Try to simulate it in Micro-cap. Unless i've overlooked something it should look like this:

Very interesting situation... It seems only I see the negative feedback here...

Vasile, the emitter follower (formally called common-collector stage) is the simplest circuit with negative feedback where the transistor changes its collector-emitter resistance so that to make the output (emitter) voltage equal to the input (base) voltage. So, If we fix the base voltage, it will copy this voltage on the emitter...

Now, if we decide to change the emitter voltage by connecting another source to the emitter, the transistor will "feel" this intervention and will react trying to restore the previous emitter voltage. This configuration is formally called "common-base stage".

Boris, your curves are horizontal; so they represent a varying current source. But the emitter follower (common-collector stage) behaves as a voltage source; so its characteristics should be vertical.

It is possible to treat the common collector or emitter follower as a case of negative feedback.

However, a simpler approach is to consider that in normal operation the base-emitter junction is a forward biased diode.  From the diode equation, the slope resistance is

re = dVbe / d Ie = k T /(e Ie)

or at room temperature approximately: re =  25 Ohms / (Ie in mA)

See, e.g. Zepler & Punnett "Electronic devices and networks" (1963).

Alternatively, if you know the four terminal network parameters for the common base configuration, then the output impedance for a common collector with low impedance base drive is the same as the emitter input inpedance for a common base transistor with zero AC load impedance.  This should be more accurate as it will use device-specific parameters and include the effects of current gain and base input resistance.

(Edited in reponse to Cyril's post below).

Dear Alan,

Really, in the first moment, when you apply the input voltage to the base of the transistor, some small initial current begins flowing through the base-emitter junction. But immediately, a big current begins flowing through the collector-emitter junction, adds to the initial base current thus "helping" it. This current will increase until the emitter voltage becomes almost equal to the base voltage... and then, reaching the equilibrium, will stop increasing. This explanation, based on the negative feedback idea, is quite convincing.

Voltage-driven common-emitter and common-base stages are identical excluding the unpleasant fact for the input source, that the output current flows through it.

My bad, I simulated the wrong circuit. The emitter follower's output characteristic is quite linear. Nothing interesting about it.

Cyril: "This explanation, based on the negative feedback idea, is quite convincing."

I am familiar with the argument, but it does not give quantitative predictions without specific device parameters and some circuit analysis.  The 25/Ie rule of thumb works pretty well over a range of emitter currents for a wide range of devices.  At 5 mA bias current the output impedance of a common collector is around 5 ohms, so the output voltage is almost independent of current - an almost vertical line if you plot current against emitter voltage.

Cyril: "Voltage-driven common-emitter and common-base stages are identical excluding the unpleasant fact for the input source, that the output current flows through it."

Sorry for the confusion.  I wrote "common emitter" where I meant "common collector" - now corrected in my previous post.

Boris... well, it is not interesting... but still is it vertical?

Cyril "Boris... well, it is not interesting... but still is it vertical?"

Almost, but not quite vertical.

Typical output admittance is 200 mA / V at 5 mA emitter current.

To make it exactly vertical, we can put the emitter follower in the negative feedback loop of an op-amp voltage follower:)

Dear Cyril,

you didn't specify the condition of measurements, but if they are analogous to the previous cases (a current generator at the input and a voltage generator at the output) I don't see why the output characteristic should be vertical.

In my opinion they should be horizontal, since the base voltage will follow the changes of emitter voltage without affecting the current.

The only effect would be the lowering of Vce voltage of BJT and so a lowering of the output current (IE) due to the Early effect.

Anyway I attach a simulation to better explain.

sim

Dear Vasile,

You've never seen it because you looked bad  :-)

First of all, input impedance of a CC depends heavily on the load. This is the first alarm bell of the presence of a feedback.

If you exclude the parasitic capacitances, CE input impedance never depends on the load. CB input impedance depends negligibly, through Early effect, on the load.

Anyway in the attached photo you can find the Inverse hybrid representation (the representation that fits a voltage amplifier) of the CC.

I neglected the Early effects on the BJT model since it is not important for the purposes of demonstration.

If I didn't make errors, you can clearly see the feedback in the presence of the gr parameter.

regards

S.

Cyril, the output characteristic of an emitter follower is linear if the load is linear. 

Q: Why?

A: Because the output voltage is between the emitter and ground, or in other words the output KVL loop doesn't include the transistor.

@Vasile Surducan

CC is voltage follower and carry negative feedback. The topology of feedback is Voltage series(voltage controlled voltage source). Feedback factor Beta is ratio of voltage across RE. Since this Voltage is same feedback factor is Unity.

@ Cyril Mechkov

The output characteristics of CC is of course unexplored? I am too doubtful about it.

Dear Cyril,

Dear colleagues,

I see a hot discussion on the topic sharing in it dear colleagues.

In order to solve this problem we have to return to precise definition to the output characteristics of the CC circuit configuration of the BJT.

At first we have to agree that there is the DC and small signal AC characteristics.

The static  output characteristics of the CC BJT transistor circuit is to be defined in the same way as the characteristics of the the CB and CE such that:

IE= f ( VEC , IB ),

with the symbols have their usual meaning. In this configuration there is no emitter resistance, but there is two sources, IB at the input which will be given discrete values as in the case of the CE, and VEC at the output which will be changed continuously as in the case of the CE circuit.

So, accordingly we get the output characteristics in form of transistor curves every curve has different IB value.

Let us now derive the CC characteristics from that of the CE, using the basic circuit laws:

VEC = - VCE,

- IE = IC + IB,

So, given the CE curves one can calculate the CC curves according to the two simple.equation above. 

Implementing these calculations, and plotting the results they will have the the same shape as those of the common collector. 

Assuming npn transistor, VEC will be negative, IE will be also negative, and one has to add a constant value of the base current IB to every collector current, which means that the collector current will be shifted upward with constant value, thereby preserving its shape and its slope.

In order to solve for the bias point of any configuration, the CE DC characteristics is sufficient. This can be accomplished by the universal biasing circuit  with an emitter resistance and for bias point stabilization and  a hard potential divider to bias the base a well as a collector load resistance.

The  common point then concerns the AC common point between the input port and the output port.. In the case of universal biasing the CE is achieved by AC by passing the emmiter resistance to the ac ground of the input source. The by passing netiquette i used also to get AC common base and common collector. Practically one uses a by pass capacitor.

Thank you

@ Vaibhav Bharatrao Vaijapurkar

There is more than one way to think about an emitter follower.  One can indeed treat it as a common emitter current amplifier with 100% voltage feedback. 

Alternatively, one can treat it as a common collector stage with unit voltage gain but whose input current (and hence input impedance) depends on the load.  With a finite source impedance, increasing the emitter current reduces the voltage provided to the base, and hence the voltage at the emitter output.

In all cases the output behaviour and input loading are coupled, as explained by Simone and by Vasili.  A full treatment will include both base spreading resistance and Early effect.  Early effect is an internal feedback mechanism caused by an increase in collector depletion width (and decrease in effective base width) as the collector voltage increases.

Regarding text book treatments, Zepler and Punnett's 1963 "Electronic devices and networks" describes the relationship between 4-terminal and T-parameter descriptions, and includes tables to convert between the equivalent Z, Y and H 4-terminal parameters for each of grounded base, emitter and collector configurations.

As @Vasile said: "You cannot analyze ... a transistor circuit by taking it out from the real word and from the complex circuit where it works".  We tend to forget this because modern integrated circuits can hide much of the complexity.  Designers also learn to think in terms building blocks which minimise interactions for particular applications, such as using a Darlington pair or a cascode configuration instead of a single transistor.  With modern semiconductors it can be easier (and often cheaper) to use a multiple transistor or integrated circuit solution, rather than carry out a detailed analysis which depends on strict manufacturing tolerances for an individual component.

The reason why output characteristics of CC configuration of BJT is not explored much may be because, CC is used as voltage buffer or unity gain voltage amplifier. Even though the current gain is very high, the low output impedance and very high input impedance make it not possible for current amplifier purpose. As stated in previous answers, CC configuration is VCVS (voltage controlled voltage source). So its output characteristics which is a plot of Ie and Vce does not catch much attention. But if we are to plot it, it will look like horizontal lines with a slope as in figure attached.

Thanks,

Arun

Hello everybody here,

I was away last days and now I read your interesting comments. As I see this issue of the basic transistor technology really turned out topical and even controversial...

Simone, I have not thought so far that an emitter follower can be controlled by current...  but as I can see from your picture below, it is possible. I agree with you that in this case the emitter follower will act as a current source. But I do not see what would be the benefits of such a current source - perhaps that the emitter sources the current while, in the conventional case, the collector sinks the current? Something more - if we connect another load in the collector, the transistor will simultaneously sink the current by the collector and source the current by the emitter...

Nevertheless, this is no easy way to control the transistor by a current. Here (CC configuration) the current source should be perfect with minimum compliance voltage (since the emitter "floats") while in the case of a CE configuration it can be a humble resistor type (the emitter is fixed).

Vasile,

IMO even when including a current source in the emitter, VOUT will be decreased with VBE... and the load should consume significantly less current than the current sank by the emitter current source. The role of this source is to set the desired quiescent current from the side of the emitter and to ensure extremely high input (differential) resistance. This trick is widely used in emitter-coupled pairs (differential amplifiers, op-amp input stages).

Now let's describe (for the students, in the lab:) the operation of this more sophisticated emitter follower by means of the negative feedback principle.

Here there are even two negative feedback systems that interact between each other - the first (the emitter follower, i.e. the humble transistor) sets the voltage; the second (the current source connected between the emiter and VEE) sets the current... and they mutually help. Let's see how when the input voltage changes...

Imagine the input voltage increases. The transistor should also increase its emitter voltage; so it should increase the current through the load connected in the emitter. For this purpose, it decreases its present collector-emitter "resistance RCE" (non-linear but still resistance)... and the current really tries to increase... But the current source "feels" this change and begins to restore the previous current magnitude (here we should only specify that actually this is not a current source but sooner a current-stable non-linear resistor). So, to decrease the new value of the current to the previous one, it begins increasing its internal resistance until restores the current. Thus the current source "helps" the transistor (the emitter follower) to change the voltage... and this combination of two active elements forms a kind of a cascode circuit.

We can explain the operation of this odd circuit in a simpler but striking way (to impress students:) by presenting it as a kind of a voltage divider where the upper transistor acts as R1 and the lower current source - as R2. The two "resistors" change their "resistances" in opposite directions like the two halfs of a potentiometer. So these are not humble resistor; they are dynamic resistors - the upper is a voltage-stable resistor, and the lower is a current-stable resistor.

Dear Abdelhalim,

You have said, "The static  output characteristics of the CC BJT transistor circuit is to be defined in the same way as the characteristics of the the CB and CE such that IE= f ( VEC , IB )".

But we have seen that it is quite oddly and not typical to control an emitter follower by a current. This is an emitter voltage follower, a voltage-to-voltage converter or a voltage-controlled voltage source. So the most natural is to use VBE as one of the input quantities (the parameter). Then the output quantity is also voltage... and the whole circuit acts as a voltage source. We know the IV curve of a voltage source is a vertical line; so the IV curve of an emitter follower is a vertical line...

I have said "circuit" since really we can think of the humble transistor (a component) here as of a simple circuit. And even something more - this is a circuit with negative feedback. Here we can see the unique ability of the negative feedback to modify the device properties. In this case, it has converted a current-stable element (the transistor) into a voltage-stable device (the emitter follower)... and its original horizontal output characteristic into a vertical line.

Vaibhav,

You have said, "The output characteristics of CC is of course unexplored? I am too doubtful about it."  Well, let's then ask Google to show some "output characteristic of common collector" (images):

https://www.google.bg/search?q=output+characteristics+of+common+collector&safe=off&biw=1280&bih=647&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIm7ixxLaLyAIVgQ4sCh3LQQD4#safe=off&tbm=isch&q=output+characteristic+of+common+collector

I do not see any characteristic... at least one I had seen... but there is no one... it is interesting why...

Arun,

IMO if the "CC configuration is VCVS (voltage controlled voltage source)", and the ordinate Y represents the current while the absciss X - the voltage, then the CC output characteristics would be vertical lines.

Here is another related question...

https://www.researchgate.net/post/Is_the_low_output_resistance_of_the_emitter_follower_a_result_of_negative_feedback