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MOSPOWER Class AB musings

Discussion in 'Electronic Design' started by Active8, Dec 20, 2004.

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  1. Active8

    Active8 Guest

    I was thinking I could set up a current controlled Vbe multiplier to
    control MOSFETs idle current. Ib, entering upper left of figure
    below, is supplied by an xCCS which provides enough current for the
    divider with extra for the NPN to shunt - the amount of which
    effects a greater/lesser drop across the divider via its effect on
    Vbe.

    The log nature of this setup allows finer control over the bias
    voltage compared to a resistor.

    Assuming a way to sense MOSFET idle current, is this approach even
    worth considering?

    Not much in the IRF ANs. Anyone know of other ANs covering this?
    BTW, that LT1166 is interesting, albeit lacking in THD spec.

    TIA

    View in Courier font, of course. Did Google Beta get that
    straightened out?


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    created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
     
  2. John Larkin

    John Larkin Guest


    Why a Vbe multiplier? Mosfets don't need the temp tracking like
    bipolars do.

    I've mused on this, too. How about a simple bypassed resistor, pumped
    by a current source? Some side circuit would know when the signal
    level was small or zero, measure fet current, and tweak the bias. A
    small uP and a dac, maybe. The hard part is measuring the smallish fet
    idle currents... maybe a sense resistor in one of the power rails,
    shunted by a hefty diode. I like having no source resistors, except
    for the current-limit thing.

    Or just use a pot!

    John
     
  3. Active8

    Active8 Guest

    I wasn't talking about Vbe mult biasing of a bjt CE amp. The Vbe
    mult in the circuit has nothing to do with temp tracking.
    Why the bypass? Maybe I'm not following you.

    As for a resistor, as I said, there's no fine control. a 9 mA change
    through 5k is 45 V whereas that same change through the Vbe mult
    only results in mV changes.
    Is 100 mA typical?
    Not, both? But why in the drain?
    How do you limit current without a much greater R than the typical
    0.1R source R?
    And a robotic arm to turn it?
     
  4. Mac

    Mac Guest

    I started a thread similar to this one about a year ago. The subject was:

    "quiescent current in audio MOSFET class AB output stage"

    It seemed that the consensus was to just use a vbe multiplier and a trim
    pot and be done with it.

    Someone also recommended Douglas Self's book the _Audio Power Amplifier
    Design Handbook_, and after I read that I didn't want to use MOSFET's for
    the amp anyway, and started thinking about doing it with BJT's.

    Now I'm so conflicted I might not ever do it. In the mean time I'm working
    on a pre-amp.

    --Mac
     
  5. Mac

    Mac Guest

    Of course you mean 4.5 Volts.

    --Mac
     
  6. Ban

    Ban Guest

    It will change its value with temperature like a multiplied diode junction,
    so when it was 4.5V at 25°C it will be only 3.96V at 65°C. A constant
    current source with a resistor alone is independent of temperature.
    50-70mA is typical, to have some accuracy we would need at least 0.1V
    voltage drop, which corresponds to 2 ohm. Now if the max. current is 15A,
    there would be 30V loss, that is why the resistor has to be bypassed with a
    hefty schottky diode. You would then sample the voltage whenever the output
    goes through zero. You need only one resistor, because when the output is
    zero, the current in the other arm must be of equal value. If you put the
    sense resistor in the source line the varying voltage drop affects the bias
    setting.
    you didn't get it, sorry
     
  7. Active8

    Active8 Guest

    5000 * .009 = 45

    Best Regards,
    Mike
     
  8. Active8

    Active8 Guest

    I started that. Need an amp to close the loop :)

    What I'm seeing - and I just plonked down IRF530 IRF9530 for
    convenience - is that when Vgs goes up just 0.07 Vdc, Id almost
    doubles.

    Best Regards,
    Mike
     
  9. Active8

    Active8 Guest

    Thank you.
    LTI1166 data sheet disagrees - works for their app.
    Yeah. You'd need resistors on at least half of the sources then.
    Thanks.
    A joke?
     
  10. Mac

    Mac Guest

    You are right, of course. My apologies.

    --Mac
     
  11. gwhite

    gwhite Guest


    "A Simple Direct-Coupled Power MOSFET Audio Amplifier Topology Featuring Bias
    Stabilization" by William "Bill" Roehr, IEEE Transaction on Consumer
    Electronics, Vol. CE-28, No. 4, Nov. 1982, pp.546-552.


    "One of the major design problems has been providing a means of stabilizing idle
    current level because short-channel FET's exhibit an increase of drain current
    with temperature at the desired idle level."


    I'm not going to comment on Vbe multipliers in this application, but
    short-channel MOS AB amps probably require bias stabilization. I don't design
    these amps and never have, so I don't know the SOTA. The article claims long
    channel FETs have poorer linearity. I think Roehr may have worked for General
    Semiconductor.
     
  12. Pooh Bear

    Pooh Bear Guest

    John Larkin wrote:

    Actually, the hard bit is distinguishing idle current from load current !


    Graham
     
  13. Active8

    Active8 Guest

    I'm ok with that. I just need to level shift a drop WRT the rail up
    to where I can use the measurement to control the idle current.
     
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