I've been told that common emitter and Common collector and Common Base amplifiers are so named because the inputs and outputs either share a common emitter, common collector, or base. I see where this is the case with the CE, and CB amplifiers, but the CC amp to me just seems to be a CE amp with the load attached to the emitter. So wouldn't it still have the emitter in common with the input and output. Could someone please tell me what I am not understanding?
Thanks for any light you can shed on this subject.
If you look closer at he circuits you should see the common element is connected (AC or signal) to the ground or common of the input and output sides. While the cc and ce look similar, the coupling capacitor for the output is moved and also the capacitor for the emitter in the CE circuit is removed and placed from the collector to the ground.
Each circuit has differant ammounts of current and voltage gain. Also the impedance for the inputs and outputs are differant.
If you draw your cc circuit with a pnp transistor, ie. with the collector connected to the ground, it will then "look" common. In fact as far as the ac signal is concerened all the power rails are common.
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 : | : | ----| : |>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.
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