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Using reverse biased zeners to correct for mosfett turnon voltagein a push pull stage

Discussion in 'Electronic Design' started by Adrian Nievergelt, Aug 10, 2012.

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  1. Hi

    I'm trying to use mosfets for a push pull current gain stage on a dc
    coupled amplifier with opamp feedback. The feedback is supposed to
    operate in the high MHz range, so I'm looking for ways to statically
    reduce crossover distortion even before feedback to alleviate the load
    on the opamp and get a better THD. After looking around for a way which
    does not involve a lot of components (and with that a lot of parasitic
    capacitances) I believe using zener diodes in reverse bias might be
    "cleaner" than stacking multiple diodes in forward bias to get a level


    I haven't seen anyone else doing it this way and I suspect there to be a
    reason. Where might I run into problems with this scheme? Is there an
    easier (high frequency compatible) version I might have overlooked?

  2. mike

    mike Guest

    You just built a smoke generator.
    You have no way to compensate for component variations or for temperature.
    You'll have similar problems with series diodes.
    Typical solution for bipolars would be emitter resistors.
    It's more complicated with fets because of the variations in threshold
    You're describing a voltage amplifier circuit using the term "current
    gain stage".
    How many is "high" MHz.?
    THD is certainly a valid term, if you put a number on "better", but it
    may obscure your objective.
    What are you trying to accomplish? Is your load always purely resistive
    100 ohms?
    The devil is in the details.
  3. legg

    legg Guest

    The dominant 'stray' capacitances will be Cgs and Cdg, regardless of
    the number of parts you hang onto the gates, whether the power device
    being controlled ia a mosfet or an IGBT (as your schematic actually

    With a very fast op amp and no local compensation, you'll run into the
    situation where the Op amp will try to charge the gate capacitances
    through the zener body diode, something that will not happen with a
    diode string.

    Cross-over distortion is not a 'static' characteristic; I'm guessing
    you're just looking for a linear transfer and stable DC bias. Unless
    you know why the first technique was used (with all those 'extra'
    components), it's not really possible to speculate on the benefits or
    advantages of a modification.

  4. Joerg

    Joerg Guest

    As others have mention it's better done with BJT. FETs have way too
    large a production tolerance in the thresholds. It'll either distort
    badly or go phssst ... *PHUT* because of excessive quiescent current.

    Another common trick of the trade is to have a resistor between opamp
    output and load, to (somewhat) tide over the dead zone.
  5. I was afraid it would be something like this. I'll try to elaborate
    some: With current gain stage I mean the push pull output, as it's
    ideally a follower. The speed question is more difficult. The load for
    the amp is a piezoceramic with a capaciance of around 90nF. The resistor
    in the schematic was just for the purpose of my simulation. I use a
    series resistor with the piezo to decouple the reactive load from the
    piezo. Now the overall bandwidth should excell 1MHz piezo drive. In most
    cases the output will not oscillate, but follow another controllers
    signal. To achieve 1MHz output however I've found (simulation and board
    testing) that i need quite fast opamps (400MHz GBP up) so that i can
    afterwards silence the oscillation of the whole amplifier with
    capacitive load with the series resistance. With deadbugging I've noted
    that the THD with unbiased fets is around -35dB at 10kHz, which i would
    like to reduce with biasing.

    I hope that clears it up some.
  6. The opamp i use is in a suspended rail configuration, so up to +-100V
    output swings are no problem.
    I don't think i understand your suggestion yet, but it sounds
    complicated to do as my supplied to the opamp are already floating.
    Since my load is inherently purely reactive (dampened some by the
    addition of a series resistance) the blowing up is always a concern.
  7. I initially used BJTs as output instead of the mosfets, but as the
    output load can spike to 6-10A easily at full speed and full swing i've
    found that I achieve higher bandwidths with a fet output, at the cost of
    some headroom and unfortunately also some crossover distortion.
  8. Joerg

    Joerg Guest

    That is what I'd call a "serious load". You'll probably have to
    pre-bias. Assuming that you transmit ultrasound pulse trains you should
    have that running in a servo in order to mitigate temperature drift.

    Then I'd add some sort of fast current sense so you can detect an
    overload before molten solder and TO220 pieces splatter about. Or maybe
    even a simple SWR-bridge so you can detect if the transducer loses
    coupling to the medium, to avoid putting a big crack into the piezo.

    Question: Why do you need to drive a tranducer with a linear amp? Even
    PZT-5H with a good backing material isn't more than 50% in BW.
  9. Ideally E'd run it in a charge amplifying configuration to even
    linearize the piezo motion which has some hysteresis. The reason for
    using a linear amp is that I'm not interested in transmitting power, but
    for the use as a nanopositioning device. It's a part of a homebuilt
    atomic force microscope that is supposed to allow for at least 100kHz,
    ideally even up to 500kHz topography feedback. I can already drive the
    load with the version of the amplifier that I already have, however i
    stagger around 300kHz and I'm currently redesigning it to allow for
    ideally >1MHz operation.
    I am fully well aware that driving a 100nF load at MHz is a more than
    serious load and I know that doing full swings at that frequency is
    utopic, but with 6-10A output transistors it's possible. The only thing
    I'm as mentioned stuggling with is getting dc precision, bandwidth and
    signal integrity to a compromise.
    The actual integrated up power load on the amp is not that staggering,
    but the peak loads are a bit extreme.
  10. I need +-60V upwards unfortunately, but cost is really not an issue, i
    can spend as much as i want as long as i get it to work.
  11. Joerg

    Joerg Guest

    As much as you want? Now that is a nice situation to be in :)

    Well, then ...

    Digikey had them in stock. But sit down before looking at the price. Of
    course, the usual precautions for capacitive loading apply but that is
    the case for any amplifier.
  12. Yeees, that's just the thing. They're not fast enough. That's where I
    started out, using, but with them
    i never got the noise as far down as with my suspended rail highspeed
    opamp. Also i've had trouble getting more than 200kHz closed loop
    bandwith out of them. That's the reason i started building my own
    amplifier in the first place.
  13. Joerg

    Joerg Guest

    Well, then the path looks pretty clear to me. You have to refine your
    own design. Maybe start with stabilizing the bias.

    Can you stop the scan of the microscope once in a while when it hapens
    to be somewhere in the middle, or let it run towards the middle,
    auto-correct the bias and then continue the scan? Essentially similar to
    how blacklevel-clamping operated in the good old analog TV days. The
    days when TV actually worked all the time.

  14. I think based on the suggestions i got here what i will try is going
    with ofsetting at least a part of the bias statically. Since i have the
    luxury of just having to get to run individual amps and not mass produce
    them i could match the bias to the individual transistors. I might be
    able to reduce the crossover distortion enough by forcing the deadband
    from 8V down to 1-2V. As transistor offset voltage is linearly dependent
    on temperature I might get away with it as long as i don't heat the zeners.
    I'll have to think more about if the condition that the opamp, if fast
    enough, will charge the transistors itself is not actually something i
    want, as it will lift the slew limitation while simultaneously relieving
    the capacitive load over the deadband on the opamp. I might be wrong,
    but I don't yet see where there fallacy is there, so I'd be glad if
    someone points it out before I do it.
    The other way to go would maybe be to use a level shifter (bjt with two
    resistors and a current source) to bias the push pull.

    Thanks for all the suggestions though.
  15. Tim Williams

    Tim Williams Guest

    Meh, not so terrible:
    Just tedious.

    All those mirrors would look better in an IC, of course. Add cascodes,
    improved mirrors (Wilson, etc.) where desirable.

    Simulations of this circuit show it goes great up to 10MHz or so, though the
    phase shift is far too great at that frequency to close the loop. Still,
    bandwidth in the 100s of kHz is easily achieved, and probably pushing it to
    1 meg closed loop is possible. To get that bandwidth at low distortion will
    require lots of loop gain somewhere, as the OP mentioned; compensating that
    shall be left as an exercise for the end user. :)

    Also, as posted elsewhere, cost is apparently no object :)

  16. Joerg

    Joerg Guest

    Probably best to post a schematic here showing how you want to build it.
    The risk with zeners like in the example you brought is that they charge
    the gates fast but the discharge is slow. That does not make for a very
    linear operation.

    Also, the threshold drifts with temperture and since the FETs are the
    parts that will become the hottest that needs to be reckoned with.

    Personally, I'd servo it. Since the piezo is a capacitive device it
    cannot have a DC part in its current so it should be possible to lock
    the quiescent current.
  17. Joerg

    Joerg Guest

    If one is willing to do it in discretes it is not such a cost concern.
    After all, back in the days of fast analog scopes they had pretty high
    voltage plate deflection amps and those had to be pretty darn stable in
    the DC and linearity.
  18. The configuration i would try to build right now would be
    with the zener voltages below the respective threshold voltages.

    I don't exactly get what you mean by servo it, care to elaborate? Google
    only turns up that servo seems to be the same as buffering.
  19. Tim Williams

    Tim Williams Guest

    It's funny, back in the 10MHz Tek days, I bet they would've loved having
    some of the later TV tubes -- video output amps and such, huge
    transconductance (some of them comparable with JFETs in current capacity,
    gain and cutoff voltage, at a few times higher terminal voltages). By my
    estimate, 7KY6 for example (which is usually pretty cheap even these days)
    can do 5MHz bandwidth pretty easily, which means they'd be able to do 10 or
    20 with some effort.

    But, the price for the cutting edge demands that one cannot wait, and thus
    distributed amps and hybrid circuits were used.

    But even those caught up, and the newest vertical deflection circuits were
    quite involved indeed. Pull ups, pull downs, pulls up and down for the
    pullups and pulldowns, etc. Double complementary double balanced and who
    knows what else...

    ....Then they introduced distributed deflection plates and, with MCP, pretty
    much finished the history of analog scopes. :)

  20. Uhh... i _really_ like the idea of using low frequency integrators to
    correct dynamically for any threshold voltage. The two additional opamps
    will introduce a tiny bit of noise, but really nothing to write home
    about. This should get nearly completely rid of any THD. I'll try if i
    can get this to run in my suspended rail configuration to make it swing
    Thanks a lot for the suggestion. If you're interested I'll post some
    results and actual schematics when i get them.
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