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high voltage "opamp"

Discussion in 'Electronic Design' started by mw, Apr 13, 2005.

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  1. mw

    mw Guest

    Is there a standard circuit connection that can be used to raise the
    output swing of an opamp to a higher voltage ? For example, a TL084
    opamp with a +18V / -18V supply can give an output swing of about +12V
    to -12V. But what I am looking for would permit a voltage swing of +36V
    to -36V. I would of course expect to provide higher voltage power
    rails, and add a few discrete transistors, etc., to massage the signal.

    It seems like this circuit has been used as an example in an app note or
    text book somewhere, but I haven't located it.

    Ideas, anyone?
     
  2. Chris

    Chris Guest

    Possibly you're thinking about National Semiconductor AN-272, "Op Amp
    Booster Designs". Look particularly at Figure 6.

    http://www.national.com/an/AN/AN-272.pdf

    But there are many high voltage op amps that will do the job for you as
    a one chip solution. One you might want to try is the TI (formerly
    Burr-Brown) OPA452, which is available and in stock from Digi-Key for
    $5.10 USD in single quantities. It comes in a 7-pin TO-220 package.
    The +/-40V max supply voltage (+/-36V output) OPA452 has a GBW of
    1.8MHz, it's unity gain stable, and can source or sink 50 mA
    continuously. It also has current limit and thermal shutdown.

    http://focus.ti.com/lit/ds/symlink/opa452.pdf

    There are several subtle and not-so-subtle difficulties with output
    voltage boosters for op amps that degrade performance. And you have to
    pay some extra attention to op amp input protection, particularly
    transients. You'll almost always end up looking better and getting
    better performance with a single chip solution if it's available.

    Good luck
    Chris
     
  3. Pooh Bear

    Pooh Bear Guest

    One method is to drive the op-amp's power pins ( only works for a single
    device ) with emitter ( or source ) followers driven by voltage dividers
    between the op-amp output and the high voltage supply rails.

    You can source more current if required by adding emitter followers to the
    output pin directly.


    Graham.
     
  4. mw

    mw Guest

    Yes. That's the one.
    I just now simulated various versions of the National circuit and did
    find that they do not perform well at high frequencies (>20kHz). I
    think it is because impedances are too high. It's the trade off between
    speed and power.

    Thanks for yours (and Pooh Bear's) help.
     
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