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Amplifier oscillation possible solution?

Discussion in 'Electronic Basics' started by Dummy, May 24, 2004.

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  1. Dummy

    Dummy Guest

    Half carrier spur was seen in one of the PA design.
    Spur amplitude is quite high, at about -45dBc.
    Would it be caused by insufficient phase margin or some other factors?
    Between gate and drain, there's a series RC feedback.
    I've tried tweaking the RC values, but to no avail.
    Frequency of interest is 30MHz - 42MHz.
    Any good articles out there regarding amplifier oscillation, so that I
    could put some possible fixes on use. Please help.
  2. Tim Wescott

    Tim Wescott Guest

    It's called the "varactor effect" or "parametric oscillation". I've
    never had to deal with it myself, so everything I know about it comes
    from books.

    The one reference I can find on short notice is "Radio Frequency
    Transistors, Principals and Practical Applications" by Norm Dye and
    Helge Granberg, published by Butterworth-Heinemann, (c) 1993. They say
    that there's nothing to be done (I think Wes Heyward had some solutions,
    but I can't remember), and suggest filtering the output. They also say
    that it only happens on class C or B amplifiers, that it's a result of
    the wide bias swings on the collector-base diodes, and that it doesn't
    happen on class AB or A amplifiers.
  3. John

    John Guest

    Destroy subharmonic resonance's in transistor PA circuits
    Sprat magazine

    Part 1 issue 91 page 6
    Part 2 issue 107 page 9

    Available on CD containing all back issues 1 to 109
    Sprat click on Club Sales
  4. Robert Baer

    Robert Baer Guest

    Well, if it *is* due to Vcb variations, then that might be fixed with
    a cascode configuration.
  5. ddwyer

    ddwyer Guest

    Before TTL dividers used it to induce 5MHz xtal oscillator to give 1 MHz
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