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Class AB common source mosfet power amplifier

Discussion in 'Electronic Design' started by John Devereux, Nov 7, 2007.

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  1. Hi,

    OK, I have been simulating away but am now all spiced out!

    I am trying to come up with a linear (i.e. not switched) power mosfet
    amplifier that works with the output device drains tied together. This
    is mainly so they can be bolted to the same heatsink without using
    insulators, although an extended voltage swing would also be good.

    I.e. generally like this:

    .. V+ --------------------------------------------
    .. |
    .. [R] (optional)
    .. |
    .. |<
    .. ------|| P-Channel
    .. |-
    .. Drive |--------------------[LOAD]--->GND
    .. |-
    .. ------|| N-Channel
    .. |>
    .. |
    .. |
    .. [R] (optional)
    .. |
    .. V- --------------------------------------------

    Any ideas?

    This is not for audio, so don't need super low distortion - but I
    don't want any cross-over distortion to be *too* obvious either.

    Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

  2. John Larkin

    John Larkin Guest

    That's the topology I use in my NMR gradient amps, which range from
    +-12 volts at 3 amps up to +-180 at 100 amps. I've posted pics of the
    big fet/heatsink assembly a couple of times.

    I use an opamp per fet (up to 32 of them!) to close a loop on the
    voltage drop across the source resistors. That gives very tight
    control of each fet's current and lets me use small sense resistors.

    The driver has to intelligently split positive and negative drives and
    maintain idle currents, and shift the drives up to the power rails.
    That's not trivial to do right.

    Note that this config is like an ldo regulator: it has a high
    open-loop output impedance, so loop dynamics is a lot more sensitive
    to loads than a more conventional source-output amp.

  3. This topology can be problematic because of the wild variation of the
    gain depending on the load impedance and current. I assume you are
    running in the class AB, right? Nevertheless it can be worked out
    however you may have to add the heavy correction capacitors between
    drains and gates (and maybe some nonlinear elements too it the goal is
    the high performance).

    But why this topology? It is unlikely that you need R-R operation at
    +/-80V. Why not a trivial pnp-npn emitter follower?

    BTW, if you can drop the rail voltage to somewhat +/-50V, you can
    probably use an audio IC like TDA7294.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  4. I don't *need* rail to rail. As I said, it is mainly so I can bolt the
    devices to the same heatsink without insulation which would reduce the
    heat transfer. Power mosfets have the drain connected to the tab, so
    this implies the topology above.

    Not for this - the final thing will have 8 mosfets paralleled for each
    shown above, and will also use a copy driven in antiphase to get full
    bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!
  5. That was my first idea, then I got diverted by all the audio power
    amplifier designs out there! You even have the same number of mosfets
    (my final version will be a full bridge configuration).
    That's where I started looking for other ideas...
    Makes sense, thanks.

    I do have a common drain version - that simulates OK - but common
    source would simplify construction.
  6. Joerg

    Joerg Guest

    Wow! Who's playing lead guitar during that concert?
  7. Yes - should drive a decent size sub-woofer!

    Its actually for a vibrator. (Not that sort! A voice-coil actuator for
    vibration testing).
  8. John Larkin wrote:

    So the output stage operates as the current source and the dynamic
    response is completely determined by the parameters of the load.
    Assuming the unconditional stability, closing the global feedback loop
    by the output voltage would be problematic because there is only ~45
    degrees of phase available for everything.
    Yes, this is not very trivial as this schematic is prone to the
    different kinds of ill behavior.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
  9. John Larkin

    John Larkin Guest

    One other note: mosfets spec'd for switching use have a bad habit of
    exploding when used in linear mode. We tested a lot of "300 watt"
    power fets, bolted to a solid copper block, and most of them couldn't
    dissipate 300 watts for 100 milliseconds when running at higher drain
    voltages. Imagine loud noises. It has to do with nonuniform heating or
    something. IXYS has some fets specially designed to not explode in
    this mode.

    So test your candidate fets to destruction to make sure you have
    margin! Or go switchmode.

  10. John Larkin

    John Larkin Guest

    We're driving an NMR gradient coil with a current source, and feeding
    back on sensed current. So having a high-z output stage makes us
    nearly insensitive to load impedance. So use a low-z stage if you're
    feeding back on voltage, and a high-z stahe if you feed back on
    Well, we did do it right.

  11. The datasheets do show SOA for various duration pulses, which I will
    Will do.
    That seemed even more complicated (and I do not need to save power).
  12. See

    Actually, properly done, this can be a very stable
    amplifier configuration. Here's an example of
    such an amplifier I designed, with a little help
    from Tony Williams, R.I.P. It has been discussed
    here on s.e.d. several times, including when I
    was doing the design. It uses BJTs instead of
    MOSFETs (which I came to regret, SOA, ouch!),
    but you may find it useful to study anyway.
  13. Phil Allison

    Phil Allison Guest

    "John Devereux"

    ** For heaven's sake, forget stupid switching fets and use complementary
    LATERAL mosfets for linear apps.

    The case is the source, they operate in parallel like a dream, are not prone
    to thermal runaway and inherently protect themselves if the chip goes


    These are 16A, 250W dual chip devices.

    Same numbers without the "D" are single chip versions.

    ....... Phil
  14. They seem quite rare, likely expensive, and I was not too worried
    about distortion.
    ....but you make a good case!
    Thanks Phil - I had not realised you could get anything with case
    connected to source. I will investigate these tomorrow.
  15. Wow, thanks, I will study it. Am I right in saying this is a variant
    of the circuit warned against here (fig 16)?


    (Hey, it's like one of your "bad circuits" sections!)
  16. Phil Allison

    Phil Allison Guest

    "John Devereux"
    "Phil Allison"

    ** Farnell have them.

    ....... Phil
  17. John Larkin

    John Larkin Guest

    Doggone, U6, with its power pins cascoded into the complementary HV
    transistors, is hauntingly familiar. Not to mention D3 and D4, except
    that I like to add LED's there, too.

  18. Tim Williams

    Tim Williams Guest

    LOL. So, I'm reading this thread, and the guy is asking if he can solve
    this problem where the transistors all have to be mounted on the same
    heatsink (or two). And then another guy chimes in with a solution that
    completely removes the problem of heat, and the guy shoots it down...


    C'mon...switching ain't all that hard...hell, I did it with tubes!
    Rak(on) ~= 100 ohms on that 38HE7. :)

  19. D from BC

    D from BC Guest

    I found an old schematic I created in 1997.
    I quickly drew it in Ltspice..

    Don't ask about the output network.. I've completely forgotten why I
    did that and it's looks weird to me now..

    D from BC
  20. Winfield

    Winfield Guest

    Don't be silly. First, in reality the configurations are
    totally different. Second, do the calculations and see the
    damaging effect in OUT1 of a too-small emitter resistor.
    My output stages are mirrors - do you see any mirrors in
    either OUT1 or OUT2 --- no I didn't think so. That's OK,
    I knew you were pulling my leg.

    Go read the circuit discussions in s.e.d.
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