Beginner's question about basic amplifiers

[John0922]

Hi- I'm learning about basic amplifier configurations, and I'm having trouble understanding the term "common" used in that context. Please see my attachment.

For example, the common emitter configuration shows the output and what is presumably the output ground, but in the real circuit there's a power supply in between the emitter and the output ground, and that output ground is actually the positive terminal of the power supply.

I always thought the term "common" meant a node with the same voltage potential, but that is not the case here. I'm trying to understand the bigger-picture context-- is the output ground somehow connected to ground in other parts of the circuit? If so, wouldn't that short out the power supply?

Thanks in advance for your help.   -John

[Seekonk]

Just think of it as some point that doesn't change with the signal.

[PA0PBZ]

"Common" is in relation to the signal input and output, look again at the drawings. Signal input and output are 2 connections, and for instance with common emitter both input and output use the emitter as one of the connections. So it is "common" for ac signals and it doesn't say a lot about the dc.
(yes I know, this is simplified but I hope it helps to understand)

[John0922]

Thanks for the replies. I think I get it.

[Brumby]

I like this explanation:

"Common" is in relation to the signal input and output, look again at the drawings. Signal input and output are 2 connections, and for instance with common emitter both input and output use the emitter as one of the connections. So it is "common" for ac signals and it doesn't say a lot about the dc.
(yes I know, this is simplified but I hope it helps to understand)

I might also like to point out that this is talking about one element of a circuit topology - or "shape", if you will - and some fundamental characteristics.  It's not a full circuit design.  You mention "ground" and power supplies, but these are not important just yet.

If you are thinking "common" means "ground", then you are getting confused with commonly observed usage where numerous components are connected to a single rail.  This tutorial uses the term "common" in the context of a single transistor and the connections to its input and output.

[John0922]

There's some good clarification there, thanks Brumby.