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Emitter Follower

Discussion in 'Electronic Repair' started by Nitin, Feb 16, 2006.

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  1. Nitin

    Nitin Guest

    HEllo Guys,
    I am trying to design a Emitter Follower. I am trying to give the load
    resistance as 50 Ohm.
    Could some one tell me how to calculate the Biasing resistance on the
    Base circuit of the transistor.I am assuming that the Short circuit
    current gain (Beta = 100). I am using a npn 2n2222a transistor for my
    purpose. Voltage supply on the collector is 12V. Voltage supplied to
    the base is from the output of a sound card or a signal generator.So
    please help me in getting the answers
    Regards
    Nitin
     
  2. Dave D

    Dave D Guest

    I'm curious- why a 50 Ohm load? Is this a college homework assignment by any
    chance?

    Dave
     
  3. Arfa Daily

    Arfa Daily Guest

  4. Guest

    Dave;

    I really hope it's not college level, this should be in high school.
    Common collector amps are the simplest to design.

    Actually I'm a bit squeaky when it comes to common emitter and
    especially common base stages' actual input impedance. In general in
    those stages I use a heavy handed approach, make it damped (shunted)
    and linear.

    Anyway, now that you got me on the subject, people have looked at me
    funny for saying this; All bipolar transistors operate as common
    emitter. No exceptions.

    This was in response to the inability of a buddy of mine in trade
    school to understand the concept. I had to draw it about 15 different
    ways but he finally got it. I almost went into sign language !, that is
    I was to the point of holding up three figners and saying sternly "OK,
    my thumb is the emitter ok ?" I didn't have to resort to that but it
    was close.

    It was a success, and he actually doesn't make more money, but he moved
    from 90 hours a week to 40 hours a week to make about $50,000. Not bad,
    it's just about a whole dollar every day.

    To the OP;

    In common collector (emitter follower) the resistance presented by the
    base is approximately the load resistance times the current gain of the
    transistor. This current is applied (V / RL X hfe) in the direction of
    whatever is biasing the emitter. As long as the collector is solidly
    connected to the Vcc, and input is reference to a propely bypassed Vcc
    return this is close enough. When the collector voltage is allowed to
    vary, like by a resisor, this is not accurate.

    I am not abject to giving help on school projects, but if it is please
    say so. For one, this cheap and dirty formula dfoesn't work with
    anything else. If you don't learn it the right way you will fall flat
    on your face later.

    Also, if RL=50 ohms, your wimpy little transistor will have to
    dissipate at the very least 720 mW even during quiescence. A 2N2222 is
    going to have a problem with that, you will at least need a heatsink.
    In fact you might need Freon !

    If you're talking about buffing a soundcard output, this is not the way
    to go. If you simply get an older soundcard that doesn't require
    powered speakers you are fine. They will feed 50 ohms no problem. A sig
    generator, OK, mine for example is 30 volt @ 600 ohms output. But then
    what would stop you from just feeding that to the soundcard ?

    JURB
     
  5. It's not the correct transistor to drive a 50 ohm load.
     
  6. Dave D

    Dave D Guest

  7. Asimov

    Asimov Guest

    "" bravely wrote to "All" (16 Feb 06 13:11:27)
    --- on the heady topic of "Re: Emitter Follower"

    ZZ> From:
    ZZ> Xref: core-easynews sci.electronics.repair:358523



    ZZ> Dave;

    ZZ> I really hope it's not college level, this should be in high school.
    ZZ> Common collector amps are the simplest to design.

    ZZ> Actually I'm a bit squeaky when it comes to common emitter and
    ZZ> especially common base stages' actual input impedance. In general in
    ZZ> those stages I use a heavy handed approach, make it damped (shunted)
    ZZ> and linear.

    ZZ> Anyway, now that you got me on the subject, people have looked at me
    ZZ> funny for saying this; All bipolar transistors operate as common
    ZZ> emitter. No exceptions.

    ZZ> This was in response to the inability of a buddy of mine in trade
    ZZ> school to understand the concept. I had to draw it about 15 different
    ZZ> ways but he finally got it. I almost went into sign language !, that
    ZZ> is I was to the point of holding up three figners and saying sternly
    ZZ> "OK, my thumb is the emitter ok ?" I didn't have to resort to that but
    ZZ> it was close.

    ZZ> It was a success, and he actually doesn't make more money, but he
    ZZ> moved from 90 hours a week to 40 hours a week to make about $50,000.
    ZZ> Not bad, it's just about a whole dollar every day.

    ZZ> To the OP;

    ZZ> In common collector (emitter follower) the resistance presented by the
    ZZ> base is approximately the load resistance times the current gain of
    ZZ> the transistor. This current is applied (V / RL X hfe) in the direction
    ZZ> of whatever is biasing the emitter. As long as the collector is solidly
    ZZ> connected to the Vcc, and input is reference to a propely bypassed Vcc
    ZZ> return this is close enough. When the collector voltage is allowed to
    ZZ> vary, like by a resisor, this is not accurate.

    ZZ> I am not abject to giving help on school projects, but if it is please
    ZZ> say so. For one, this cheap and dirty formula dfoesn't work with
    ZZ> anything else. If you don't learn it the right way you will fall flat
    ZZ> on your face later.

    ZZ> Also, if RL=50 ohms, your wimpy little transistor will have to
    ZZ> dissipate at the very least 720 mW even during quiescence. A 2N2222 is
    ZZ> going to have a problem with that, you will at least need a heatsink.
    ZZ> In fact you might need Freon !

    ZZ> If you're talking about buffing a soundcard output, this is not the
    ZZ> way to go. If you simply get an older soundcard that doesn't require
    ZZ> powered speakers you are fine. They will feed 50 ohms no problem. A
    ZZ> sig generator, OK, mine for example is 30 volt @ 600 ohms output. But
    ZZ> then what would stop you from just feeding that to the soundcard ?


    JURB,

    If his card has only Line-Outs then he would be better off with a pair
    of amplified speakers. If he insists on DIY then perhaps an 8-pin itty
    bitty LM386N etc, pair might do. All the preamp gain he needs and it
    will drive 1 watt into 8 ohms quite happily without too much fuss.
    Just don't use much gain because the little bastard has a noisy input.

    If you insist on just 3-pins, then checkout the application note for
    the TL431 precision voltage reference which makes a radical use of it
    as a 400 milli-Watt phono-amplifier. However it uses a small audio
    transformer 330:8 ohms. The beauty of using a transformer is that at
    idle it dissipates the maximum power it ever will. So if it is too hot
    then bolt it to a big heatsink and it will be safe.

    A*s*i*m*o*v

    .... A fail-safe circuit will destroy others.
     
  8. Arfa Daily

    Arfa Daily Guest

    Dave - I was referring the OP to that site, and not replying specifically to
    your question about the 50 ohm load. The site has information relevant to
    his query about how to calculate the values of bias resistor for a common
    collector stage, with BIGGGG simple diagrams ...

    Sorry if it confused - I was just tacking onto the bottom of the thread for
    the benefit of those following it, rather than sticking my bit in the
    middle.

    Arfa
     
  9. Dave D

    Dave D Guest

    No worries, Arfa, we've all done it!

    Dave
     
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