Is BJT a Voltage controlled device or Current controlled device?

Question

I have searched already for this question, but everywhere I see debate instead of clear answer. Though it is not that important of a matter whether we call it a voltage controlled device or current controlled device. I believe it can be called both voltage controlled or current controlled depending on the situation. Still I believe it requires a little clarification. Which part it really is in view of the core operating principle?

Answer 1

The answer depends on your perspective.

A physicist might say that the fundamental action in a BJT is that an electric field across the base-emitter junction decreases the width of the depletion zone. It is this electric field, measured in volts, that controls the movement of charge carriers. Therefore the BJT is voltage controlled.

An electronic engineer might say that the most useful model for a specific circuit design is the current-amplifier model.

Answer 2

So called "common emitter current gain" is a range not a constant. Good designs don't depend on it.

Short answer: the Ebers-Moll model gives a relationship between the collector current and the base-emitter voltage. So you can view the base-emitter voltage as being controlled by the collector current or as the collector current being controlled by the base-emitter voltage.

Many people make the incorrect claim that there is a useful relationship between the base current and the collector current, and thus mistakenly claim that a transistor is a "current controlled current source." A transistor is not a current-controlled current source.

Long answer:

The confusion about whether a BJT is current controlled or voltage controlled comes from two sources. The first is that the equations we use in describing electric circuits are not definitions of one variable in terms of several others. Rather they are describing a constraint between several variables. Take Ohm's law: \$V = IR\$. This is not a definition of voltage. Nor is \$I=V/R\$ a definition of current or \$R=V/I\$ a definition of resistance. Rather it says that in any circuit (involving an ohmic device) this equality will always hold. No matter how we change the current, the voltage will always stay proportional to the current. No matter how we change the voltage the current will always stay proportional to the voltage. (True story: I once received a resume from a gentleman who listed as one of his qualifications that he knew, and could use, Ohm's law "in all three forms.")

The most important constraints in describing how a transistor works within a circuit are the Schockley diode equations used in the Ebers-Moll model. In the active mode this results in the constraint that: $$ I_E = I_{ES} (e^{V_{BE}/V_T} - 1) $$

where \$I_{ES}\$ is a constant that describes the transistor, and \$V_T\$ is the thermal voltage (about 26mV at room temperature). So this is describing a relationship (constraint) between the emitter current, \$I_E\$, and the voltage between base and emitter, \$V_{BE}\$. Yes, the current is on the left hand side and the voltage is on the right hand side, but this is only because the \$-1\$ makes it a little difficult to write the other way around. In fact, when \$e^{V_{BE}/V_T}\gg 1\$ it is sometimes useful to write \$V_{BE}=\frac{1}{V_T} \log(I_E/I_{ES})\$.

Nonetheless, the physics behind the Ebers-Moll model, is usually thought of the way that @RedGrittyBrick describes it: the voltage between base and emitter controls the current of minority carriers into the base (given the relative dopings of emitter and base).

The second source of confusion comes from another statement that people make about transistors that is just completely false. This is a statement that a transistor has a well defined "common-emitter current gain", or \$h_{FE}\$. I will write this really large so people don't miss it:

A transistor has no (well defined) common-emitter current gain.

It is definitely the case that there is a flaw in bipolar junction transistors where there is always a leakage current through the base, but the leakage current is not well defined between a pair of the same type of transistors, nor is there any simple linear relationship that describes the base current in terms of the emitter current in a specific transistor. The current through the base is caused by a number of factors, like relative doping levels of the base and emitter and the width of the base, that are difficult to control during manufacturing. Let us take a look at the datasheet for the Fairchild PN2222. You will see that \$h_{FE}\$ is given as a range. It is somewhere between 100 and 300 (a factor of 3 difference !) when the collector current is 150mA. But \$h_{FE}\$ is not less than 35 when \$I_C\$ is at 0.1mA. Another factor of 3 different! So \$h_{FE}\$ is not like the measured resistance of a resitor. \$h_{FE}\$ is not a constant and is not a useful description of the transistor's gain.

When designing an amplifier the only thing you use \$h_{FE}\$ for is to decide whether the transistor's leakage current will be bearable for you or not. If the \$h_{FE}\$ is too low for your use case you'll either have to choose a different (probably more expensive) transistor, or replace the single transistor with a Darlington pair.

Now I will write this really large again so people don't miss it:

A good design never depends on \$\beta\$ (\$h_{FE}\$) having a particular value.

Try the following Wilson current mirror to see how you build a current-controlled current-source. Q3 is specifically included to reduce the dependence on \$\beta\$. I encourage you to change all the 2N3094s to 2N3055s (or to any of the other transistors that has a different \$\beta\$ than the 2N3094) to see that the output current is always about 2x the input current.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Please enjoy an answer in the form of rhetorical questions:

Does a current through a resistor cause a voltage across it, or does a voltage across a resistor cause a current?

Does a motor spin your car's wheels, or do the wheel resist the motor's spinning? Does shifting to a lower gear increase the engine torque at the wheels or does it reduce the resistance from the wheels at the engine?

Does a college education make your expected salary higher, or are successful people more likely to attend a college?

Did the decline in pirate population cause global warming or did global warming kill off the pirates?

Point is, an explanation of how a BJT is "controlled" is a fallacious attempt to assign a cause and effect relationship between voltage and current when really there's only a correlation (except unlike pirates and global warming, the correlation is very strong and observable). We can think of this correlation as cause and effect when it suits our needs, but it's only a model to help our reasoning in a particular case. Both explanations (a BJT is voltage controlled / current controlled) are valid models, each appropriate in a different context.

Answer 4

One of the most important formulas to describe a BJT is \$I_C = I_B × h_{FE}\$ , which clearly makes it a current controlled device.

On the other hand Ebers-Moll says: \$I_E = I_{ES} \left (e^{\frac{qV_{BE}}{kT}} - 1 \right)\$, where I_ES, q, k and T are constants and V_BE is the "input voltage" between base and emitter, which would turn it into a voltage controlled device.

Usually for large signal analysis it is considered a current controlled device and for small signal analysis it is considered a voltage controlled device.

Answer 5

It is true that you can't have current (movement of charge) without an electric field to move it (voltage) unless you have a superconducting material, which semi conductor is not. It is also true that you can have voltage without current such as the the voltage across a charged capacitor.

The input to the junction transistor is modeled on a diode. Consider what would happen if a 6V battery was directly connected to the base-emitter junction. This would cause a very large current to flow and destroyed the device.

Compare this to the gate-source junction of a MOSFET device - no damage would occur.

Any VOLTAGE signal used the junction transistor base circuit must be converted (or limited) to a current of suitable size using a resistor or other device.

Hence the junction transistor is a current controlled device that can be made to operate with an input voltage and external circuit components.

Answer 6

Consider an NPN transistor and it is forward biased so the positive terminal of the battery is connected with the emitter and negative terminal of the battery is connected to the collector terminal. The electrons present in the emitter will move towards base region some of them will diffuse in the base region and some of the will recombine in the collector region. as in the base region movement of charge carrier is because of the electrons which is minority charge carrier in the base region. So a BJT can said to be minority current controlled device.

Answer 7

A junction BJT is voltage controlled device, period. All the above explanations that say otherwise are wrong and faulty. Let me explain why. First of all, discard all explanations using models. Models are useful for obtaining results, but they usually work with effects, not the cause. To really find out how a device works, you have to delve into the physics of the operation. In other words, you have to find the cause, not the effect.

OK, let's hook up a correctly biased NPN BJT and see what happens. The electrons travel from the emitter to the base by diffusion, where they are swept onward to the collector by the higher positive voltage. These electrons leave behind ions whose charge causes a back-voltage which neutralizes the diffusion voltage and inhibits the flow of electrons from the emitter. The voltage applied to the base reduces the back-voltage and expedites the electron flow from the emitter to the collector. That is why a BJT is voltage controlled. Without the forward base voltage, the back-voltage caused by the ions would choke off the flow of electrons to the collector. Because the voltage controls the current through diffusion, the output current is exponential instead on linear like it is for FETs and tubes.

Unfortunately not all the electrons make it to the collector. Some are attracted to the base circuit as waste current. Within a reasonable operating range, the collector current is somewhat proportional to the waste current. This proportion is called "beta". You can hook up a current source to the base and get come nice collector current curves. Then you might say, "A transistor is a current amplifier." If you do, you should be sued for libel. You don't have a single transistor anymore. You have a transistor CIRCUIT using the high resistance of the current source. The transistor itself still operates according to its Vbe. You would not call a a ordinary op-amp a current controlled device if you made it part of a current amplifier, would you?

If you want an example of a current controlled device, check out magnetic amplifiers. The Nazis used them extensively during World War II. Compared to solid state, they are big, ugly, and rugged.

Answer 8

Base current flows between base and emitter in the BJT to induce a larger current flow between emitter and collector. When no current flows between emitter and base only a leakage current, nearly completely unaffected by any input signal, flows between collector and emitter. This makes the BJT a current controlled device.