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Switching 800mA at 10Mhz?

Discussion in 'Electronic Basics' started by [email protected], Oct 25, 2005.

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  1. Guest

    I am attempting to switch an 800mA load at 10Mhz (e.g.with a square
    wave). I started with an assortment of BJTs and FETs placing a 10 ohm
    carbon resistor load in the collector, (common emitter config). The
    squarewave gets pretty sloppy. I added a "speed up" cap to the base
    with some improvement however not good enough.
    I am lost in a pleathora of differential drivers, totem poles,
    cascodes and so forth, but before I proceed I would be interested in
    any designs that may already perform this basic task.

    Thanks,
    Fritz
     
  2. w2aew

    w2aew Guest

    Probably the biggest problem is that you're letting the transistor
    saturate, which will severely limit your speed. Try using a resistor
    in the emitter to set the current. By controlling the base voltage,
    you can set the emitter current via the emitter resistor. Then, you
    can select a collector resistor to ensure that the transistor doesn't
    saturate. This should get you a lot closer...
     
  3. mike

    mike Guest

    You're gonna be wanting to disclose your minimum requirements for:
    risetime
    falltime
    edge linearity
    delay rise
    delay fall
    symmetry
    transient response
    voltage compliance
    load impedance
    A description of what you're driving might be helpful.

    What are you using to measure the current waveform?
    Taking a wild guess that you're not measuring the current at all,
    but rather measuring the voltage across the resistor. How are you
    hooking that up. Probing technique can mess up your measurement.
    What's the bandwidth of your measurement system?
    mike

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  4. Guest

    Thanks for the replies. Here are some more details. I intend to
    modulate a "bright" LED which draws about 600-800mA at 10Mhz. (At some
    point I will convert an ethernet signal too OOK and use this as a
    transmitter). Conventional devices for laser diodes usually move less
    that 50mA.
    This LED has a fast response time of about 10nS so we are comfortable
    that this will not be a limiting factor. The vendors of course don't
    give a lot of information for these devices. The forward voltage is
    3.7V. Dynamic resistance is 0.8ohms. (I presume this is the slope of
    the forward voltage). See: http://www.lumileds.com/pdfs/DS46.PDF
    At 10Mhz I am assuming that I can transmit a reasonable square wave
    if I can pass frequencies of 100Mhz thus I am looking at rise/fall
    times of about 10ns. These comply with some of the FETs and BJTs out
    there.
    I am measuring the waveform with a 10x probe via the voltage accross
    the resistor. Also picking it up from a PIN diode set up as a
    reciever-they seem to comply. Since I get a clean square wave using the
    same probe at the sig-gen and base I assume that this is not the issue?
    At any rate I expect it will be easier to operate the resistor or a
    diode first as a starting point. I presume driving 800mA at these
    frequencies is not uncommon and probably there are some circuit designs
    already available as a starting point. I can't find much in my library
    -Horowitz & Hill alludes to some things but I would prefer a complete
    circuit to start with. I think I have tweaked the most from my present
    simple design-a Darlington driven by a buffer-it gives a much better
    waveform than a simple common emitter transistor circuit.

    Thanks,
    Fritz
     
  5. John  Larkin

    John Larkin Guest


    Try a 2N7000 (or 7002) n-channel fet. Drive from an HC-family gate
    running at 5 or even 6 volts, maybe a couple sections in parallel. 800
    mA is close to the edge here, so maybe use two fets in parallel, with
    a couple HC gate sections driving each. Should be plenty fast.

    Whole thing (one hex HC gate, two fets) should cost about 80 cents.

    Heck, two 74AC04's with all 12 sections in parallel would do it all by
    itself.

    John
     
  6. mike

    mike Guest

    The secret is not to let the voltage move.
    Bias the LED on with the most current you can tolerate for the minimum
    intensity. Then drive the thing with the collector or drain of a
    transistor. The voltage will only have to move far enough to drive the
    dynamic resistance. The base/gate drive you need depends on the
    reverse transfer capacitance of the device. For bipolars, I'd look into
    the improved versions of the 2N3866. Don't remember the numbers of the
    newer parts. Low Cob is what you want. You might find a cascode
    connection beneficial depending on the turnoff edge. I've achieved an
    amp/nanosecond slew rate with 4 IRF530s in parallel
    and 100 ohm gate drive resistance for a power supply dynamic tester.

    mike
    --
    Wanted, Serial cable for Dell Axim X5 PDA.
    Return address is VALID but some sites block emails
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    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Bunch of stuff For Sale and Wanted at the link below.
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