common emitter/base/collector -- remembering which one is which

[John Popelish]

I've heard that the easy way to remember which configuration is being
used is that the "common" node is basically the node that is not being
used as the input or output. It seemed to help me but didn't know if
there are "traps" or "warnings" in using this to remember.

I think this is a very wrong way to look at the
configurations. The common node is common to both input and
output.

Base is always part of input, collector is always part of
output, but:

In a common emitter configuration, the input is applied
across base to emitter, and the output is taken across
collector and emitter.

In the common base configuration, the input is applied
emitter to base, and the output is taken across base to
collector.

In the common collector configuration, the input signal is
applied across base to collector, and the output is taken
across collector to emitter.

The common terminal is just the one that essentially holds a
constant voltage (an AC common) while the other two swing
with the input and output signals.

 

[vlsidesign]

I've heard that the easy way to remember which configuration is being
used is that the "common" node is basically the node that is not being
used as the input or output. It seemed to help me but didn't know if
there are "traps" or "warnings" in using this to remember.

 

[John Larkin]

I've heard that the easy way to remember which configuration is being
used is that the "common" node is basically the node that is not being
used as the input or output. It seemed to help me but didn't know if
there are "traps" or "warnings" in using this to remember.

That's about right; there's no signal on the "common" node.

One trap would be something like a phase splitter, where the base is
the input but both the emitter and collector are outputs. Or some RF
oscillator circuits where everything is everything.

I seldom hear people say "common collector." Most say "emitter
follower", which really should be "base follower."

But these categories are gross simplifications; the transistor does
what it does.

John