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.