As far as I understand, a BJT in any given circuit must fall in exactly one of the following configurations:
-Common emitter.
-Common collector.
-Common base.
So my question is:
The three basic configurations (CE, CC, CB) don't cover all possibilities but they are essential to quickly determine the behavior of a circuit.
The configuration is always determined with respect to a given input and given output and therefore depends on the signal path that is being analyzed.
The determination of the type of configuration is actually quite simple. The name of the topology is determined by the node which is at AC ground, that is the node that carries no signal.
We have to start by identifying the input and the output of the single transistor circuit, which already excludes two terminals. The remaining terminal determines the name of the configuration.
For example a CE configuration has the input at the base (we exclude the base), the output at the collector (we exclude the collector), so that only the emitter remains. Hence, we have a CE configuration.
Might sound like a strange approach but works very well.
Are there any other configurations a BJT can be in?
A differential pair has each transistor simultaneously in both common base and common emitter and sometimes one of these two transistors will be in common collector: -
R2 helps fine tune the performance but can be a short circuit hence T2 is kind of in all three circuit topologies.
Some circuits have an output from both collector and emitter when producing an in-phase (emitter) signal and an inverted signal (collector) for feeding some types of output transistors in audio amplifiers. What category would you put these in?
How can I determine which configuration a BJT is configured to from a given circuit diagram?
Not important - maybe an analogy is what the firing order is for a 4 cylinder engine. That is detail that is useful but doesn't take anything away from recognizing many more important topological things about the engine used.
