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this should be easy

Discussion in 'Electronic Design' started by Mook Johnson, Jan 23, 2005.

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  1. Mook Johnson

    Mook Johnson Guest

    I'm looking for a sine wave driver that can deliver 2 volts p-p 30Khz sine
    wave into a 3 ohm load. +/- 4.5V rails are available and sine wave will be
    the input. Distortion needs to be reasonable but not ultra tight. >1%

    I have a design using complementary NPN and PNP biased to class AB (audio
    amplifier) with a opamp completing the loop and providing feedback to
    minimize the residual xover distortion.

    Thermal runaway is a high concern. I'm using large 2 ohm emitter resistors
    on the output and diode biasing to compensate for temperature affects.
    Anything else I should be looking at? both xisters will be mounted to a 6
    layer PC board with lots of copper underneath.

    This amp will be running above 125C (ambient) so the audio amp ICS with the
    thermal cutoff will be of no use to me.

    Just looking for different opinions..


    P.S. Class D is also out due to noise issues in the immediate areas.
  2. Ban

    Ban Guest

    This is probably not so easy because of the +125°C operation limit. If you
    want something stable and with adjustable current limit, I recommend the
    OPA547. With 1.5W dissipation and moderate heat sinking the die temperature
    will be still below 160°, where it shuts down. Not too cheap tho, but
    certfied up to 125° operation temp.
  3. Mook Johnson

    Mook Johnson Guest

    Ambient temperature may go higher than 160C 125 is the minimun temrpature of

    the thrustof my question is if anyone knows of an audio type IC tat doesn't
    have thermal shudoff or it can be disabled?

    If a discrete design must be used, what would be the most stable biasing
    scheme for the most reliable operation overwide temperature swings? (hence
    my concern about thermal runaway)

    In my business I know for a fact that garden variety transistors, opamps and
    other components will work for several hours at 200C if they are well
    selected. Thats all i need.
  4. Jim Thompson

    Jim Thompson Guest

    RS-232 to TTL??? You have something against 1488 and 1489 ?:)

    ...Jim Thompson
  5. Luhan Monat

    Luhan Monat Guest


    You have not said what this whole thing is for. From the description
    you have given, it could be an electric cigar lighter. You get better
    directions if you let people know just where you are going.
  6. Mike Fields

    Mike Fields Guest

    OK, sometimes I have trouble "reading between the lines", but how
    did we get from a 30khz amp at 120 deg C to RS-232 ??????????

  7. Mook Johnson wrote...
    That's 330mA delivered from a 4.5V supply with a 3.5V rail-to-output
    drop allowed, right? What's wrong with unbiased NPN and PNP followers
    inside an opamp feedback loop? There won't be any thermal runaway,
    and < 1% distortion shouldn't be any problem with high opamp f_T.
  8. Jim Thompson

    Jim Thompson Guest

    You didn't check out Luhan Monat's website ?:)

    ...Jim Thompson
  9. Luhan Monat

    Luhan Monat Guest


    Those are fine for putting in a design itself. This unit was for quick
    hookups to bench prototypes so each one does not need rs232 just to try
    out an idea. Also, it provides for 'network' type mixdown to a single
    ttl with 'echo suppression'.

    All of this could be done with the chips you suggest, or, as most of my
    other later designs did, just a single pnp transistor for transmit, and
    a cmos inverter gate for receive.
  10. Mark Jones

    Mark Jones Guest

    One thing. Consider this when using multi-layer boards:
  11. Ban

    Ban Guest

    I once encountered a similar problem, but with 180°C temp spec. Impossible
    to do reliably with silicon, I had to assemble these brittle GaAs chips on a
    hybrid. It was supposed to be for "Telecommunications".
    By chance I found out later that this circuit was mounted above the gates
    of those F16 bunkers and was used to arm the atomic weapons in the last
    possible moment. The afterburner was heating up that area, that is where
    that spec came from. Well telecommunication was not exactly wrong...
  12. Ken Smith

    Ken Smith Guest

    .... and highish frequency transistors. At these currents that should be
    no problem. This is one application where you don't want the TIP35/36
  13. Maybe reduce the dissipation in the Tr's with a resistor in
    series with each collector. Up to 10 ohms looks possible.
  14. Tam/WB2TT

    Tam/WB2TT Guest

    This sounds reasonable to me. The class B operation will also cut down on
    dissipation. I would look for semiconductors intended for automobile
    operation. It gets pretty hot under the hood. There would also be some
    advantage to transformer coupling; that would let you match the 2V P-P to
    the +/- 4.5 V supply. Lastly, for a single frequency amplifier, why not
    tuned class C (or class B) ?

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