# Why is a BJT considered "current-controlled"?

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? • could you please post the jpg as a png and use the image tool? Or draw the circuit with the circuit editing tool? Aug 17 '16 at 15:57
• Just to complicate your life, a BJT isn't current controlled. See full set of simplified DC-only Ebers-Moll model equations here (injection, transport, and non-linear hybrid-pi): electronics.stackexchange.com/questions/252197/…
– jonk
Aug 17 '16 at 16:29
• zen22142.zen.co.uk/Theory/re_model.htm Aug 17 '16 at 17:07
• good grief... in circuits like those depicted in the drawing, nobody is thinking about Ebers-Moll or Hybrid-pi models. You pretty much have to be doing AVLSI to be concerned with that stuff. Aug 17 '16 at 20:53
• @Raj, remove R1, and then we'll see that the interior of a BJT is indeed controlled by Vbe. But that design approach is mostly for discrete diff-amp designs (such as the inside of modern DC-coupled audio amplifiers.) Instead we can ignore the interior BJT physics, and pretend that Ib determines Ic directly, even though it really doesn't. This also avoids having to deal with any nonlinear xfer functions produced by diode junctions. Aug 18 '16 at 5:18

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:

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

• Nitpick: the voltage drop across Vbe will always be around whatever the datasheet says, which could be as low as 0.3V for some BJTs. Aug 17 '16 at 16:59
• What really happens is the following: R1 realizes - together with the base-emitter path - a voltage divider. And the signal voltage Vin causes a corresponding voltage drop across the B-E path which controls the collector current. Hence, it is NOT the base current Ib which determines Ic. Just the opposite is true: Ib and Ic are both caused by Vbe.
– LvW
Aug 17 '16 at 18:36
• The first sentence is false and the equation given at the end is an approximation that ignores the logarithmic dependence of $V_{BE}$ on $I_B$. $$V_{BE} = V_T \ln \frac{\beta I_B}{I_S}$$ So, while it is true that $V_{BE}$ doesn't change by much, it isn't true that $V_{BE}$ doesn't change at all. Aug 18 '16 at 13:55
• I am not sure why this answer is the top most rated. It's a good estimate but doesn't answer OPs question about why (or why not) its current controlled. Aug 22 '16 at 3:18
• @lvw it's called a current source, which is what mmize described. The current is fixed. It's not a voltage divider because vbe doesn't really change based on a change in vin, which is the definition of a voltage divider. Aug 22 '16 at 3:35

Preamble

Let's start with a little digression: what makes a generator a current generator instead of a voltage generator? Look at the V-I characteristics: the one with mostly constant voltage (almost horizontal in the I-V plane) will be called a voltage generator, the one with mostly constant current (almost horizontal in the V-I plane) will be called current generator.  (Pictures taken from the Electronics Tutorials website)

This is because the 'accent' is on the constant quantity (the voltage or current supplied - while the other quantity is variable depending on the load and the compliance of the generator). (Note 1)

In a controlled device, the accent is on the variable quantity. Given the exponential input characteristic, that leaves Vbe almost constant, it is current you would like to see as the controlling variable. This is 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). Picture taken from "An introduction to error analysis", Taylor and distorted to fit the purpose

The bottom line is that it's easier to distinguish between 10 e 40 uA (1 to 4 ratio) than it is to separate 0.65 and 0.67 V (1 to 1.03 ratio). (Note for the less flexible minds: like the more extreme values I used before this edit, these are made up values intended to show the contrast between a discernible change in what you want to see as the controlling variable - the current entering the base - and the feeble change in the voltage between base and emitter).

The simplest thing

You can see why that is called current control by pushing it to the limits by adopting the simplest model for a BJT, as shown by Chua, Desoer and Kuh in their "Linear and Nonlinear Circuits": in the following pictures 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. 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 where, by changing ib (you can't change Vbe in this model, since it's fixed) you change the value of Ic. You can see how you can make ib change by intersecting the input characteristic with the input load line By changing E1 (not part of the BJT) you change ib (part of the BJT). Then you can find the value of ic corresponding to that value of ib, select the corresponding output characteristic and finding 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?"

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 this 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 you 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. (And this is why I made that digression at the beginning of this answer).

You might want to compare 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 V-I 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.

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 (on page 475 is shown a common base configuration):

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.

Who plays in base, first?

Now, back to (almost) real transistors. This is the first vbe chars that I came up with after a Google image search: Dunno if it's real, but it looks plausible. 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 of the B-E junction. Let's say you want to use this BJT to produce 10 mA on odd days and 15 mA on even days. You have a German lab measure the beta of the particular transistor in your hand and it came out as 250 over the range of interest. Let's say you have a current and voltage generator with an accuracy of 10%.

Current control : You can use ic = beta ib to find the value of ib that you have to set. The nominal values of 10 and 15 mA of ic require nominal values of 40 e 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

Voltage control: You don't believe in beta, so you must specify a voltage that create a vbe of... Yes, of what? Go read it on the above graph (but then you'll have to accept the dreadful ic = beta ib relation). I guess you'll have to use the Ebers-Moll model to compute the values to the desired values for ic. But let's say we determined it's precisely 0.65 and 0.67V (just like I have used a precise value for beta, above) When we try to set those precise values, our china-made 10% accurate generator will supply the following voltage ranges

0.585 - 0.715 V -> back to Ebers-Moll, to compute ic, ... too bad the uncertainty will be exponentiated...

0.603 - 0.737 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.

I guess it's better to resort to current base as a means to control the collector current.

With current control, even if I allow a 10% error on the measured value of beta, I can still (barely, but still) make out the two ranges of current (8.10-12.10 mA vs 12.15-18.15 mA) corresponding to odd and even days.

With voltage control, if you add a 10% error on the computed (or read from the diagram) value of voltage (and I am being generous since that error is going to be amplified), you are already lost in uncertainty. This is basic error propagation theory.

Intermission

This post is taking time, I'll come back another to add something more. Let me just address the question of the religion war you might have witnessed. What is that all about?

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 models tend to express relevant quantities in function of potential (difference)s. The reason I wrote

Note: 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.

is that voltage and current are closely related too: they are coupled quantities of the effort-flow sort, so that basically you can't have one without the other. It is a delicate matter though, and I guess one should also consider what it means to create a voltage difference. Is it not created by displacing charges (by electrochemical reaction in a battery, by electromagnetic interaction in a mechanical generator). I suspect that in the end all devices are basically charge controlled: you move charges from here to there and get a certain effect.

I suspect the 'voltage control' crusaders are assuming the 'current control' counterpart have learned electronics on Forrest Mims' books and have never seen a quantum physics, solid state or semiconductor devices book. They seem to ignore the meaning of controlling variable as the variable one chooses to set to actuate a control. I hope the quote from Eisberg & Resnick (two 'solid' physicists if you allow me the pun) will show them this is not the case.

Note (1) The ideal generator curves are just that: ideal. Try to picture a transition from an ideal voltage generator to an ideal current generator passing through good, average and lousy voltage generators, then lousy, average and good current generators.

• In your Note, the first sentence is simply false! The Ebers-Moll model does not "imply" something - instead, it is in fact a cause-effect relationship. Please consult W. Shockleys patent document. You are right, you can always create inverse functions (on paper) - so what? Do you think you can interchange cause and effect on paper? By the way: Did you ever design transistor stages (because you are mentioning some funny Vbe voltages). Are you familiar with emitter degeneration (current-controlled VOLTAGE feedback) ?
– LvW
Aug 18 '16 at 14:59
• I made up those values to exemplify the difference between trying to set veeeeeery veeeeeeery close values of Vbe and discernible values of Ib (I also added in the edit comment that I wanted to make those values more extreme). I did not want to waste time to find plausible values, but later for those who do not have enough mental flexibility, I will add a picture or two. As I wrote above: try to control the BJT by removing Rb and by supplying a pure voltage to Vbe. Good luck. (Oh, by the way: the simplified model cannot be used to explain the Early voltage, too.). Aug 18 '16 at 20:34
• It seems you have overlooked my mentioning of emitter degeneration. More than that, did I spoke about supplying a "pure voltage" to the base? You should try to be fair. As you have mentioned the Early effect. Are you aware that the explanation of this effect prooves voltage-control? Have you ever heard about the tempco -2mV/K ? Have you ever thought about the meaning of this value?
– LvW
Aug 19 '16 at 7:09
• I like this comment: 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 Aug 20 '16 at 17:16
• @LvW what you did is technically called "mutatio controversiae". It is a well known technique. I suggest you re-read my post with more attention, especially the quote from Munari. BTW, regarding the circuit in the question (not another one, the one in the question), you still have not said what values of vbe you would set to produce 10 e 15 mA in collector current (and how do you plan to set them). Why is that? Aug 21 '16 at 17:07

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).

• Spehro Pefhany, regarding your first sentence: I think, for bias point determination we must not "in general" imagine that the BJT would be current controlled. The classical biasing method using a voltage divider at the base node is certainly based on the voltage-control view.
– LvW
Aug 17 '16 at 18:52
• @LvW If you consider Vbe fixed at 0.6 or 0.7V and evaluate the voltage drop from the divider based on Ic and $\beta$ you'll get the right answer, close enough for most purposes. Aug 17 '16 at 20:05
• Art of Electronics II goes into this issue in depth, giving examples of design fails caused by "the hfe-think" taught by most other texts. The main issue is the variability of hfe among transistors, and across large temperature range. Relying on hfe is fine for one-off hobbyist designs that remain at 20C deg. But in a mass produced product with transistor hfe between 80-300, and automotive temp range, most will fail unless hfe effects can be removed (removed using voltage-based design philosophy common to op-amp innards.) Aug 18 '16 at 4:59
• @wbeaty: whats up with the BJT physics crusade? The OP asked why it is considered a current controlled device, not SHOULD it be considered a current controlled device. Plus the answer mentions this is for large signal analysis. Aug 18 '16 at 11:51
• @wbeaty It's not uncommon to specify the beta bin more closely in volume production. For example, C1815Y (was very popular in Japanese designs) has 120-240 range. Aug 18 '16 at 13:04

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 ($V_{BE}$).

• It's so useful an approximation that any BJT datasheet you will ever look at will characterize beta. Aug 17 '16 at 16:40
• Yes - beta is specified. So what ? From this fact, do you really derive that the BJT would controlled by the base current ? Or do you have some other arguments? I doubt.
– LvW
Aug 17 '16 at 17:27
• @vicatcu Devices can be characterized in any number of ways, including parameters that are fictitious, or functions of other, more primary parameters.
– Kaz
Aug 17 '16 at 18:13
• @Kaz: I think it's wrong to say a BJT is not current controlled just because the base current can be expressed as a function of base-emitter voltage. Actually it is current controlled because physically the base current matters. Otherwise you could also say a BJT is temperature controlled instead of current controlled...
– Curd
Aug 17 '16 at 19:29
• > ...will characterize beta. Yes, they guarantee that the value for hfe falls somewhere between 80 and 300! Aug 18 '16 at 5:25

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.

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...

• They are confusing the "superior mindset for proper analog design considering process variations" with "reasonable ways to think about things" Aug 22 '16 at 4:56

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.

• so R1 is varying u say
– Raj
Aug 17 '16 at 16:41
• The voltage drop on the base is typically 0.6-0.7V Aug 17 '16 at 16:41
• R1 is external to the BJT I say. Aug 17 '16 at 16:41
• @vicatcu I'd say it's typically 0.3-0.7V, and yeah, that's what I call small for the sake of simplicity. Aug 17 '16 at 16:43
• @horta I tried to make my quote more international-friendly. Aug 17 '16 at 19:06

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.

• Regarding the problem of control it is important to distinguish between (1) the "naked" transistor (voltage-controlled transconductance device) and (2) a working circuit, which consists of the BJT and surrounding resistors. Such a circuit can (can, but not necessarily) be seen as current controlled. This is the case when in the above example the series resistor R1 is very large if compared with the transistors input resistance at the base node.
– LvW
Aug 17 '16 at 19:12

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.

• OF COURSE IT'S NOT A RELIGION, instead it's physics/engineers versus the incorrect beliefs taught in grade-school. We're supposed to give up those simple models when we get to higher levels (undergrad EE.) Diff amps cannot be explained by hfe-based models. Neither can current mirrors. Neither can cascode amps. IMPORTANT: if you believe that ib controlls Ic, then for you modern audio amps will be forever behind a barrier of confusion and ignorance, since DC-coupled audio circuits use voltage-based BJT designs where hfe is irrelevant. Similar situation: look at the interior of TL071 etc. Aug 18 '16 at 18:34
• @wbeaty: what's ridiculous is that I agree with both LvW and you, on how BJT's work. Yet, I can still understand that the BJT requires current to function. Plus, I feel like the V-I dichotomy is just a duality in this case, as shown by taking the natural log of the shockley diode equation. But, I guess two slightly opposing thoughts are too much for your head to handle (with all that theory packed in there!!) Aug 18 '16 at 18:43
• I suspect that much of the debate here hinges on what one means by control. Since a simple SPICE simulation will confirm that one can control the collector current by controlling the base current, the statement "collector current is controlled by base current" is indisputably true in that sense. If, like LvW and wbeaty appear to, one chooses to insist that such a statement is false in any sense, I will simply point to this: i.stack.imgur.com/LqFx1.png Aug 18 '16 at 20:31
• @LvW, I am disappointed in, though not entirely surprised at, your very weak response. Aug 20 '16 at 1:10
• @LvW: then you are beyond reasoning with. the model you have married your soul with is just that: a model. progressively more complex models arise as we realize further, deeper interactions. the shockley diode equation is based on other empirical exponential formula, namely the Arrhenius equation. This does not account for the micro level of quantum mechanics, but gives very predictable results (statistics). Alas, it is just a model. Physicists cannot even agree on whether energy is stored in a field; you claiming to have complete understanding of the p-n junction is quite laughable. Aug 20 '16 at 15:30

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.

• Sorry - this is not a correct description of the working principle of the BJT. Have you ever heard about Shockleys exponential equation Ic=f(Vbe)? Do you know that the transconductance gm=d(Ic)/d(Vbe) is the key parameter for the amplification process? Do you know that two different transistors with different beta values (100 and 200) will provide the same voltage gain (identical quiescent current Ic)?
– LvW
Aug 17 '16 at 19:37
• @LvW I think the point jbord39 is making is that you can't have voltage without current and vice versa. Therefore, by the strictest definition, nothing can truly be a current or voltage controlled device (alone). Therefore he/she's trying to answer the question of why textbooks even bother to make the distinction. A BJT's output is very much dependent on the input current unlike a MOSFET, which is I'm assuming why textbooks state that certain devices are current or voltage controlled (when in reality that's never truly the case). Aug 17 '16 at 19:41
• horta, it is simply not true that the BJTs output is "very much dependent on the input current". Each reliable (!!!) book and home pages from leading US univesities can tell you the opposite. Nobody denies that a base current does exist but it can be seen simply as a "nuisance or a defect" (as mentioned by the well-known BJT specialist Barrie Gilbert).
– LvW
Aug 17 '16 at 19:54
• @LvW: On top of that his question is not "Is a BJT current controlled" but "Why is a BJT considered current controlled". Aug 17 '16 at 20:03
• @LvW electronics.stackexchange.com/questions/201533/… Since voltage and current in devices don't exist without eachother, you cannot say that the BJT is truly a voltage-controlled device. Even the Ebers-Moll model is nothing more than a model (an approximation that humans use to abstract away messy details of the real world). Aug 17 '16 at 20:13

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.

The collector current is, by definition / physics, a function of the base current (and implicitly the load current demand). The governing formula of a BJT is $I_C = \beta \cdot I_B$. Where $\beta$ is the gain, $I_B$ is the current through the B-E junction, and $I_C$ is the (maximum) current through the C-E junction.

The base voltage (i.e. the voltage measured at the base terminal with respect to GND) is actually more or less a constant (at least in saturation), as characteristic of a diode forward voltage drop.

• It`s interesting - a wrong answer gets one point. Perhaps, becaus the answer was so simple? (“I think it's much more interesting to live not knowing than to have answers which might be wrong.” R. Feynman).
– LvW
Aug 17 '16 at 17:25
• vicatcu - are you sure to be right? Are you aware that you are completely wrong? The key parameter for amplification is the transconductance gm which is the SLOPE of the exponential curve Ic=f(Vbe). What makes you think that Vbe is a constant? My recommendation: Consult a reliable textbook before giving false answers.
– LvW
Aug 17 '16 at 17:36
• @vicatcu wrong. The physics clearly shows that Ie (and Ic) is controlled by the potential barrier of the BE junction, and NOT by the base current. However, base current and BE potential are linked together by the diode equation. Put simply, base current controls Vbe, and then Vbe directly controls Ie (and hence Ic.) In other words, the current-gain equation isn't fundamental physics, since there is no mechanism where Ib can directly affect Ie or Ic. Ib does have indirect control of Ic (via Vbe variations,) so "hfe" is a very useful concept. But hfe is not fundamental BJT physics. Aug 18 '16 at 5:09
• @wbeaty: Dude, I have read the book and I do know analog electronics ( taught two semesters as well). Yes, both voltage and current are required to forward bias the b-e junction. You can always take the ln() of the shockley equation and you will now have V as a function of I. It's nonsense what you are saying. If you can get a BJT to work without any base current flow, then you will have a point. Until then, you are just going on about nothing, on some strange crusade of your own (correct) knowledge mixed with ignorance and ego. Aug 18 '16 at 18:33
• Bottom line, in the circuit from the OP, the voltage source very likely 0/5v output of a controller, and the resistor is selected to set the base current, not the base voltage. Nobody is contesting the fundamental physics of a BJT, it's just a practical context-specific application construct. Aug 20 '16 at 15:52