Does a Transistor Amplify?

In this story, I have collected links pointing to my materials (questions, answers, comments...) about the basic transistor property to "amplify".

My answers

My comments

ResearchGate

Below are my comments to RG questions. They are chronologically arranged (ResearchGate does not have a possibility to link to them).

Is a transistor really an amplifying element?

The main task of an amplifier is to produce voltage... it is a voltage-controlled voltage source... How does it produce voltage? By attenuating some else's (power supply's) voltage... How does it attenuate this voltage? By series-connected variable resistor (transistor)... But if there is no load connected (open circuit)? So, another resistor (transistor) is needed... and the two resistors (transistors) form a voltage divider. In this voltage divider configuration, the input voltage source can control either the first or the second resistor (transistor) or the both (the powerful push-pull idea)... but this is still a voltage divider.
But can a variable resistor (e.g., a carbon microphone) act as an amplifier? If we couple a speaker with the microphone? Well... but still we can think of the combination "speaker+microphone" as of a kind of an "electromechanical transistor".
So, there is no amplification... it is impossible... there is only attenuation... The so-called amplification is only an illusion... where we do not (want to) see the additional power supply... We see only the input power at the port 1-2 and the output power at the port 3-4... then make the ratio between them and name it "gain"...
A new thought is that we began thinking about the "amplifying" (controlling, regulating...) element as another load that dissipates power. Thus actually we have two loads dissipating two kinds of power - useful and harmful... as though the power is steered between them...
"Attenuation" expresses the relation (ratio) between the partial load power and the total supply power... while "amplification" gives the relation (ratio) between the partial load power and the input source power. From this perspective, it seems that "attenuation" is more real and true term since it gives the proportion between two powers produced by the same power source... while "amplification" gives the proportion between powers produced by different power sources... although there are exceptions... A similar situation is when using terms as "resistance" and "transresistance"... "conductance" and "transconductance"...
The output (collector-emitter) part of the transistor does not produce current... it is not a source... Then what is it? It is something that impedes (opposes, resists) the current caused by the power supply in the output part of the circuit... it is a kind of resistance... non-linear, current stabilizing... but still resistance. From this viewpoint, the transistor is a kind of a voltage-to-resistance converter... a voltage-controlled (nonlinear) resistor... Only, in terms of geometry, the input base-emitter voltage controls not the angle of the IV curve but its horizontal position...
According to my considerations above, here is a possible definition of "amplification": The amplification is an electrically controlled attenuation. The trick is that we attenuate a more powerful (than the input) source... so the resulting partial power is always bigger than the input one... as though it is an "amplified" input power... and we take it as output power...
I also cannot imagine how a "smaller quantity can directly control/determine the value of a larger quantity of the same kind". IMO the "smaller quantity" can do it by means of a third element having the property of resistance that dissipates some part of the supply power; the last part of the power is dissipated in the load thus forming the "larger" quantity. As I have already said, at my last lecture, my students and I began thinking of the third (controlling) element as another "load" that dissipates power.... thus actually we have two loads dissipating two "complementary" powers - useful and "harmful". In some cases we use the voltage drop associated with the useful power (across the load), in other - the "harmful" (across the controlling element). We can use event both or more - as in the case of the phase splitter (VRC, VCE, VRE).
In some amplifying arrangements, there is no input voltage source... there is only one supplying voltage source... The input voltage is "produced" by a voltage divider with a sensor (e.g., a carbon microphone) connected instead the one of the resistors... and this voltage divider is supplied by the power supply. So, in this arrangement, an (input) attenuation controls another (output) attenuation... as though there are two cascaded voltage dividers... and this is an amplifier assembled by attenuators.
As you probably know, the humble carbon microphone can "amplify"... acting as a kind of a "transistor"... but actually, it attenuates. It seems the attenuation is an ubiquitous operation.
IMO the main question is: Does any electrically-controlled attenuator "amplify"?
Now I would like to clarify my question above, "Does any electrically-controlled attenuator 'amplify'?" As a result of this discussion, we have come to the unanimous opinion that "amplification" actually means "controlled attenuation". We implement this attenuation by replacing the one, other or both elements of a voltage divider by an "electrically-controlled resistor". Regarding the output part (collector-emitter, drain-source...) of this controlling (regulating, modulating) element, it can be linear (ohmic) or non-linear (constant-voltage or constant-current).
It is interesting to discuss what kind of characteristic is better but, for now, let's focus our attention only to the fact that, generally speaking, there is a kind of an "electrically-controlled resistivity". As any "trans-element", this electrically-controlled "resistor" also has its own transfer ratio ("gain")... and it should be expressed by some ratio - a sort of a resistance/voltage. My thought is that the value of this ratio determines the kind of the operation: If the ratio is bigger enough, we will observe an "attenuatior acting as an amplifier"... otherwise we will see an "attenuatior acting as a follower"... or even an "attenuatior acting as an attenuator":) In other words, to obtain an amplification, the element should vigorously cnange its resistance.
Thanks for the definitions... but all they consider the relation between two quantities (powers) - input and output... This arangement consists of three components - an input power source, an attenuator and a load... and we are interested in the ratio between the output quantity and the input quantity. Here we are talking about more sophisticated atenuation... a controlled attenuation... In this arrangement, we have four components - an additional input power source, a supply source, an attenuator and a load... and here we are interested in the ratio between the output quantity and the new input quantity. Also it is interesting to compare the three kinds of "voltage-controlled resistors": linear (R), constant-current (T) and constant-voltage (D)... and to see why, in practice, we use mainly constant-current nonlinear elements (transistors).
All they have "resistivity", i.e., they oppose the current... and their resistivity (IV curve) is controlled by the input voltage in different ways: R rotates, T moves vertically and D - horizontally. Thus, in the first case, thе input voltage controls the angle of rotation while, in the last two cases - the position. In the first case, the output current depends only on the input voltage - IOUT = f(VIN). In the second case, the current depends on both the input voltage and the instant resistance - IOUT = f(VIN, R); in the third case - the output voltage depends on both the input voltage and the instant resistance - VOUT = f(IIN, R). The interesting question is, "Which of these three ways is best and why?" IMO in the last two cases (non-linear elements) we have linear transfer ratio... so they are preferable.
I suggest we to put such a motto of our discussion: TO AMPLIFY, YOU HAVE TO ATTENUATE! ... or even the simpler: TO INCREASE, YOU HAVE TO DECREASE! ... or in a broader aspect: TO WIN, YOU HAVE TO LOSE!
The picture represents three, superimposed on the same coordinate system, IV curves corresponding to three generalized elements - a resistor R (ohmic resistance), transistor T ("I type" nonlinear resistance) and diode D ("V-type" nonlinear resistance). Examples of such elements can be: a FET transistor (in the initial part of the output characteristic), a bipolar or FET transistor, and some kind of a "voltage-controlled Zener diode" (e.g., such a circuit). You understand that I allow myself to fantasize a little:)... but actually, if now there is no such an element, it could ever be invented. It is supposed these elements are voltage controlled; so when the input quantity changes, their IV curves respond somehow to this "intervention": R IV curve rotates around the origin of the coordinate system, T moves vertically... and D - horizontally. We can immitate these elements by moving the sliders of rheostats in the corresponding way... see this Wikibooks story dedicated to understanding the differential resistance: Virtually Zeroed Resistance, Virtual_Infinite_Resistance.
Obviously, active elements are somehow related to sources... but they are not sources... they are elements that can control sources. They do that by dissipating power like passive elements... but they are not simply passive elements... they are controlled passive elements with changeable resistance.

StackExchange

Below are my comments to SE EE questions. They are chronologically arranged and can be accessed by links.

2021

Differential pair active load contradiction. I use such a scenario to explain what a transistor is: First, I say the transistor is a "resistor" because it dissipates power. After, I clarify that it is not the 19-century Ohmic resistor but a "non-linear resistor" (draw its output IV curve). Then, I show that it is actually a "dynamic resistor" (draw the rotating IV curve of its instant static resistance). Finally, I begin changing Rc (your comment) and illustrate graphically how the intersection (operating) point moves almost horizontally...

Differential pair active load contradiction. I am most annoyed by "explanations" such as "the transistor changes its collector current". How does it change it? Generally speaking, at constant supply voltage, the only way it can do this is by changing its resistance (no matter linear or non-linear)... there is nothing else to change. But in short, we say directly the end result - that the current is changing and not the reason for this change...

Differential pair active load contradiction. Exactly! Simply put, clear and unambiguous! What prevents people from expressing themselves in this way? The transistor is a variable resistor... so the network of a transistor + collector resistor is a variable voltage divider with one varying resistance, the other is static… in the simple active load stage, the other resistance is dynamic (self-varying) so the network is a dynamic voltage divider... in the more sophisticated (OP's) current-mirror active-load stage, both resistances are differentially varying and dynamic... so this network is a fully dynamic voltage divider...

Is a transistor truly an amplifier? The problem with accepting a transistor as a controllable resistor comes from the fact that it has a nonlinear (current stabilizing) resistance in the horizontal part of its output characteristic. But the carbon amplifier is an example of amplifier implemented by a controllable linear (ohmic) resistor.

Is a transistor truly an amplifier? I consider Rc in the common-emitter stage as a load and Re in the emitter follower as a load as well. So, in both cases (and in any case) there is a voltage divider configuration of two elements in series - a variable "resistor" (the CE part of a transistor) and steady resistor.

Is a transistor truly an amplifier? True, but it does not explain the mechanism of what is called "amplification". Sounds like a definition... OP does not want a definition but an explanation of how this happens...

Is a transistor truly an amplifier? The OP's question (and answer) is conceptual. It is about what exactly the so-called "active devices" do. To answer such general questions, you do not need to know what exactly is inside the specific device. This is a concept... and concepts do not depend on the specific implementation (carbon microphone, tube, transistor, tunnel diode, etc.).

Intuitive way to think of transistor saturation current? Great explanation! This is the way to explain and understand, step by step, basic ideas behind semiconductor components and circuits - through simple equivalent electrical elements and circuits. Indeed, I prefer to use a varying resistor (rheostat) for this purpose but there is not much difference. The important thing is the transistor shows some opposition against the supply voltage and part of this voltage is lost across it...

How can a transistor amplify current in a circuit? I have nothing against using the terms "current amplifier" and "voltage amplifier"... and I regularly use them... but I want us not to forget what lies behind them. The purpose of my answer is to reveal the idea of what we call "amplification" by showing that, at first glance, it is absurd but inevitable in analog electronics. The absurd thing is that we get gain through attenuation: we attenuate the voltage of the supply voltage source but still this voltage is many times higher than the input voltage... and we say that this is an amplified input voltage.

2020

Does a transistor amplifier produce negative voltage? Exactly! "Voltage controlled resistor" is the most intuitive notion about the transistor that gives a chance to understand/explain what it does in the circuit. It is a passive element and the only thing it can do is to dissipate power in a form of heat. Indeed, the transistor is a "nonlinear resistor" but this is not essential in the case. Eventually, when the collector voltage is zero, it has used up all its resistance and its collector voltage could "go down" below zero only if its resistance becomes negative (if there were a negative voltage source inside it... as I joke with my students:-)

Amplifiers without vacuum tubes or transistors? It sounds fantastic and brought us back to the romance of invention… I can't stop admiring the "elegant simplicity" of 2-terminal amplifiers. What a powerful and intuitive idea - a dynamic resistor that changes its resistance under the control of the input voltage and thus controls the current in this same circuit. What perfect simplicity - four elements in series (input source, power supply, negative resistor and load) - Negative Differential Resistance.

Amplifiers without vacuum tubes or transistors? The power supply is a 'constant voltage source' but we need a variable (voltage-controlled) voltage source. To make it (in a silly way:), we connect a variable (voltage-controlled) resistor in series. Thus, the combination of the two elements acts as a 'variable voltage source'. The smarter way to do this is not to dissipate excess power but to control the conversion from non-electrical to electrical energy in the power supply itself... as is done in power plants... and how we do in life (we do not regulate the power by wasting it, e.g. in heating).

Amplifiers without vacuum tubes or transistors? Another example is the carbon ampifier (a stickied speaker and carbon microphone) that was used in telephony.

Common-Source Amplifier. Do you know what a "potentiometer" and "variable resistor" (rheostat) are? And that a potentiometer can be assembled by two oppositely changing resistors in series? The total resistance and current are constant; only the two partial resistances and the voltage drops across them are redistributed (these are ideas from the 19th century). CMOS stage is such but very sensitive electronic "potentiometer" assembled by two oppositely varying "resistors" (NMOS and PMOS). This is the simple idea behind this topology...

Common-Source Amplifier. Razavi's book is not the place where you can find the intuitive explanation you need. Two concepts can help you to understand the complementary (CMOS) pair - "voltage divider" and "dynamic resistance", that can be combined into a "dynamic voltage divider". So think of the two (drain-source parts of) transistors of as the two halves of a "dynamic voltage divider". When the input voltage changes, their resistances vigorously change in opposite directions. As a result, the common current does not change but the voltage drops vigorously change.

How this current mirror act as a replacement to resistor Rc? Exactly! Or maybe a "potentiometer"? Of course, CMOS is a better (exact) example of a "dynamic voltage divider" suitable for graduate students. "Potentiometer" is a simplified electric analogy where the two resistances are linear (lower gain); it is more suitable for 6-year old boys (Einstein).

How this current mirror act as a replacement to resistor Rc? The "dynamic voltage divider" consists of two "dynamic resistors" that change their resistances oppositely (in a differential manner). I have illustrated it in the pictures above. The power of this viewpoint is that it is very intuitive, "material" ("variable resistor" is a well-known concept). But, of course, it is only one possible way of understanding this phenomenon in addition to the existing.

How this current mirror act as a replacement to resistor Rc? ... I agree with you that we are talking here about the current-to-voltage part of this amplifier stage... that is resistance (dynamic and static). I will say it again: The dynamic resistance is responsible for the extremely high gain. The static load resistance (in parallel) only decreases the gain. I am expressing myself again in terms of resistances instead of currents. Why? Simply because the resistance is before the current. When you say "the current has changed" it means that the resistance has changed before that... the current change is a result of the resistance change...

How this current mirror act as a replacement to resistor Rc? When saying "resistance" I mean the present (current, chordal) "static resistance" RA = VA/IA in the current operating point A (look at the pictures above). During the circuit operation this resistance varies and creates an illusion of lower, higher or, in some circuits, negative "differential resistance". I wonder how such an intuitive concept is so difficult for people to perceive it... and it causes negative reactions...

When does a transistor act as a switch, and when as an amplifier? Nice explanation... I have read it with pleasure (maybe because I am a teacher and the way of explaining is important for me). To pique the interest of my students, when reached the point of saturation, I usually ask them, "How can we continue increasing the collector current beyond the saturation? Let's dream..." And we consider the situation in terms of resistances...

The potential drop in a pn junction diode. ... this is only in circuit theory where the source can be not a source; in real life where we are, the source is source producing power and the resistor is resistor consuming power. Every engineer (including Transistor) and technician dealing with real elements and circuits (not only with their abstract mathematical models) imagine the Zener diode as a non-linear (dynamic) resistor forming a (dynamic) voltage divider with another resistor that is supplied by the genuine power source. If you want, think of it as of "constant V" and don't even draw it with this misleading symbol.

Why don't I need a resistance when testing a light bulb circuit in a breadboard? I have illustrated the idea behind the voltage-stabilizing dynamic resistance in my answer above. It is based on the believe that basically, the diode is a kind of "resistor" but non-linear... and this non-linear resistor can be thought as of a dynamic (self varying) static resistor...

The potential drop in a pn junction diode. My goal is understanding and in the name of this I have introduced these intuitive elements. The difference between the two explanations is that from mine they will understand how the element does it while from yours they will only know what it does. It is much clearer and more understandable to explain how a Zener diode maintains a constant voltage drop across itself by presenting it as a dynamic resistor. I am curious how you will be able to explain it by modeling it with an ideal voltage source. Or will you just tell the poor students that it is by definition so?

How does a transistor make a signal copy A very absurd way of amplifying... A low-energy input source controls a high-energy power source by throwing away some of its energy... but still the rest of energy is greater than the input energy... and we say that the energy is "amplified"...

What is the difference between differential resistance and resistance? And why is it called "dynamic resistance"? What is "dynamic" here?

Triangle wave input with Zener and diode. This is another confirmation of my thesis that diodes should not be represented by voltage sources and transistors by current sources because this is misleading for beginners. The most accurate general idea of them is they are simply dynamic (nonlinear) resistors that have the ability to change their resistance so as to maintain the voltage across or current through them invariable.

Emitter resistor and biasing voltage. Source models do the job but in principle they are incorrect and often misleading. There is a need for two new symbols (and names) to indicate those passive elements that have behavior of sources but are not sources. They become sources when combaining them with the power supply...

Emitter resistor and biasing voltage. Do you need to know anything about semiconductor physics just to judge that there are no any sources inside these semiconductor elements? They are just as passive (from an energy point of view) as a resistor, rheostat or potentiometer... and it is quite strange and misleading to attribute energy to them. Even a capacitor or inductor deserves it more.

Emitter resistor and biasing voltage. Then why they model the forward-biased diode by a voltage source? Or the transistor by a current source? Is this more true? I think it is more correct to define at the very beginning these two types of elements... to add them to the library of elements... and then model the diodes and transistors through them... rather than through sources... what they are not...

Is the voltage at this node 0 volts (quick yes or no) There are many not so accurate analogies which, however, are widely used. For example, we represent the forward biased diode through a voltage source even though it is not a source at all but a voltage-stabilizing nonlinear resistor. Similarly, the emitter current source in a differential pair is not any source but a current-stabilizing nonlinear resistor. Sources accumulate energy but these resistors do not accumulate anything. More about the mechanical analogy above: the spring elasticity is proportional to its resistance and the length of the stretched spring to the voltage drop across it.

2015

Basic questions about transistor amplification. From this "energy viewpoint", the transistor does not amplify; contrary, it attenuates the power of the source... it does not produce energy; it consumes energy.

Basic questions about transistor amplification. Nice explanation... To move it to the electrical domain, we can simply say the transistor is an "electrically-controlled resistor" inserted in series ("rheostat") or in parallel ("shunt") to the load. Thus it forms a voltage or current divider. To be more precise, we can only add that this "resistor" is non-linear, and it is controlled both by the side of the input source and the load. And also, the transistor is a passive, not active device (regarding the power). From this "energy viewpoint", the transistor does not amplify; contrary, it attenuates the power of the source... it does not produce energy; it consumes energy.

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