Common Collector, emitter follower. What it really means.

[Thecomedian]

Some lit will call it common collector, some will call it emitter follower. Some will call it one and also say "it's also called ...".

from my understanding, what makes Emitter, Base, or Collector in common with the controlling and controlled voltages, is that the particular side is the only one that has no or equal resistance to both the input and the DC.

Every "perfect circuit" schematic, which is largely oversimplified versions of the thing, as well as schematics for "real application" circuits maintain this rule (apart from the variable cap in fuzz faces). This is where it gets confusing. Could you not build a Common Collector and take the output from Vc instead of Ve?

[R.G.]

Some lit will call it common collector, some will call it emitter follower. Some will call it one and also say "it's also called ...".

Common collector is more correct. The "follower" name came from the fact that it's a nearly-unity gain buffer like the earlier "cathode follower". The guys naming these things were TUBE designers.

from my understanding, what makes Emitter, Base, or Collector in common with the controlling and controlled voltages, is that the particular side is the only one that has no or equal resistance to both the input and the DC.

Not quite. For a three terminal device, you can stuff input into one terminal, take output out the other. The third terminal plays a part in both input and output sides, so it's the "common" terminal between input and output.

For common emitter, the input is the base, the output is the collector, and both the base current and collector current flow through the emitter.

For common base, the input signal flows in the emitter, some of it  is siphoned off to the base. The base region's conduction enables the collector to pass current. So the base is common to both the input and output operation.

For common collector, the base voltage signal is relative to ground; so is the emitter. The collector is held fixed with respect to ground, so it's common.

Could you not build a Common Collector and take the output from Vc instead of Ve?

Not in simple circuits. There are some contortionistic circuits with driver and output transformers that blur this. But in a simple circuit, no; the collector voltage is fixed with respect to ground. You can take the output there, but there won't be any output to take.

[PRR]

> For a three terminal device, you can stuff input into one terminal, take output out the other. The third terminal

In different words:

Every circuit is a complete LOOP. To connect it into something, you need two wires.

2-pin wall-plug
tip-ring guitar plug.

We have an INput circuit and an OUTput circuit.

Therefore we have _4_ wires.

Tubes and transistors have _3_ legs.

One leg must be used for *both* input and output.

Which leg is that? Call it "common".

"Common (EBCGKP)" is the canonical system of ALL possible connections.

"Follower" is a nickname for one handy corner of the full set.

[Thecomedian]

that makes much more sense.

[merlinb]

"Common collector" is the general term for a circuit where the collector is, quite literally, 'common' to both the input and output 'ports'. (A port is the two wires the form an input or output).

"Emitter follower" is the name for just one specific type of common collector circuit.

In other words, an emitter follower is a common collector amplifier, but not all common collector amplifiers are emitter followers.

[PRR]

me> "Common (EBCGKP)" is the canonical system of ALL possible connections.

I mis-stated that.

Here is the cannonical table.

You have an input (2 wires) and an output (2 wires), on a 3-leg device. One (or two) leg must be "common"; this is inferred from the leg not mentioned. If input and output are the same leg, no power gain is possible, "useless" (with ordinary devices in simple connection; ignoring Dynatron).



This is for vacuum tubes. JFETs are very similar, few differences. A similar table for BJTs has several differences and is left as an exercise for the student.

{"Inverse Mode" is a very special case. Pretty much vacuum-tube only. It has few uses, and fewer good uses. It can take a high voltage with very low loading and convert to a low voltage load for measurement. It appears to also have use in low-voltage "power" amps but is rarely better than some other connection.)