# Common-Base amplifier explanation?

Discussion in 'General Electronics Discussion' started by Sohllivan, Mar 26, 2013.

1. ### Sohllivan

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Mar 26, 2013
Very early morning to you all! I'm currently studying basic electronics and the theory behind it all and such and I took a 3 month class earlier last year that covered it all pretty much however it's been a long time since I've used that knowledge and have gotton a bit rusty on the mechanics of it. below is a simple common-base amplifier circuit that I found in a book and my question is,

The book states that the voltage being applied to the Emitter is negative because of the voltage drop across R2. What I'm having trouble understanding is why that voltage is negative even though there's what appears to be an AC input connected directly to the emitter? and how does R2 effect the emitter if it's supposed to take the path of least resistance?

I already get how transistors work with regards to the biasy and how the emitter has to be more negative than the base, what I am confused on is how this circuit makes that happen.

Sorry if my question is confusing, for me learning how it all works makes understanding it alot easier so any help would be appreciated!

http://oi50.tinypic.com/14wvwk.jpg <- the circuit mentioned

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Post the relevant schematic so we can understand what you are referring to.

3. ### Sohllivan

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Mar 26, 2013
My appologies, it was in a book so I had to draw it with a tablet, that's the best I can do at the moment, and I posted the link at the bottom of my post :]

4,960
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May 8, 2012
5. ### Harald KappModeratorModerator

11,196
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Nov 17, 2011
I think your schematic is not correct. You will need a negative bias for the emitter. There's no way a negative voltage drop can occur across R2 unless the AC input is negative. But the positive part of the AC input will be clipped.

In addition to Chris' link, here's another descriptin of the common base circuit.

6. ### Sohllivan

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Mar 26, 2013
See I was thinking the same thing, I didn't get how it could have possibly been negative BECAUSE of the AC input, when there's also a DC battery that has the negative terminal connected to the emitter as well. How is it clipped? I looked over the tutorial provided here and to be honest it confuses me somewhat, I get how Common Base Amps work, what I didn't understand was how it was working on the schematic provided, that's literally an exact replica of the one in the book, no values were listed. Thank you for the help though, How would the AC be clipped exactly? is it because of its NPN nature that just sends it into cutoff on the positive half of the cycle?

I think my problem here is trying to figure out which direction current is flowing because I'm not sure whether to read the emitter voltage as coming from the DC battery or the AC battery, and then what happens to the other, etc.

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Harald, this is exactly the identical information we have in my link.

Chris

8. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Electron flow is always from negative to positive regardless of NPN or PNP, Common Base, Common Emmitter or Common Collector.

Note: Electricians in the U.S. are taught that current flows from positive to negative but this is in opposition of higher level electronics teaching. It really doesn't matter which direction you think it's flowing as long as you make it a constant in all your studies and design, For instance my spice simulator uses the positive to negative convention. After 55 years of using negative to positive I amazingly had no trouble adapting positive to negative.

Chris

9. ### BobK

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Jan 5, 2010
Current flows from positive to negative. Because electrons have a negative charge, current flows in the opposite direction from electrons. But that is just a convention.

Bob

10. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
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May 8, 2012
I still have old text books that teach electron current flow from negative to positive. As I said earlier either convention will work as long as they're not mixed. After all I'm never going to see an electron.

Chris