How to tell what configuration a transistor has in a complicated circuit

Question

Apparently transistor Q15 in the following circuit is in common collector mode (well, actually it says it's an emitter follower but my understanding is that this is the same thing as common collector mode). This isn't obvious to me at all, although my textbook offhandedly says this as if it should be obvious. How can you tell that it's in common collector mode?

Answer 1

It's not immediately obvious, but Q15 and Q19 form a darlington pair. Replacing all the gumph around them with basic equivalent elements, you are left with (on the left):

^{simulate this circuit – Schematic created using CircuitLab}

Everything in the blue box is a darlington pair emitter follower, which is sometimes called "common collector". In this case, the collector doesn't look very common to anything, especially considering its collector wobbles up and down, and is actually the output, just prior to buffering by the push-pull output stage. I understand your confusion.

For me, the big giveaway was the low value of R1, 50Ω, indicating that its function is to modulate current through R1, and consequently throughout the entire vertical path via I1, V1 and Q19. The voltage across R1 (and therefore also the current through it) varies in proportion to input potential at node A, a classic "voltage-controlled current sink" architecture, employing an emitter follower.

Since emitter followers are also called "common collector", that's probably why your professor named it such.

I suppose R2 could be confusing too. Its role here is simply to improve Q19's turn-off speed, by providing a path for Q19's stored base and capacitive charge to evacuate, when Q15 turns off, which would leave Q19's base floating.

The other potentially confusing element is Q22, shown above right. This seems to be a simple current limiter. When 12mA or more flows down through R1, it develops 0.6V, enough \$V_{BE}\$ to switch on Q22, and prevent A from rising any further in potential.

Answer 2

Q15 by itself is more or less an emitter follower (common collector), yes.

More precisely, it's in a Darlington pair with Q19, a composite structure, which is itself common-emitter. (Which is also current-limited by Q22.)

Which way is more useful to look at it, depends on what you're after. These circuits are complex enough that an analytical description isn't really feasible, so we tend to use hand-waving descriptions like "common-emitter", and rougher descriptions like, corresponding the input/output voltage/current ranges (discounting nonlinearity of the device(s)), making assumptions about certain devices being "on", "off", in or out of saturation, etc.

I wouldn't think there would be much academic merit in labeling these as CE or CC, but if your instructor is going to stake the success of a test on it, say, I would suggest paying attention to whatever definition they give.

Answer 3

Q15 is in an emitter follower, or common collector circuit.

The signal comes in via base and is output from emitter, and the collector is supplied with constant current from a current source circuit.

Answer 4

One way to tell which transistor electrode is the common one is by looking at whether it's connected to a voltage rail or current supply line OR if the other two electrodes serve as signal input and output.
The Q15 in this case has the signal coming into its base electrode and going out of its emitter into the base of Q19 (darlington connection), which would, by the process of elimination, make its collector the common electrode.

Answer 5

It is difficult to label Q15. An emitter follower has a voltage output at its emitter. In a Darlington configuration which Q15,Q19 are in, the emitter supplies current to the base of the second transistor. There is no signal output from the Darlington emitters to the load. The 50K resistor is only there to discharge Q19 bas-emitter capacitor. The 50 ohm resistor is used to sense current for current limiting and so is not used as a signal output.

So the emitters are essentially grounded. The Darlington pair is used as a common emitter gain stage.

Since the emitter of Q15 is not used as a voltage output but the collector is, then Q15 is not configured as a common collector amplifier, but is more like a common-emitter configuration.

Like Tim Williams....Call it the way your instructor says.

Answer 6

I've had another look through my textbook, having considered some of the answers here. It seems that they consistently use 'common collector' to simply mean that the input is at the base and the output is at the emitter. Similarly, they use 'common emitter' to mean the input is at the base and the output is at the collector, and 'common base' to mean the input is at the emitter and the output is at the collector. So, that is how I am going to think of it from now on. I'm no longer going to worry about why, if it's common collector, there is a path between base and emitter even though they aren't connected at all in the very basic diagrams of the common collector, or think that the collector is common to both the 'input circuit' and the 'output circuit' and try to identify where the 'input circuit' and 'output circuit' is (try doing that for the diagram I posted - very difficult indeed!)

Thank you all for your help.