Confused about transistor coupling

[Dazzle]

Hello,

I have been studying a transistor circuit and was informed that it is of the common collector type because the collector is directly connected to the power supply rail. However, this is confusing to me because it appears that the current from the power supply would primarily influence the output, rather than the input. Intuitively, I would classify this circuit as a common emitter configuration since the emitter is connected to ground, even though it's through a resistor.

If someone can explain the rationale behind identifying the collector as the common element in this transistor coupling, I would be most grateful. Many thanks!

[magic]

The collector is connected to a fixed supply rail, so it is neither the input nor the output. The emitter is the output.

In CE, the emitter is neither the input nor the output and the collector is the output (and its voltage is allowed to vary).

[RoGeorge]

If someone can explain the rationale behind identifying the collector as the common element

The naming (common "something") comes from the AC (Alternative Current) analyses of the circuit, not from the DC (Direct Current) schematic.

AC analyses is looking only for the AC signal, and for the AC signal, a voltage power supply is nothing but a short circuit.  So, when we draw the AC model of the posted schematic, the V1 (which is DC, a fixed 10V power supply) is replaced with its AC model:  a short-circuit.  Thus, in the AC model of the posted schematic, after we replace V1 with a short-circuit, the collector appears tied to ground.

Now, looking at the AC model of the schematic, the only terminal of the transistor that is common to both the input and the output is the collector, hence the name:  common collector (CC) amplifier.

[ejeffrey]

Because current always flows in loops, input and output each need two terminal.  A transistor only has three terminals so one must be "in common".  Once you pick the common terminal the other two are constrained because the transistor only has gain one way.  This this leads to the classification of the three basic topologies of single transistor amplifier: common emitter, common collector, and common base.  There are also FET and vacuum tube equivalents with different names but the same topology.

[MarkT]

Common collector is normally called emitter-follower which is more descriptive.  There are other variants, for instance you can use both emitter and collector resistors to have emitter and collector as anti-phase outputs, and some of the two-transistor topologies.

[magic]

Because current always flows in loops, input and output each need two terminal.  A transistor only has three terminals so one must be "in common".

Mentioning current loops is misleading because there are always two obvious current flows: one from B to E and another from C to E, so E is "common" in every circuit configuration. This is obviously not what these names are about. The problem is that emitter followers are never ideal buffers and they "leak" significant input current to the output, creating this confusion.

Situation is more clear in FETs (or vacuum tubes, where these "common X" names originally came from) - there is no DC gate-source current, only some AC current flows into capacitances of the device, and if the source follower is lightly loaded, the gate-source current is lower than the gate-drain current, which is independent of output load and can be regarded as the one that controls the output of the device. In this light you can see that the gate-drain voltage/current is the control input to the amplifier, the output is referenced to ground and the drain is either directly grounded or a fixed offset away from ground, hence the drain is common to both.

It may help to imagine a very similar circuit, where the transistor is PNP, the collector is literally grounded, and the emitter is fed by a positive current source or a resistor connected to a positive voltage rail. Then there should be no doubt that ground is the common reference to the input (base) and output (emitter) signals and that the collector is the "grounded" or "common" terminal.

[Dazzle]

Because current always flows in loops, input and output each need two terminal.  A transistor only has three terminals so one must be "in common".  Once you pick the common terminal the other two are constrained because the transistor only has gain one way.  This this leads to the classification of the three basic topologies of single transistor amplifier: common emitter, common collector, and common base.  There are also FET and vacuum tube equivalents with different names but the same topology.

Thank you all for your insights.

From what I gather, the term 'common' in these configurations relates to AC analysis, where the power supply is effectively a short circuit for signal paths. Consequently, for AC signals, the collector is common to both the input and output, which contrasts with a DC analysis perspective. However, I'm still not clear on what the AC signal paths would look like in this circuit. Perhaps examining a circuit specifically designed for AC amplification would help? In a DC analysis, it appears to me that the currents overlap at the emitter, as shown in the figure below. This is likely always the case for any NPN-transistor configuration, as magic pointed out in a recent post.



Could someone clarify how the AC signal paths differ from the DC current loops I've drawn? Following magic's suggestion, I have tried to read about FET-amplifiers on this page:

https://www.electronics-notes.com/articles/analogue_circuits/fet-field-effect-transistor/fet-circuit-configurations.php

However, it is not clear to me how they work either. I do not understand what the N-channel and P-channel are, or even where the power for amplification comes from. Perhaps someone can suggest a more basic introduction?

Additionally, ejeffrey mentioned 'gain one way,' and I do not fully understand what this means. How does this concept relate to signal amplification in the transistor, and how does it influence our understanding of 'common' terminals?

While it seems we can often identify the common terminal in practice as the one directly connected to a power source or ground, as suggested by magic, it does not seem to always work. Here is an example of a common emitter amplifier where neither terminal is directly connected to the power source or ground, with one terminal connected to the power source through a resistor and the other to ground through a resistor and a capacitor in parallel.

https://www.electronics-tutorials.ws/amplifier/amp_2.html

As you can tell, these basic concepts are rather difficult for me to grasp, and I very much appreciate your help in sorting them out!

[magic]

AC current is any variation in current flow due to changes in signal voltage. Here varying voltage across R1 causes emitter current to vary, which splits into two components: varying collector current (your red path) and varying base current (your green path). But this is all due to load, and if you replaced R1 with an ideal current source these AC currents wouldn't be there.

Less obviously, there is an AC current into base-collector capacitance caused by changes in B-C voltage. It is much smaller than the base current under most circumstances, but it's always there regardless of emitter load. This current returns back to V2 through V1 and ground. It is this current which really enables/causes base voltage to change, which is how the output voltage is controlled.


The simple rule is really "whichever terminal is neither the input nor the output". The last example that you linked could be changed into a common collector amplifier simply by moving the "Vout" label from the collector to the emitter. It will work, because the emitter does follow changes in base voltage almost 1:1 as long as Re is not too low and C2 not too high. Although in practical circuits of this type C2 will typically be too high for significant AC voltage to exist at the emitter at the frequencies the circuit is designed to work with and therefore it will be a poor emitter follower for those frequencies, it could still work effectively as a follower (common collector) at lower frequencies.
 
You will also sometimes see common collector amplifiers / emitter followers with a resistor between the collector and the supply rail. This may be done to protect it from conducting "infinite" (i.e. too large to be healthy) current when the emitter gets accidentally shorted to ground.

And as remarked by MarkT, you may also see cases where there are resistors on both E and C and both outputs are used at the same time. In such case the emitter provides noninverted signal with 1:1 voltage gain and the collector provides inverted signal with arbitrarily chosen voltage gain. The transistor can be seen as a CC stage for circuits driven by the emitter and a CE stage for circuits driven by the collector, all at the same time.