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Why the common emmiter

Discussion in 'Electronic Basics' started by [email protected], Apr 24, 2007.

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  1. Guest

    hello every body, i have a quistion. Why do we use the common emmiter ?
  2. Bob Myers

    Bob Myers Guest

    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.
  3. Eeyore

    Eeyore Guest

    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.

  4. Charles

    Charles Guest

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

    John Larkin Guest

    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.

  6. Eeyore

    Eeyore Guest

    Except in the 'old days' of course.

  7. 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

    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.
  8. Eeyore

    Eeyore Guest

    Ahem !

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

    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.
  10. 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.

  11. John Larkin

    John Larkin Guest

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

  12. Bob Myers

    Bob Myers Guest

    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.
  13. Bob Myers

    Bob Myers Guest

    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.
  14. 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.
  15. (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. ;-)
  16. 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 shit 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 ;)
  17. I seriously thought that was a good answer of yours. Because it
    pointed the way, without actually providing the answer.

  18. John Larkin

    John Larkin Guest

    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.

  19. 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. ;-)
  20. John Larkin

    John Larkin Guest

    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

    I draw mosfsts sort of like IGBTs...

    --| | n-ch mosfet

    because the "proper" symbol is a mess.

    You write awfully well for being dyslexic.

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