Beginner's question about basic amplifiers

• Beginner's question about basic amplifiers
  Posted by John0922 on 20 Jan, 2017 19:15
• 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

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• #1 Reply
  Posted by Seekonk on 20 Jan, 2017 19:31
• Just think of it as some point that doesn't change with the signal.
• #2 Reply
  Posted by PA0PBZ on 20 Jan, 2017 19:43
• "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)
  
• #3 Reply
  Posted by John0922 on 20 Jan, 2017 22:05
• Thanks for the replies. I think I get it.
• #4 Reply
  Posted by Brumby on 21 Jan, 2017 00:05
• I like this explanation:
  

  Quote from: PA0PBZ on 20 Jan, 2017 19:43

  "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.
• #5 Reply
  Posted by John0922 on 21 Jan, 2017 00:39
• There's some good clarification there, thanks Brumby.