads, that is near the V axis on the bottom right, it looks more like a voltage generator with a high series resistance (a few hundred ohms.)

Let's try to replicate the 40 to 60 uA ideal constant current generator used to set I_C to 10 and 15 mA in the previous examples. We will have to keep the cell in minimal lighting condituon, and we will focus for brevity on a 50 uA value. Suppose we have only one cell, with Voc = 0.45 V and Isc = 50 uA. What is the operating point when we place it across the BE terminals of a BJT? We can try to determine it graphically: we draw the generator characteristic on the same VI plane with the BE diode characteristic.

_{BJT circuit powered by a single cell: bad operating point}

It's clear that this cell alone won't be operating in the constant current region, but it will instead appear as a weak voltage generator, incapable of driving the transistor as we wish.
But if we put two such cells in series, their combined characteristic will have the same short circuit current and double the open circuit voltage. Now the intersection of generator (solar cell) and load (the BE diode) will be in a better position, near the region where the current appear to be constant. If we use three cells in series, we achieve a reasonably stiff constant current generator: it will deliver 50 uA for a wide range of loads, including the load represented by the BJT base-emitter junction.

_{A double cell circuit results in a better operating point}

Now, this is not only a current generator from the point of view of the characteristic line in the VI plane, but it is also a current generator in that we set the current by setting the illumination of the cell. N photons per second striking the active area of the cell will result in a current of k N electrons per second flowing in the copper terminals cross section. The value of the voltage across the series, on the other hand, depends on the load. And when we have 50 uA supplied by a series of 3 cells, we get -essentially - the same current we would get simulating an ideal generator.
Change the illumination and with the number of photons per second, the number of electrons per second will vary as well, causing a different value of collector current. It's going to be hard to assert that the voltage across the BE junction is causing the correct number of photons per second per unit area to fly into the solar cell.

_{With three solar cells, the BJT is driven by a constant current generator with current determined by cell illumination}

I call this a practical example of almost ideal current control.
But wait! There is more...

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In reality we apply both current and voltage

Time to address the elephant in the room: the voltage/current religion war you might have witnessed. What is that all about? Some people point to the Ebers-Moll model, whose equations are usually expressed in the form of currents as a function of voltages, and say "see? the voltages determine the value of the currents, and this has to be a cause-effect relationship". Others go even a step further and state that "voltages cause currents and not the other way around".

I personally believe this point of view originates from the path usually followed when explaining transistors to students: at first it's the simplest model, where IC = beta IB and no math is required; then the exponential characteristic I = I(V) of the PN junction is explained (not mentioning that it can be expressed as a logarithmic relation V = V(I), as well), and then the Ebers-Moll model (in its I = I(V) form) is usually paraded as the ultimate transistor model where the truth lies. Furthermore, transistor are solid state devices whose inner working needs to be explained using the laws of quantum physics. Given the band structure of the energy levels of electric carriers in solids, it is natural to resort to energy levels to depict the inner workings of these devices. Energy and potential are closely related with each other, so most advanced models tend to express relevant quantities in function of potential (difference)s.

It is therefore easy, when following this path, to think that current control is a white lie told to first year students, while experienced and savvy engineers know Ebers-Moll and device physics, and it's clear to them that it's voltage that controls the currents in a BJT. Well, not so fast! For starters, the dependence on Vbe shown in the Ebers-Moll model is not implying a cause-effect relationship. It's just simpler to write the equations in that way. Nobody forbids you from using inverse functions. And this is not an opinion: the expression of the Ebers Moll model in the form V = V(I) can be found no less than in the original 1954 Ebers and Moll paper (and Millman's textbooks). And then there are charge control models of the BJT that go beyond voltage or current control.

Finally, the actual elephant: voltage and current are coupled quantities of the effort-flow sort, so that basically you can't have one without the other. It is a delicate matter, and one should consider what it means to create and sustain a voltage difference. Is it not created by displacing charges (by electrochemical reaction in a battery, by electromagnetic interaction in a mechanical generator)? I have ultimately grown the conviction that, in the end, all devices are basically charge controlled: you move charges from here to there and get a certain effect: current is the result of the movement of charges, voltage is the result of the presence of charges in space.

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I have already written way too much, therefore for further information on this specific topic (that is tangential to the question about how a BJT can be "considered" current controlled) I direct you to the following questions/answers:

Does voltage cause current or does current cause voltage?
General discussion on the what causes what problem.

Why is base current needed after transistor is switched on
For an hydraulic model of the BJT that exemplifies how pressure and flow are intertwined (disclaimer: I am referring to an answer of mine)

Working of Bipolar Junction Transistor with electron flow
On models using the inverse relations, and how certain device physics textbooks explain current control (disclaimer: I am referring to one answer of mine)

Synthesis has never been my thing.

Answer 3

In general you could imagine the BJT to be a current-controlled current source when finding the bias point in a linear application (large signal). \$I_C=\beta I_B\$

It's more useful to think of it as a voltage-controlled current source when you are doing small-signal analysis, such as for an amplifier- using the hybrid pi model.

Neither is particularly useful when you are evaluating switching applications since the base current will be high enough that the collector current is determined by the external circuit and not by the transistor characteristics (the first helps somewhat in ensuring that condition exists).

Answer 4

A BJT isn't current-controlled, but, to a useful approximation, it behaves that way. Under more accurate models of the BJT, like Ebers-Moll, the collector current isn't a function of the base current but of the base voltage (\$V_{BE}\$).

Answer 5

Other answers have expressed opinions on whether the BJT is voltage controlled or current controlled or both. In my answer, I wish to address instead this:

when it's obvious that changing the voltage controls the current through the collector?

Consider the following alternative circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Is it not obvious that

$$I_C = \beta_{DC}I_B$$

and

$$i_c = \beta_{ac}i_b$$

and thus that the base current controls the current through the collector?

Yes, you might object that changing \$I_B\$ necessarily changes \$V_{BE}\$ etc. but that is a two-edged sword since the objection works both ways, i.e., a change in \$V_{BE}\$ necessarily changes \$I_B\$.

So no, it's not obvious, by your example, that the BJT is voltage controlled.

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Addendum: there is quite a bit of argument in the comments regarding the question of whether the collector current of a 'stand-alone' BJT is fundamentally controlled by \$v_{BE}\$ or \$i_B\$. It's easy to confirm with SPICE that one can control the collector current by controlling the base current with a current source:

Similarly, one can confirm that one can control the collector current by controlling the base-emitter voltage with a voltage source.

Regardless, a couple of users have strongly expressed their position that the BJT collector current is plainly voltage-controlled and that to suggest otherwise is beyond the pale.

It's been a while since I studied solid state physics so I decided to consult my library of EE textbooks. The first textbook I pulled off the shelf is "Solid State Electronic Devices", 3rd Ed.

Here's an extensive quote from section 7.2.2:

It remains to be shown that the collector current \$i_C\$ can be controlled by variations in the small current \$i_B\$.

In the discussion to this point, we have indicated the control of \$i_C\$ by the emitter current \$i_E\$, with the base current characterized as a small side effect. In fact, we can show from space charge neutrality arguments that \$i_B\$ can indeed by used to determine the magnitude of \$i_C\$.

Let us consider the transistor of Fig. 7-6, in which \$i_B\$ is determined by a biasing circuit. For simplicity, we shall assume unity emitter injection efficiency and negligible collector saturation current. Since the n-type base region is electro-statically neutral between the two transition regions, the presence of excess holes in transit from emitter to collector calls for compensating excess electrons from the base contact.

However, there is an important difference in the times which electrons and holes spend in the base. The average excess hole spends a time \$\tau_t\$, defined as the transit time from emitter to collector. Since the base width \$W_b\$ is made small compared with \$L_p\$, this transit time is much less than the average hole lifetime \$\tau_p\$.

On the other hand, an average excess electron supplied from the base contact spends \$\tau_p\$ seconds in the base supplying space charge neutrality during the lifetime of the average excess hole. While the average electron waits \$\tau_p\$ seconds for recombination, many individual holes can enter and leave the base region, each with an average transit time \$\tau_t\$. In particular, for each electron entering from the base contact, \$\frac{\tau_p}{\tau_t}\$ holes can pass from emitter to collector while maintaining space charge neutrality. Thus the ratio of collector current to base current is simply

$$\frac{i_C}{i_B} = \beta = \frac{\tau_p}{\tau_t}$$

for \$\gamma = 1\$ and negligible collector saturation current.

If the electron supply to the base \$(i_B)\$ is restricted, the traffic of holes from emitter to base is correspondingly reduced. This can be argued simply by supposing that the hole injection does continue despite the restriction on electrons from the base contact. The result would be a net buildup of positive charge in the base and a loss of forward bias (and therefore a loss of hole injection) at the emitter junction. Clearly, the supply of electrons through \$i_B\$ can be used to raise or lower the hole flow from emitter to collector.

Now I'm almost certain that those firmly in the voltage-control camp will interpret this as confirmation of their position as will those firmly in the current-control camp. So I will just leave it at that. Let the barking begin...

Answer 6

I think you got it backwards. \$V_{in}\$ is controlling \$I_{B}\$ via Ohm's law (assuming the voltage drop on the base is small): \$I_{B} = V_{in}/ R_1\$. The BJT is in turn controlled by this current: \$I_C = \beta \cdot I_B\$.

In the end there is a linear relationship between \$V_{in}\$ and \$I_C\$, but this is only true for as long as \$R_1\$ remains constant. Since \$R_1\$ is not part of the BJT, you cannot assume anything about it when discussing BJT characteristics, and you cannot say the BJT is controlled by \$V_{in}\$.

Perhaps an example would explain it better. Imagine I drive a car, and its speed depends on how hard I push the gas and for how long. But I don't want to get any fines, so I always respect speed limits. Now you come along and say:

Why do they say cars are controlled by gas pedal, when in reality their speed depends on flat metal objects with numbers painted on them?

So what you say is true in this particular case, but that doesn't change the fact that cars don't care in the slightest about flat metal objects in their surroundings.

Answer 7

If you made Vin a constant and R1 a variable would you say BJT's are resistance controlled devices?

In your setup you appear to have control of a voltage and observe it is able to effect the collector current. It's reasonable to use this as proof this circuit's current is voltage controlled, but it's not reasonable to say this means that all BJT's are voltage controlled.

You have to make a distinction between the whole system and a component in the system, even when it's the most interesting component or even the only interesting looking one.

Answer 8

I think it makes sense to call a BJT current controlled when you compare it to the MOSFET.

The MOSFET has a gate, and the higher the voltage on the gate (which draws essentially no current), the higher the conductance from drain->source. So, this is a voltage controlled device.

Alternatively,

A BJT has a base. The higher the conductance from collector to emitter, the higher the base current.

As a practical example which really highlights the difference:

• Flash Memory

This memory topology is impossible to implement with BJT's, because a constant base current is required for conduction. In a MOSFET, charges can be injected into an insulated gate. If they are injected, they will stay there, and keep the MOSFET conducting all the time. This conductance (or lack thereof, if no charges were injected) is sensed, and used to read the stored bit-state.

Answer 9

Up to now, I count 10 answers and a lot of comments. And again I have learned that the question if the BJT is voltage- or current controlled seems to be a question of religion. I am afraid, the questioner („Why do textbooks state that BJTs are current controlled“) will be confused because of so many different answers. Some are correct and some are totally wrong. Therefore, in the interest of the questioner I like to summarize and clarify the situation.

1) What I never will understand is the following phenomenon: There is not a single proof that the collector current Ic of a BJT would be controlled/determined by the base current Ib. Nevertheless, there are still some guys (even engineers!) which again and again repeat that the BJT - in their view - would be current-controlled. But they only repeat this assertion without any proof - no surprise, because there is no proof and no verification.

The only „justification“ is always the simple relation Ic=beta x Ib. But such an equation can never tell us anything about cause and effect. More than that, they forget/ignore how this equation was originally derived: Ic=alpha x Ie and Ie=Ic+Ib. Hence, Ib is just a (small) part of Ie - nothing else. (Barrie Gilbert: The base current is just a "defect").

2) In contrast, there are many observable effects and ciruit properties which clearly show and proof that the BJT is voltage-controlled. I think, everybody who knows how a simple pn diode works should also recognize what a diffusion voltage is and how an external VOLTAGE can reduce the barrier effect of this fundamental property of the pn junction.

We must apply a proper VOLTAGE across the corresponding terminals to allow a current through the depletion zone. This voltage (resp. the corresponding electrical field) is the only quantity which delivers the force for the charged carrier movement, which we call current! Is there any reason that the base-emitter pn junction should behave completely different (and does NOT react upon the voltage) ?

Upon request I can list at least 10 effects and circuit properties which can be explained solely with voltage control. Why are these observations so often ignored?

3) The questioner has presented a circuit which deserves an additional comment. We know that an opamp (undoubtly voltage driven) can be wired as a current-in-voltage-out amplifier (transresistance amplifier). That means: We always have to distinguish between the properties of the „naked“ amplifier unit and a complete circuit with additional parts.

For the present case, that means: The BJT as a stand-alone part is voltage-driven - however, viewing the whole circuit (with a resistor R1) we can treat the complete arrangement as current driven circuit if R1 is much larger than the input resistance of the B-E path. In this case, we have a voltage divider driven by the voltage Vin.

Answer 10

Implicitly, two questions:
1. why can it be considered “current-controlled”, and
2. why is it convenient to consider a BJT “current-controlled”.

First question. Mathematically, the device imposes two equations on the space of parameters, that comprises two voltages and two currents (one may add temperature, some time-related stuff to account for transient effects, but it won’t change the number of equations). The system can be equivalently expressed in different forms. Unlike a FET, where on/off modes don’t differ in the gate current, in a BJT any control change results in certain shifts on both voltage and current planes. Each plane accounts for two degrees of freedom. So, we can consider two voltages as independent variables, or two currents. Or, say, \$V_{\mathrm{BC}}\$ and \$I_{\mathrm E}\$, with other parameters dependent on them. No difference.

Second question. According to the common sense, it is reasonable to treat as control such a parameter whose small changes result in large (but predictable) change in the mode of operation. Moreover, controlling a transistor occurs largely or entirely in the forward-active region, useful for its gain. Most obvious candidate parameters are \$V_{\mathrm{BE}}\$ and \$I_{\mathrm B}\$, whose small changes (in forward biased B–E) result in great changes of the characteristic of collector. But effects of \$V_{\mathrm{BE}}\$ are strongly non-linear, whereas (for fixed \$V_{\mathrm{BC}} \approx V_{\mathrm{EC}}\$) currents in a BJT depend on \$I_{\mathrm B}\$ almost linearly. That’s all.