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transistor selection

Discussion in 'Electronic Design' started by merco, Feb 14, 2007.

  1. merco

    merco Guest

    hi,
    I need some suggestions in complementary transistors selection.

    I have to design a class AB amplifier (push pull) with IOut max = 1
    App for a >=100Khz and <=170KHz sine wave signal.

    Any ideas ?
    or link to sites ?

    Thanks
     
  2. Eeyore

    Eeyore Guest

    What load impedance ? And what volts ?

    Most of the modern audio fast output transistors from the likes of Toshiba will
    do that fine ( fT >= 20 MHz ).

    Graham
     
  3. merco

    merco Guest

    4 ohm / 7Vpp...
     
  4. John  Larkin

    John Larkin Guest

    Why do you "have to"? What's the application? What's the load?

    John
     
  5. Eeyore

    Eeyore Guest

    Wow. I'm not used to looking at anything that low power normally.

    Are you looking for an SMT part ?

    Graham
     
  6. Rich Grise

    Rich Grise Guest

    Probably because if he didn't he wouldn't get a good grade. ;-)

    Cheers!
    Rich
     
  7. Tom Bruhns

    Tom Bruhns Guest

    That's only about a watt and a half. What power supply voltage will
    you run from? It should be easy to do with a single supply, capacitor
    coupled output; a 4.7uF polypropylene cap would have around a third of
    an ohm reactance at 100kHz, and very low loss. The 44H11/45H11 pair
    should do well at this current and power level, and have ample voltage
    ratings.

    You might even find that an integrated "audio" power amplifier would
    do fine, at a cost savings if you consider the sheer number of parts
    needed in a discrete design and the cost of putting them on a board.

    Also, over that small frequency range and low power, you could easily
    transformer-couple the output, and use a pair of NPNs push-pull. It's
    likely that one of the small MiniCircuits "RF" transformers would work
    fine; at 4 ohms instead of 50, you'd get response to lower frequency,
    and you'll find that even at 50 ohms they have quite a few models that
    cover to below 100kHz. You could easily run from a single 3.3V or 5V
    supply, or pick a higher impedance ratio to run nicely from 12V or
    other voltage. For example, with a 12V supply, you can deliver 7Vp-p
    with a 36:1 impedance ratio (3:1 turns ratio from _each_half_ of the
    primary to the secondary). Lest you think that those small
    transformers couldn't handle the power, I can say I just tested one
    (TT1-6) at about half a watt, which is all my RF signal generator will
    do easily, and it behaved well at 100kHz. I can't guarantee that that
    particular one would handle 1.5 watts, but I'd expect some in their
    lineup would.

    Cheers,
    Tom
     
  8. David Starr

    David Starr Guest

    First compute the heat developed in the transistor. That's equal to Vce
    * Ic. Assume VCE is a sine wave and Ic is a sine wave 180 out of phase.
    Either evaluate that function and allow for the 180 cutoff (transistor
    is off for half the cycle. Or dig out somethine like the Radio
    Amateur's Handbook and find a formula for power in class AB amplifiers.
    Given the power requirements select your PNP transistor. Holes have
    lower mobility than electrons so all things being equal the PNP
    transistor will have lower Ft than a corresponding NPN. Go for the best
    Ft you can find in a device that can handle to power.
    Then use 0ja (thermal resistance case to ambient) to compute
    junction temp on your selected device, with the heatsinks you may or may
    not need at the ambient temp you expect. I feel that junction temps
    should be less than 125C, preferably 100C, but others have run devices
    as hot as 200C. If you cannot show a safe junction temp, you need a
    bigger package or a bigger heat sink or both.
    Given a good PNP device, scan the selector guides for a reasonable
    match in NPN. You want the same case style, the same Ft, the same
    breakdown voltage and collector base capacitance. Don't worry about hfe
    or beta.
    Finally make sure the devices you have selected are really
    available. Can you get them from Digikey? Are they on the shelf?

    David Starr
     
  9. Tom Bruhns

    Tom Bruhns Guest


    I assume by "best" Ft, you mean one adequate for the job, not the
    highest one. Picking one too high will just invite stability
    problems. Certainly I can get parts that have very high Ft's that
    will handle the power; doing so seems a waste and an invitation to
    trouble.

    Cheers,
    Tom
     
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