Common collector is more usually called emitter follower. That is a more
helpful name IMHO.
Your idea about the terminology is mistaken. For 'common'
emitter/collector/base it may prove helpful to think *earthed/grounded*
emitter/collector/base. Where 'earthed/grounded' means the ac signal is
'earthed or grounded'. For example in a typical common-collector ( emitter
follower ) configuration the collector is connected to the supply rail which
is an ac ground.
It's the common terminal that *doesn't* have a 'signal' connection on it. It
is in a sesne 'common' because both input and output use ground as 'the
other side of the signal'.
These are good points and bring up to mind a confusion of my own, some
time ago. And anything connected to any voltage rail is _supposed_ to
not have any "signal" on it. If it has a signal there, the voltage
rail isn't doing its job!
I had not allowed myself to "see" a +5V rail as being similar to the
"ground" or 0V rail. To me at that time, they were very different
things, separated at least by a "battery," which was some complex
thing but certainly not "nothing" in my mind. Even the fact that I
used Thevenin equivalents to simplify circuits didn't clue me in,
though it should have.
The text that changed my mind (after some thinking) said that the
battery has an effective zero impedance and that in this sense there
is a "dead short" between 0V and +5V, for example, via the battery,
with the important detail being the voltage change. This started me
thinking more closely about exactly what the phrase, "voltage source,"
meant and to start clearing up my internal ideas of both "voltage
source" and "current source." I'd read texts saying this and that,
but in my confusion I hadn't bothered to think more closely about
them, and just left myself in a general state of conflation and
confusion.
Just to point out a circumstance which will "test" internal ideas
about what all this means is a cascode BJT pair. The node between the
two BJTs (the one connecting the collector of one to the emitter of
the other) is held (at least, in terms of the signal) at a relatively
stable voltage even though one does not actually see a voltage rail
there.
:
: ++V
: |
: |
: \
: / R1
: \
: /
: |
: +----> signal_out
: |
: Q2 | Q2 operates as a 'grounded'
: |/c base, passing along the
: +V-----| signal-generated current
: |>e from emitter to collector
: |
: | <--- relatively stable
: | voltage point located
: Q1 | here has a voltage of
: |/c about (+V - Vbe)
: signal_in >----|
: |>e
: | because of the above stable
: | voltage above, Q1 operates
: \ as an emitter follower
: / R2 (a good voltage controlled
: \ current sink, here, because
: / it's collector is 'clamped')
: |
: |
: gnd
I'm just a hobbyist and still learning slowly, though. So keep that
in mind.
By the way, when I was learning to draft electronic circuits in a
course I took on the Tektronix campus, I was taught to organize the
schematics so that "signal" flowed "left to right" on the sheet, that
hole flow (or reversed electron flow) should always cascade from the
top of the sheet to the bottom, and to _not_ bus voltage rails around
(the argument here is that busing voltage rails _confuses the eye_ and
makes it seem as though those wires are somehow important to
understanding the schematic.) So one of the helpful things for me was
to take schematics found in magazines, which rarely followed any of
these rules if ever and are horribly confusing to read, I think, and
to rewrite them into a form with the above rules applied.
Jon