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Clapp oscillator

Discussion in 'Electronic Basics' started by Andrew Holme, May 18, 2005.

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  1. Andrew Holme

    Andrew Holme Guest

    I found this past paper whilst surfing:

    I'm a bit confused by the 0.5 ohm loss resistance shown in figure A1 on
    page 2. The capacitive reactances add up to -j160. If the inductor
    was +j160 to resonate, the unloaded Q would be 160/0.5 = 320, which
    sounds a bit high even for unloaded Q. It says Q=80 on the diagram,
    which is more like it. Did they calculate 80/160=0.5 by mistake? Is
    80 supposed to be the loaded Q? What am I missing??

    BTW if you go up a few directories on that URL, you can browse all
    their past papers.
  2. Hi,
    By giving you the inductor's Q (= 80), they are indirectly telling you
    its total loss resistance (RF and DC) which I make to be 2 ohms. Add-
    in the 0.5 ohm parasitic and you finish up with Q(tot) = 64. This value
    doesn't include any loading by the bias and emitter resistors but then
    that is the beauty of the Clapp. This oscillator dates back to the
    30's but J. K. Clapp published an article on it in QST as late as
    October 1948.
    What I love about these old exam papers is that they remind one just
    how much has been forgotten over the years and forces you to dig out
    the books once again - good old Terman, et al...

    Cheers - Joe
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