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

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.

Reply to
John Popelish
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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.

Reply to
vlsidesign

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

Reply to
John Larkin

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