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BJT voltage amplifier question, transistor basics

Discussion in 'Electronic Basics' started by panfilero, Mar 11, 2007.

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  1. panfilero

    panfilero Guest

    I have a small voltage.... millivolts, like 20mV, that I need to
    amplify to around 1V, how would I do this? I was thinking about using
    a BJT, and feeding this voltage into the Base, but then I remembered
    something about the Base Emitter voltage having to be over 0.7 V in
    order for the BJT to be on, so I'm not sure how to turn on the BJT to
    have it amplify my small signal.

    thanks
    Joshua
     
  2. panfilero

    panfilero Guest

    it's DC, it's actually to feed into an Analog to Digital converter
    input on a microcontroller, I need to amplify my signal before feeding
    it in there.
    Oh, sounds complicated, I thought it would be simpler
    thanks, I'll look into that option, so then would using a BJT not be a
    good solution for this?
     
  3. MassiveProng

    MassiveProng Guest

    Why is your PC clock ALWAYS fucking wrong?

    Are you just on the other side of a date line?

    Or do you have some lame Usenet retard desire to see your post
    appear at the top of a new header pull?
     
  4. Jamie

    Jamie Guest

    ..............................................Lived with out? :))
    just poking! :)
     
  5. PLONK
     
  6. MassiveProng

    MassiveProng Guest

    snip

    FIX YOUR FUCKING PC CLOCK, BOY!
     
  7. Is this a very low frequency 20 mV (essentially DC) or some
    definite AC frequency?

    Amplification is simpler for AC signals, because you can
    capacitively couple successive gain stages, blocking the DC
    bias voltages between stages. DC amplification requires not
    only canceling the bias voltages, but compensating for
    temperature caused changes in those bias voltages.

    Operational amplifiers (opamps) were invented to do those
    last two things, and if you can get them, they would
    definitely make life a lot easier for something like this.

    Here is a basic tutorial on opamp applications. It shows
    how to program them for all sorts of uses, including gain
    blocks. At DC or low frequency AC, one would have little
    trouble amplifying 20 mV to 1 V (gain of 50).
    http://www.national.com/an/AN/AN-20.pdf
     
  8. By the time you got it to work well, you would have just
    about reinvented the opamp. Much easier to buy one and
    program it with a few resistors. After you have read about
    them a bit, come back with the power supply voltages
    available in your system, an we can help you select an opamp
    that will work well in that situation. By the time you get
    this working, you will wonder how you lived with opamps
    until now.
     
  9. Yes. Thanks.
     
  10. panfilero

    panfilero Guest


    what are you talking about clock? I'm asking how to amplify a voltage
    signal, has nothing to do with my PC clock, did you even read my post?

    PLONK!


    Joshua

    PS - I think I'm going to go with the OpAmp solution, the main part
    that confused me about the BJT was that if I want to amplify a real
    small signal.... how do I get the BJT to even turn on if it need the
    Base Emitter Voltage to be 0.7 V...... no problem like this with the
    op-amp. Or maybe some kind of open collector high impedance type of
    thing.
     
  11. You have to arrange for the transistor to see the sum of
    your signal and a much larger bias voltage that changes as
    temperature changes the voltage it takes to keep the
    transistor conducting a fixed amount of collector current.
    In an opamp, this is done by using a pair of input
    transistors configured as a differential pair, biased in a
    way that keeps the sum of their collector currents fixed,
    while canceling each other's bias voltage, but combine the
    effect if the input signal on the two of them. The output
    signal is the difference between their two collector
    currents. It is essentially impossible to duplicate the DC
    accuracy of this arrangement with a string of single
    transistor gain stages.

    If you want to actually get inside an opamp to understand
    how this 'magic' works, see:

    http://www.national.com/an/AN/AN-A.pdf

    So what supply voltage is available to power your opamp, and
    what is the range of input signal voltage and the desired
    range of output voltage the opamp should produce with this
    range of input voltage?
     
  12. Rich Grise

    Rich Grise Guest

    Kind of like Women (or Men, if you happen to be a Woman):

    Can't live with'em, can't hunt'em down and kill'em. ;-)

    [ NOTE: THIS IS A JOKE!!!!! -----^ ]

    Cheers!
    Rich
     
  13. Rich Grise

    Rich Grise Guest

    Well, once you get into looking at opamp input circuits, you'll come
    to realize that the way they did this in the old days was to use a
    "differential pair" - that's two transistors with their emitters
    connected together and to a common resistor to the negative supply.

    Then, just ground the input of the other transistor, and that provides
    a reference. The output is the differential voltage between the two
    points where the two collectors of the transistors connect to their
    load resistors:

    +V +V
    | |
    [R] [R]
    | |
    +--------------------- -
    | | Differential output
    | +---------- + (might need level-shifting)
    | |
    | |
    |/ c c \|
    in -----| b b |-----+
    |> e e <| |
    | | |
    +-----+----+ 0 V ref.
    |
    [R]
    |
    -V

    But, to get around zero at the input, you need a bipolar supply. (both
    positive and negative relative to the 0V reference, usually called ground.)

    As John has said, this is pretty much the way opamps are designed, but
    with more buffering & stuff. And the R values can be important, depending
    on the application.

    Hope This Helps!
    Rich
     
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