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RF Transistor Selection

Discussion in 'Electronic Basics' started by sthim, Jul 1, 2003.

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  1. sthim

    sthim Guest

    Hi,

    I am working on a rf preamp project and I've been having trouble
    finding a transistor. Some of the transistors are not stable for the
    frequencies that I'm working on (2 different preamps for 200MHz and
    500MHz) or others do not have the NF I'm looking for (0.5db). I was
    hoping if somebody could suggest some transistors... fets, hempts,
    anything ...

    Thanks
    sthim
     
  2. if you use a high mobility, high current amplifier, stability may be
    difficult near the nearly DC frequencies you mention ;-) . You might
    find that, in general, if a transistor is well charactarized at 6 GHz,
    its going to be tough as nails to get a stable input match at 200 MHz.
    If you tell us the linearity (IIP3), current, input match and gain you
    are striving for, it will be easier to make a more specific
    recommendation. Try bipolars, JFETs, even large geometry GaAs and SiGe
    from Toshiba, NEC(CEL) IBM, Infineon Philips and Agilent. I've had
    varying degrees of success with each in different applications. The
    toughest challenge will be to try to find a device with NF min at
    optimum near the other parameters above. If your input match doesn't
    fall within the required noise circle on the smith chart, you may need
    to use negative feedback (at the expense of gain and perhaps
    stability) to change the input parameters.

    Best of luck

    Frank Raffaeli
    AOM Wireless
    http://www.aomwireless.com/
     
  3. IIP3 - 3 order Input interception point.

    This is a non-linear parameter. For low-power input, for each dB you add at
    the input you get +1dB at the output (of course) and +3dB third order
    distortion. This translates in a graphic like this:
    - x-axis: input power;
    - y-axis: output power;
    - two lines:
    - one for "signal" (1st order), "growing" at 1 dB/dB
    - one for "distortion" (3rd order), "growing" at 3 dB/dB

    These two lines cross at a given point. The abciss of this point is known as
    IIP3, i.e., IIP3 *would* be the input power you would apply in order to get
    at the output the same power for both 3rd order distortion and signal.

    I wrote *would* because for such a high input power these lines are no
    longer straight lines. But for low power they are. One should extrapolate
    the lines from low-power in order to achieve the cross point and hence the
    IIP3.

    Hence IIP3 is related with non-linear 3rd order distortion, which is the
    predominant in-band distortion.


    Ricardo
     
  4. Cost is going to be an issue here... Generic Gasfets often used in
    preamps are your best bet here... but there are some newer devices by
    Agilent that are smoking good quality.

    Under $25 per device, go with the Gasfets...

    Under $160 per device, go with the Aglient Devices. As you can figure,
    the IP3 for the Agilent Devices are much better.

    But the IP3 for the common ARR used type Gasfet is not that bad. When
    you get into "busy places" the better devices pay off. Contact me
    through the sonic server Email icons if you need additional information.

    200 and 500 Mhz... not a problem... been there, done that, coffee mug and
    tee shirt.

    cheers
    skipp
    http://sonic.ucdavis.edu

    : Hi,

    : I am working on a rf preamp project and I've been having trouble
    : finding a transistor. Some of the transistors are not stable for the
    : frequencies that I'm working on (2 different preamps for 200MHz and
    : 500MHz) or others do not have the NF I'm looking for (0.5db). I was
    : hoping if somebody could suggest some transistors... fets, hempts,
    : anything ...

    : Thanks
    : sthim
     
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