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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. legg

    legg Guest

    You don't want them in the actual drive path. Capacitive coupling
    might help. As long as they don't DC-restore.....

    The gate load, being capacitive, should return all charge as the
    signal reverses polarity.RL
  2. mike

    mike Guest

    Don't know if this is relevant. This month's edition talks about drivers.

    I don't understand the objective. Is trajectory important? Or do you
    just need
    to get from one physical location to another fast and not worry about
    the path from A to B? You can do some interesting things by
    pre-calculating charge levels and stuffing in an approximate impulse
    and let the loop take care of the details. Very messy...

    Also, I don't think you have ANY room for a dead band. If the loop
    opens, you're screwed.
  3. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Only in class A operation, otherwise your average supply current depends
    on the AC output current.
  4. Joerg

    Joerg Guest

    Hmm, that one is not very encouraging. It might work with capacitors
    across the zeners but the opamp is not going to like to drive that.
    You'll probably be better off completely rolling your own amp like Tim
    suggested. If the Apex ones aren't fast enough that is pretty much the
    end of the rope in terms of power/voltage/speed.

    Another thing you could try since you already have an Apex amp is to try
    to follow that with a pair of BJTs.

    Essentially an active circuit that sets the DC quiescent current. Since
    temperature will make that drift away it needs a regular quiet interval
    where the DC bias circuit measures the quiescent current and adjusts
    itself. That would be the servo part. But Fred is right, DC can only be
    measured while in full operation in a class A amplifier and this (or
    shuold become) is class AB2. So you'd have to briefly stop the
    microscope scan to adjust the DC bias.
  5. Joerg

    Joerg Guest

    Oui, j'ai eu tort. Was thinking of class A but this is class AB2 (or
    should be). So that only leaves the option to regularly stop the
    microscope scan. Probably not too bad since temperature drifts are slow.

    Other options might be to gauge the DC from the input signal and bick a
    low amplitude phase but that gets esoteric.
  6. Joerg

    Joerg Guest

    One sad trend is that with the demise of the CRT the nice video driver
    transistors also died away. Those would have been very useful in a
    project like Adrian's.
  7. Joerg

    Joerg Guest

    It is a good idea but you have to make sure the capacitance to ground in
    those isolated supplies is very minimal. Also, the slew rate in the
    input signal must be limited because if the opamp-FET combos can't
    follow fast enough, else ... PHUT ... *POP*

    You could diode-clamp around the opamp to avoid excursions that would
    exceed the abs max limits on its input. That should fix the startup
    blow-outs and input pulse racing.
  8. Tim Williams

    Tim Williams Guest

    I've got a couple pulls sitting around here, like the 20V 100mA 1GHz cascode
    bottom device, or the 120V 100mA 1GHz device that sat atop it! Almost
    always Sanyo parts, lovely specs.

  9. Joerg

    Joerg Guest

    Probably all obsolete or "Not for new designs" :-(

    But for Adrian that wouldn't matter since he only has to build one unit.
  10. Joerg

    Joerg Guest

    John Larkin wrote:

    This doesn't have to be an opto-isolator. You can also pump RF in the
    GHz range into a tiny RF transformer, rectify on the other side and use
    that to control the FET. Modulating the RF on the primary side is easy,
    the signal into the modulator is your control signal. On the rectifier
    side things must be fast and loaded so there will not be a slow decay.

    The capacitance between the windings of such a transformer can be in the
    low single-digit picofarads.

  11. Joerg

    Joerg Guest

    If you try to get high reaction speed out of a high-Q transducer that
    usually ends with a shattered transducer and a puff of smoke wafting
    away from the amplifier :)

    How do they get fast movements if there isn't any stiff backing
    material? The PZT transducers I dealt with (except for CW Doppler) all
    had 6dB bandwidths between 30% and 60%. The only way to get there is to
    have a lot of tungstens and stuff in the backing.
  12. Weell... make that two or three units at first. The circuit as proposed
    by John Larkin seems to me to be the currently best option i have. I'll
    be trying to combine that with the rest of the amp (voltage swing) and a
    fast overall feedback and see if I can get a better amp. The power
    handling capability of mosfets is after all quite stunning.

  13. I am well aware of these problems as I already have a simpler version of
    this physically running. Slew limiting i do simply with an RC lowpass at
    the entrance. The startup i do using a relays to ground the input
    initially and after the rails have been established (window comp)
    together with a time delay (zener and RC) i turn on both rails
    simultaneously. This already works reproducably.
  14. Ah yes this is where the whole thing becomes increasingly interesting.
    If you use dither piezos that are near MHz range with their resonances
    you don't need much any drive circuitry, you need neither linearity nor
    much power, as cantilever in air have a resonance Q of easy 300 or more.
    I use cascaded PID loop control for the z feedback. The samples are
    essentially mounted on a stack of piezos with increasingly higher
    resonance frequencies and in turn decresing maximal deflections. This
    scheme allows to boost z-feeback to well over 500kHz if done right, but
    it takes a long time to set up and control for.
    Also total z deflection measurements become a project of their own as
    you'd have to sum up all single hysteresis-ridden deflections. The
    current goal is using optical sensors instead.
  15. Jamie

    Jamie Guest

    But, don't get misled with those high amp ratings.. Heat is a killer
    of MOSFETS.

  16. That's just the beauty of it all. Using piezos as fast nanopositioning
    devices requires crazy currents of multiple amps at peak, but since you
    don't have steadystate oscillations the actual power is limited. Granted
    I will have to care for heat dissipation, but with the current amp
    scheme i can scan around 100kHz feedback without the DPAK cased hexfets
    going much above 30 maybe 35C even with deadbugging and without a heatsink.
  17. Joerg

    Joerg Guest

    Yeah ... but ... if it's swinging back and forth all the time you have
    to make a proper time allowance in the SOA and then the power handling
    capabilities are quite a bit reduced. Don't push it :)
  18. I don't understand the circuit well enough yet, but isn't it a problem
    attaching +-60V supplies this way with an opamp that can only take 30V?
    The suspended rail configuration i drew earlier hardwires the rails to a
    fraction of the supply range above and below the opamp. That one i've
    already tested on board and it works really quite nicely. I might draw
    the board flexible enough to try both schemes.
    If i understand your first suggestion correctly, setting the dividers at
    the input correctly will already keep each transistor always just
    slightly conducting, eliminating any crossover distortion as well.
  19. From all i've gathered only for Vt > 5V, below zener diodes have a NTC
    as have FETs.
  20. Jamie

    Jamie Guest

    The DC-DC converter is a isolated type and the common is being attached
    to HV side output. This way the op-amp supplies will only see +/- 15
    volts because they are basically riding ontop of the HV supply.. the
    output will look like a common/ground to the op-amp no matter the level.

    This poses a problem however, the common for your input signal..
    Something just don't look right there.. SOmething tells me the inputs
    also need to be isolated.. They do make isolated op-amps where the
    common rail for the output side can attached in this manner and the
    inputs can use a low voltage common.

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