# Is a transistor truly an amplifier?

My understanding of transistor is that in it a tiny current controls a huge current. I get that concept. If that is the case, it is not amplifying anything in my opinion. Sure it does dictate when the huge current can or can't flow but it is doesn't change the huge current in any way I think.

Is it correct to call a transistor an amplifier?

So after going through the answers and bit more reserach I think I found the answer. This is what I think is happening. In transistor, Ic is proportional to Ib (and i think this is the key). So since Ic is proportional to Ib, Ic is an ENLARGED COPY of Ib. So if Ib is our input signal, Ic becomes enlarged copy of that input signal and that is why we say it is acting as an amplifier (creating an enlarge copy of the input). Please let me know if this interpretation is correct.

• Comments are not for extended discussion; this conversation has been moved to chat. – Voltage Spike May 10 at 16:55

amplify /ˈamplɪfʌɪ/

transitive verb

1. to expand (something, such as a statement) by the use of detail or illustration or by closer analysis
2. a: to make larger or greater (as in amount, importance, or intensity) : INCREASE
b: to increase the strength or amount of especially : to make louder c: to cause (a gene or DNA sequence) to undergo amplification

Source: Merriam-Webster.

Sure it does dictate when the huge current can or can't flow but it is doesn't change the huge current in any way I think.

It sounds as though you are thinking of a transistor as a switch where it is either fully on or fully off. Many applications use it as a controllable element where it is partially on. In these cases, the base is used to control the conductance of the collector-emitter terminals and hence it controls the current in an analog manner. This fits the dictionary definition 2 above. The fact that it is controlling the amplitude of the current means that it is changing it.

Figure 1. From Horowitz and Hill, The Art of Electronics. "Transistor Man" looks at the current at the base, and adjusts the current at the collector so as to be a multiple of the base current.

It is the transistor's ability to amplify a small control signal to provide a larger output current or voltage that makes it so useful.

• All - It's clear that there are strongly-held views on this topic, which is fine, but some recent comments were getting very close to "ad hominem" arguments, instead of technical ones. That would break the Code of Conduct to be kind and respectful, so a few comments have been deleted to de-escalate things. If members want discussion on this topic (which must be calm and respectful), then please use Electrical Engineering Chat and not comments. I have locked comments on this answer for at least a day, to allow people to calm down. – SamGibson May 10 at 13:49
• Some site members want to continue a discussion in comments, despite my explanation above that discussion should be in chat, not comments. So I've moved the whole comment thread to chat (link below). This moving of comments to chat can only be done once, so any new comments posted here trying to continue / start a discussion, might be deleted without notice. Keep it in chat, please! (If there are any CoC violations in chat, please flag them (hover over that posting, click the left "bar", click the relevant flag link.)) – SamGibson May 11 at 17:55
• Comments are not for extended discussion; this conversation has been moved to chat. – SamGibson May 11 at 17:56

Other answers have explained about a small current or voltage controlling a larger current or voltage. You must also consider the end result. The ability to make the larger current, voltage and power duplicate the variation or amplify the signal is the ultimate objective. The an information signal is made stronger so that it can be transmitted over a long distance with out being changed. More power in the signal also allows it to be converted to a louder sound and still have the same sound quality. A signal variation can also be made more powerful to control motion accurately when converted to mechanical power with a motor. Ultimately we must consider that it is the signal that is being amplified by controlling voltage and current from a power supply.

A transistor is often used in configurations which amplify signals by modulating the amounts of externally-supplied energy that get directed to the output, converted into heat, or allowed to remain in the source. Because there is no such thing as a perpetual motion machine, it's not possible to amplify energy. It is possible, however, to construct a device which, upon receiving small amounts of energy at the input, will cause an output to receive large amounts of energy that it would not otherwise have received. The device will need to take that energy from somewhere, but if it is given a source of "bulk energy" it may take that along with a small signal, and from the combination output a large signal.

By way of analogy, a photocopier doesn't simply take one document and produce many copies of it. Instead, it takes an original document along with a supply of blank paper and toner, and combines those things to produce copies of the original document. In functional terms, a photocopier "amplifies" a document, but it doesn't actually turn one document into many. Instead, it uses a document to direct the process of producing documents from of blank paper and bulk toner.

The word "transistor" was coined at Bell Labs to convey the function of the device:

TRANS-fer res-ISTER

They were trying to invent the JFET - a voltage-controlled variable resistance (their boss's pet idea), but couldn't get it to work. So they shifted to their own idea, a current-controlled variable resistance.

• An interesting story... 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. – Circuit fantasist May 9 at 20:23
• Here are some historical infos regarding the name "transistor": blog.adafruit.com/2011/09/05/how-the-transistor-got-its-name and wiki2.org/en/History_of_the_transistor – LvW May 10 at 9:02

you're thinking the transistor is only a switch - yes it can switch but it can also vary the current based on the input, so it therefore 'amplifies'. The collector current is related to the base current and one of the transistor specifications is its gain usually referred to in the datasheets as hFE.

So, for a gain of 200, a base current of 1mA will allow a collector current of 200mA, 2mA in for 400mA out and so on. There is a point where increasing the base current does not cause a corresponding increase in the collector current - this is called saturation. So for switching things on or off, we would either have no base current, or the base current at saturation.

There's a load of theory that determines how the transistor operates as opposed to my simplification above.

They are not a simple device, say, as compared to a mechanical relay, so one needs to understand a fair portion of the theory to confidently choose a suitable transistor and design a circuit to operate according to expectation.

• 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.). – Circuit fantasist May 8 at 11:58

## What "amplification" means

"Amplification" is to make an enlarged copy Y = K*X (K > 1) of a quantity X.

Similar operations are "following" (making an exact copy, K = 1) and "attenuation" (making a decreased copy, K < 1).

## How to amplify

The paradox of amplification is that it is made by attenuation. Let's for concreteness consider the usual case of voltage amplification where an input voltage source produces the input voltage VIN. To amplify it means to produce a higher output voltage VOUT that is proportional to VIN.

For this purpose, first we take another voltage source (power supply) with a voltage Vcc higher than the expected maximum value of the output voltage.

Then, to produce VOUT = K*VIN, we have to remove a part of the supply voltage. We can do it by connecting an element (carbon microphone, tube anode-cathode, transistor collector-emitter or drain-source…) with a general property of resistance. The IV curve of this resistance is not so important since, as a rule, a negative feedback is applied. The only important feature is to dissipate power… and it is controlled by the input voltage. So, this element is a sort of a "voltage-controlled resistor" (they call it "active" or "amplifying" element).

For example, in a common-emitter and c9mmon-base stage, voltage drop (VCE) appears across this element and we take the rest voltage (VRc) across the load (Rc) or its complement VCE = Vcc - VRc as an output (VOUT). The latter is necessary in the case of an NPN transistor since usually the load is grounded.

Similarly, in an emitter follower, there is a voltage drop (VCE) across this collector-emitter part of the transistor and we take the rest voltage (VRe) across the load (Re) as an output (VOUT).

So, in any case, there is a voltage divider configuration of two resistor elements in series - a variable "resistor" (transistor) and steady resistor.

• It seems like your analysis only apples to the common emitter case. In the voltage follower, there need not be any passive element. Just a DC supply, an input voltage, an output voltage, and a load. And yet the voltage follower can have power gain. – mkeith May 9 at 18:57
• 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. – Circuit fantasist May 9 at 20:11

No - the "naked" transistor is not an amplifier. It is rather a kind of converter.

It converts a small change of the (input) voltage between base and emitter into a corresponding change of the output current (emitter or collector current - depending on the output node definition).

Together with some external passive components (resistors, capacitors) the transistor can be used to form a so-called "amplifier stage", where the output current variations are transferred into corresponding voltage variations.

Additional Comment: As we can see, my answer has triggered a number of comments, which are all about one single question: Is the bipolar transistor (BJT) - from a purely physical point of view - an element where the magnitude and variations of the emitter current (and thus also the collector current) is controlled/determined by the voltage between base and emitter or by the flowing base current?

I have given my answer above - and there is no shortage in quotations, explanations, observations, effects and circuit characteristics, which can support and prove this statement of voltage control.

But as we see, there are also other views and claims:

• The BJT is a current controlled element, or
• For many applications, however, it would be at least helpful to consider the BJT as a current-controlled element (regardless of the physical conditions).

However, I must state that for both views no meaningful examples have been presented yet. I think, the originator of this thread ("Is the transistor truly an amplifier?") should be informed about this controversy in order to form his own opinion.

Comment to Fig.1 (Horowitz, Hill: Art of Electronics), see contribution from the forum member "transistor" :

It is interesting to know that the co-author of the book (Winfield Hill) is not very happy with the over-simplified representation as shown in this illustration. Here is what he writes

"BJTs are transconductance devices (voltage in, current out), just like tubes and FETs, which means they are first and foremost, voltage controlled."

By the way, some lines below the mentioned figure ("transistor man") the book says:

"One warning is in order here: Dont think that the collector of the transistor looks like a resistor. It does not. Rather, it looks approximately like a poor-quality constant-current sink (the value of current depending on the signal applied to the base)."

Some lines later we can read:

"The transconductor model will be accurate enough.. ......to understand differential amplifiers, logarithmic converters, ...and other important applications you must think of the transistor as a transconductance device - collector current is dermined by base-to-emitter voltage"

I think, this information given with the figure is necessary in order to interprete it correctly.

• But the humble resistor does the same (I = V/R); in addition, it can do the opposite (V = I*R). In contrast, the more sophisticated transistor can only do the first... – Circuit fantasist May 8 at 14:18
• OK - however, there is a big difference: For a resistor, the variations in voltage and current are valid between TWO nodes only. In contrast, for a transistor the voltage is between two nodes - and the controlled current goes through a THIRD node. Question: Cana resistor really do the opposite? No - a resistor never can transfer a current into a voltage . Always: Voltage first .....current as a result. – LvW May 8 at 14:55
• If we are willing to stipulate that a BJT is a voltage-controlled device, will any of the equations describing BJT behavior be changed? Will $I_C = h_{FE}I_B$ no longer hold? If I have a circuit that works when I call the transistor a current-controlled device, will any of the voltages or currents change if I start calling it a voltage-controlled device? Does the transistor know what words we use to describe it, and change its behavior accordingly? – Elliot Alderson May 8 at 23:29
• @ElliotAlderson I think that the enthusiastic advocators of viewing the transistor as a voltage controlled device are so because of the precisely defined nature of the relationship between Vb and Ic where as it is difficult to accurately determine Ic from Ib or vice-a-versa due to the value of hFE varying so widely with variables such as temperature and Vce. A circuit design that depends on the value of hFE is a poor design and, if designing using the current controlled model, a designer should always leave a large safety margin to allow for hFE variations. So use hFE/4 for a switch design. – James May 9 at 4:50
• Circuit fantasist, I am sorry, but we should not mix two different points of view: (1) physical laws and (2) personally shaped ideas about how the BJT works in a circuit. Question: Do you believe that the BJT is the only (mystical) electronic component for which the working principle could not unambiguously and clearly be explained? There is no need to allow two different views - unless someone can show and prove that the base current is not an undesirable quantity, but has controlling properties. I have asked often enough for such examples, until now without success. – LvW May 9 at 14:31

An amplifier is an electronic circuit with gain greater than 1, or you could also say with gain greater than 0 dB. The transistor can be arranged with just one or two passive elements to make such a circuit. It is for this reason that the transistor is correctly described as an amplifier.

Gain is the fundamental thing that defines an amplifier and distinguishes it from other electronic circuits.

So, yes, the transistor really is an amplifier because it has gain when used in a circuit a certain way.

• mkeith, 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... – Circuit fantasist May 9 at 10:02