Why the common emmiter

As opposed to what?

Describe what the CE, CB, and CC configurations do in terms of voltage and current gain, and input and output impedance, and you should have your answer.

Bob M.

It's popular because it has both current and voltage gain.

We do also use the common base and the common collector (emitter follower) too you know.

Graham

Has the best power gain and moderate input/output impedances.

Just look at the transistor symbol. Obviously the emitter is heavier, so it settles down to ground.

But I'm still confused about PNP's. I figure the emitters must be helium implanted, since they tend to go up.

John

Except in the 'old days' of course.

Graham

Its the only amplifier configuration.

CB is a switch. It is normally to unstable for use as an amplifier(or you can think of it as an amplifier with positive feedback that saturates or cuts off the transistor very easily).

CC is a unity gain buffer. It has a high impedence input so that you do not load the output of whatever your taking(sorta like how a volt meter works... volt meters would be useless if when you used them they did not give you the right voltage). Its output is low impedence which means that it can drive a load.

CE is a relatively stable amplifier. It will take a voltage signal and amplify it. Here though there is a loading effect. By adding a CC at the end then you can buffer the amplifier so that you get the best of both worlds. You get gain from the CE + low loading effects.

CB and CC do not amplify(in the sense of taking a small single and making it larger) but they have other important properties.

Ahem !

Graham

Man, do you have a lot of wonderful things to learn about transistors.

Any configuration with more power out than power in has gain.

CE, CB and CC can all have power gain.

I don't know where to start. CB puts the input impedance in the output circuit as negative feedback. This makes the output more linear (less like a switch).

Good. And that current gain at unity voltage gain represents real power gain.

Of course, they do. CB amplifiers can produce voltage gain and CC can produce current gain. That is getting a bigger signal (in a power sense) out than you put in.

Now this is a roundabout way to force us to do the guy's homework.

Common base are used when one wants to do impedance transformation. So when you want to use that 8 ohm speaker as a microphone, you use a common base amplifier because the low impedance input will better match the microphone, and you will get some gain as well as a higher output impedance.

One should point out that common base amplifiers are not uncommon in radio work, because they do that impedance transformation and because their relatively low gain is a good thing. If it had more gain, it would require more effort to ensure that it wasn't unstable.

Michael

Common-base has the same voltage gain as common-emitter, and it's just as linear and sometimes more stable.

John

It most certainly is not. Take a look at the output stage of most audio amps, and get back to us, OK?

(Hint: "amplifier" does not necessary equate to "voltage gain.")

Bob M.

Well, I tried in my original response to avoid doing that, but it looks like some folks just have that overwhelming urge to explain....

;-)

Bob M.

Anyone else want to point out my ignorance and that I must have an IQ of -3? By amplification I ment voltage amplification... I thought it would be clear because usually that is how they are configured. A unity gain buffer is unity in voltage but you don't say a unity gain voltage buffer? When you talk about amplification it is almost always in the context of voltage amplification? If you talk about current "amplification" it is almost always talked about as current "gain"?

Also, aren't all transistors configured for power gain? wouldn't it be worthless if both voltage and current decreased?

I was wrong about the CC and what I ment was a base biased configuration. I was trying to give the 3 common modes that transistors are used as: Switching, Amplification, and buffering.

Now amplification means voltage amplification because buffering is current "amplification". If not then why call it a unity gain buffer and not a current amplifier? Ok, maybe some people do but some of you have acted like I commited a cardinal sin and it must mean I have no clue about what I was talking about. In fact I was atleast 2/3 right and if you stop grading me like a test then I might not be as dumb as some of you want to make me out to be. Maybe I got some termiology mixed up but it is enough to point that out instead of trying to ridicule me.

Obviously if I said a switch it must be the configuration for a switch. i.e. base biasing. So just changed CC to Base biased and that fixes that problem. Just prefix all amplifications with voltage amplification and then that fixes the other issue.

I guess 10 more people are going to reply to my original and say the same stuff the last 10 said and call me a retard? Now I really feel like a retard. Thanks guys.

You know, its one thing to point out the mistake like John Larkin did and its another to try and make the person completely feel like a retard even when the mistakes are more about terminology than anything else. Maybe I should have been more clear or re-read my post but its not like I'm writing a thesis. Sometimes I jsut get confused on the names cause I don't mess with transistors much. Actually all I have ever did was a simple *voltage* amplifier. I have read about them and actually just got finished reading again... but the details don't stay in my brain when I don't use them. The book I was reading only discussed CE and CB and base bias so I assumed CC was that base bias because I recalled that there were only 3 configurations. I'm going to have to see exactly how CC works now to see the difference.

(snip)

Don't take it so hard or so personally. I get focused on an idea (often a completely mistaken tangent) and say something stupid, almost daily. I have tried (and sometimes it is hard) to just smile and say thank you when someone points out how silly my contribution was.

You gave us all an opportunity to wax poetic on transistor configurations. Besides, its a lot more fun to criticize someone than just to dive in and and write a tutorial. Stick around. You'll get your chance to correct me. ;-)

I know... sometimes its hurts though!! ;) We all want to know everything and when someone points out that you don't it stings... and when they make you feel stupid then its even worse..

What sucks is that I just got finished reviewing the damn s*it and I should have known but I completely forgot all the names of the different configurations ;/ I do understand how the work but unfortunately the book I'm reading didn't conver CC(or if it did it used a different name or I just haven't got to it yet). I really need to get in some practice with some transistor circuits soon... hopefully I'll be able to play around more with them later.

I guess I'm more mad at myself than anyone. I hate when I try to be helpful but end up making it worse. I should atleast double check what I'm saying to make sure but I tend to be lazy and just type what ever pops in my mind...

Oh well... no hard feelings ;)

I seriously thought that was a good answer of yours. Because it pointed the way, without actually providing the answer.

Michael

Everybody makes a mistake now and then, especially so when the consequences are low, as in no planes will crash or anything like that. Just say "oops" and get on with life... it's no big deal. I see my mistakes as reminders that I should brush up on something, which then keeps me from making the same mistake later when it really matters, as when serious things might be involved.

John

They are often referred to as emitter followers, since the output voltage from the emitter "follows or parallels the input voltage on the base.

I am dyslexic, and regularly get NPN and PNP substituted for each other, when describing a circuit. I don't have the problem with schematics, for some reason.

You made it quite a bit better for the O.P. Lots of people who were not willing to help with what looked like homework, were more than happy to correct someone who was willing.

You can always offer a double your money back, guarantee.

I don't believe you, but hope you'll heal. ;-)

I always draw the emitters up on PNPs and the emitters down on NPN's. Voltages are more positive at the top of the sheet, and current flows down.

I draw mosfsts sort of like IGBTs...

| | | | |- --| | n-ch mosfet |>

| | | |

because the "proper" symbol is a mess.

You write awfully well for being dyslexic.

John