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Transistor Choice

Discussion in 'Electronic Design' started by [email protected], Jul 21, 2007.

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  1. Guest

    Hi,

    I'm having some difficulties finding the correct transistor. I'm
    designing a inductorless and transformerless RF power amplifier at
    40Mhz, 1 Watt output. The design is somewhat different I guess. There
    is no impedance matching and I'm trying to maximise linearity and
    bandwidth. I'm operating on a 24V supply. I currently have a common-
    base stage, feeding into a shunt-feedback stage (gain) then onto a
    push-pull power amplifier.

    I'm having trouble finding a relatively cheap option for the power
    stage. RF Power Transistors are generally too expensive and hard to
    obtain. Darlington are good but the poor Ft is a problem. Philips
    (NXP) have alot of good RF BJTs but the 24V supply really seems to
    limit my options. It seems I'm trying to find a transistor which isn't
    quite small-signal and isn't quite power, something in between. I've
    had a look at the older Motorola and NXP transistors and they would
    have been perfect, but these medium voltage, RF transistors seems to
    gone; replaced by small-signal RF.

    Any help would be great.

    Regards,
    Alan.
     
  2. Winfield

    Winfield Guest

    The reason you're struggling, Alan, is because this is a difficult
    problem. If you look at the CATV amplifier scene, you'll find that
    they've solved the problem, but only by using expensive, delicate
    transistors (mounted in expensive ceramic hybrid circuits), that
    are made in the fashion of complex integrated circuits. They have
    thousands of small low-voltage emitter-degenerated high-frequency
    transistors in parallel. It takes a high-magnification microscope
    to see what they've done. The result is a high-performance very
    wideband amplifier, but with fragile easy-to-burn-out properties.

    I learned that running 24-volt amplifier modules at full power with
    28V supplies (a 16% increase) was a quick path to disaster. They
    would enter an excessive-bias-current mode, and die. Only when I
    added a fast electronic fuse could I save them. Finally I lowered
    the voltage and breathed easier, having kept the electronic fuse.
     
  3. Jitt

    Jitt Guest


    The old 2n3375 had an Rb of 15 ohms, and was fairly robust.
    There was an RCA equiv., I forget the number, I'll try to
    find it today if you like.
     
  4. Guest


    Thanks Jitt, that would be great. I was wondering if there are many
    options for RF power transistors for push pull outputs ? Most I've
    seen are singular and don't have a complementary PNP device.
     
  5. Jitt

    Jitt Guest


    Sorry for delay....
    RCA had a series of those, 40279 and such, but I dont know where you'd
    get one today. As Win mentioned, these were for CATV work.
    I see the RCA 2n3375 shows Rb at 10 ohm, maybe the TI was 15-
    or maybe I'm wrong!
     

  6. Some of those old RCA types ended up with a JEDEC number.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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