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Voltage buffer

Discussion in 'Electronic Basics' started by [email protected], Apr 14, 2007.

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  1. Guest

    In a circuit I have DC signal somewhere around 10V-14V. This I would
    like to use as input for a load of 13kO. However, the output circuit
    should only be minimally loaded. Now if this signal was <6V I would
    just use whatever NPN I can find in my drawers and set it up as an
    emitter follower, however with the voltage being that large I guess
    that would kill the transistor- or would it? Any other simple
    solutions?
     
  2. Andrew Holme

    Andrew Holme Guest

    Most NPN transistors in my drawers will happily work at that voltage. I
    probably don't have anything with VCEO less than 20 or 30V.
     
  3. Chris

    Chris Guest

    The power to drive the output circuit has to come from somewhere else
    -- I'm assuming you've got a DC power supply which is greater than the
    highest amplitude of your input signal.

    If you do, why not just use an opamp as a voltage follower, like this
    (view in fixed font or M$ Notepad):

    |
    | +24V
    | |\|
    | >-------|+\
    | Vin | >---o----->
    | 10-14V.---|-/ | Vout
    | | |/| |
    | | | |
    | | === |
    | | GND |
    | '----------'
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    An LM741 will work just fine here. Or if your load requires more than
    20mA, you can put your transistor in the feedback loop as a current
    booster, like this:

    |
    | +24V +24V
    | |\| |
    | >-------|+\ |/
    | Vin | >------|
    | 10-14V.---|-/ |>
    | | |/| |
    | | | |
    | | === |
    | | GND | Vout
    | '---------------o-------o
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Either way, your output voltage will track the input within a few
    millivolts, and your signal source will be unloaded. Much better
    solution than a single transistor.

    Good luck
    Chris
     
  4. Wim Lewis

    Wim Lewis Guest

    Even the common-as-beans PN2222A has a Vceo max of 40 volts, according
    to the first datasheet I found. Power dissipation should be under
    100 mW, well under the 625 mW that the 2222 claims to be happy
    with. So yeah, I think a random junk-drawer transistor has a good
    chance of working in this circuit (since all the transistors in my
    junk drawer seem to be 2222s or BC547s).
     
  5. Brian O

    Brian O Guest

    I'm assuming there is a reason you cant use a voltage divider if the
    transistor is not rated high enough??
    B
     
  6. Guest

    Okay, this was a basic misunderstanding on my part. I assumed that the
    maximum Vbe rating is what I am allowed to apply to the transistor
    base. For the above scenario a regular 2N2222 turned out to work just
    fine.
     
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