Are you asking about small signal AC output impedance? The two figures you have show the DC output impedance for the two configurations which can be totally different from the AC output impedance.

I mean small signal AC output impedance. IMO the slopes of the curves (in the active region) in the two figures well represent these impedances.

I understand. For the DC IV curves that you have shown, it makes sense for Common-Emitter output resistance to be smaller than Common-Base. I did a small derivation (attached) to show that this makes sense from device physics perspective. The same argument holds for low frequency AC signals also.

The key to this argument is that device gain is large and is a function of VCB only (because VCB determines WB of the device).

At the moment I don`t have the time to go through all the arguments and mathematical derivations, therefore only two short comments:

* I think, it is not allowed (it makes no sense) for comparing the slopes of both set of curves presented by Prof. C. Mechkov because both sets are not based on the same constant parameter (Ie resp. Ib); more than that - where is negative feedback in the circuits? The given link applies to common collector configuration which is not the subject under discussion?

* According to my experience, I have severe doubts (it cannot be true) that the output impedances of both configurations differ by a factor as large as the current gain beta.

Yes, Lutz... you are right about "both sets are not based on the same constant parameter (Ie resp. Ib)". IMO we should distinguish between the two common-base arrangements - when the emitter is driven by a voltage source and when - by a current source.

• When CB is driven by a voltage source, there is no feedback... there is no difference between the two configurations... and both set of curves are identical... The transistor (its CE junction) will behave as a poor constant-current differential resistor... and, "looking" at the collector, the varying voltage source (power supply) will "see" its relatively high AC resistance.
• When CB is driven by a current source, there is a feedback (emitter degeneration)... and there is a significant difference between the two configurations. Now the transistor, forced by the negative feedback, behaves as a voltage stabilizer (common-collector stage) that adjusts its base-emitter voltage so that to deliver the same as the input current through the collector. Thus, "looking" at the collector, the varying voltage source (power supply) "sees" extremely high AC resistance. The whole configuration (the input current source and the CB transistor) represents the so-called cascode. We can observe this configuration in the long-tailed pair, op-amp input stages, emitter-coupled logic, etc.

The link about the output characteristic of the common-collector stage is related since here we talk about the output resistance of basic transistor configurations... and this is the third of them.

- I understand that the constant parameter is different for the two cases. Still, the fact is that the slopes are different and there is a physical explanation for that.

- I do not fully understand the negative feedback argument either.

- What I am trying to say is that the output resistance in common emitter is finite and can be calculated. This is the typical derivation in device physics textbooks. In common base, output resistance is larger by a factor of beta. This means that the output resistance is extremely large and the IC curve is flat. In the given IV curve also, the ratio of output resistances (common base/common emitter) is almost infinity (because the common base curve has practically zero slope).

To Dr. G. Gupta:

I must admit you are right as far as the output resistance in CB configuration is concerned. At the first moment, I was a bit confused in connection with  the different hybrid parameters (and their interrelations) for CE and CB configurations. But now it is clear.

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.

Then, if we connect a current source (or simply a voltage source and a resistor in series) in the emitter, we have the following situation: the current source will set the emitter (collector) current while the emitter follower will set the voltage across the current source. Actually, we have two sources (voltage and current) connected to each other... where the voltage source keeps up the voltage while the current source - the current... and this aggregate is called "cascode".

Now, if we begin varying the collector voltage (the power supply), these two sources will continue uncomplainingly perform their duties as above operating as a good current source with negative feedback... and the horizontal lines in the first picture are due to this source...

Gentlemen - I am confused and I don`t know what we are discussing here. I think, we strictly should restrict ourselfs to the main question. More than that, I think it is necessary to mention all preconditions - otherwise misinterpretations and misunderstandings are unavoidable.

First example: Dr. G. Gupta has presented the results of a calculation without mentioning explicitely that he has assumed  IE=const. (You must go into the detailed calculation to see this requirement). Of course, in this case the output resistance of the CB configuration is strongly increased - but only in this (nonrealistic) case. For a voltage injection at the emitter node  the output resistance is equal to the value which we have for the CE configuration (without any RE in the emitter path).

Second example: Cyril brings the emitter follower into the game. I cannot see where we have something like "emitter following" within the CB configuration. (Actually, we have two sources...). Without additional explanations I do not understand - sorry.

Well, Lutz... I will explain it in detail...

What is a CB stage? It is nothing else than a CC stage (an emitter follower) with a constant input (base) voltage where we have applied the input signal (connected a voltage or current source) to its output - the emitter. It sounds strange but it is true - we drive the output of a voltage follower (voltage stabilizer) with the input voltage or current source... and, of course, it react to this intervention by changing its current.

So, if we look at the CB input (the emitter), we will see the output of a CC stage (emitter follower). I think that you should agree with my argument?

Yes - but how does this view help? At first, there is no feedback effect (as in CC) and secondly, it is not a pure CB stage if the signal is applied using a resistor RE.  And this is an important point while speaking about the output resistance. Do you know what I mean?

Yes Lutz, I see... I think the "true" CB stage is voltage-driven. Then there is no feedback, and the output IV curves are inclined exactly as in the case of a CE stage.

But if we introduce even a small resistance in series to the input voltage source (connected in the emitter), the negative feedback appears... and the output curves become more horizontal. If we replace the voltage source with a current one, the curves become horizontal.

So, the explanation is: a CB stage with some resistance in the emitter, is the well-known current source with a negative feedback (emitter degeneration). When we change the collector voltage (the voltage of the power supply), this current source keeps the current constant. That is why the output resistance is extremely high, and the curves are horizontal.

As a final conclusion...

There are two CB configurations - voltage-controlled and current-controlled, with different (differential) output resistance and behavior:

• The voltage-controlled CB stage does not posses negative feedback. As a result, it has the same as the CE stage output resistance, and its output characteristics are inclined due to Early effect
• The current-controlled CB stage posseses series negative feedback (emitter degeneration). As a result, it has higher than the CE stage output resistance. Its output characteristics are almost horizontal since the Early effect is neutralized by the negative feedback
• So, the CB stage does not always have high output resistance but only in the case when it is current-driven
• The negative feedback is that which increases the output resistance of the CE stage (and CB without negative feedback). It is interesting fact that, to do this "magic", the negative feedback changes the behavior of the transistor. To keep the collector current constant, in the case of CE (CB without negative feedback), the transistor acts in the usual way - as a current stabilizer (current-stable non-linear resistor)... while in the case of CB with negative feedback, the transistor acts as a voltage stabilizer... or as an emitter follower keeping up a constant emitter voltage across the resistance of the input source.

The variation of the collector voltage when measuring the output characteristic, is a disturbance (something that tries to change the collector current) for this elementary negative feedback system. It reacts to this intervention (the transistor changes its present collector-emitter resistance) to restore the equilibrium.

• So, when measuring the output characteristics of a CB stage driven by  a constant emitter current, this CB stage is actually a disturbed (from the side of the collector) CC stage (emitter follower). Thus, actually we investigate a classical transistor current source with series negative feedback (emitter degeneration).

The common-base stage is actually a Shunt-Series (CCCS) feedback amplifier, known as current amplifier. Based on attached schematic we get: Ac = -h21/(1+h21), Zin = h11/(1+h21), Zout = h22(1+h21), so low input and high output impedance.

CC stage is a voltage amplifier (VCVS) having high input and low output impedance.