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Dumb question of the week: Mis-matching & matching

Discussion in 'Electronic Basics' started by billcalley, Apr 10, 2007.

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  1. billcalley

    billcalley Guest

    Hi Guys,

    From what I have been able to absorb now from all of your really
    terrific responses is that it is totally and completely impossible to
    LC match, even theoretically, a purposefully mis-matched active
    device*, and then look back into that mis-matched network and see a
    perfect 50 ohms for the next stage. However, the purposefully mis-
    matched active device WILL get to see the impedance it wants to see.
    And all this is due to the reciprocal nature of LC matching networks.
    Thus, it would be wise, as you have all mentioned, to design any
    non-50 ohm LNA or PA to be as close to 50 ohms as possible, or to
    design the next connecting stage so that it properly works with
    something other than 50 ohms -- 'cause there is no way to "fix" this
    mis-match issue with an LC matching network. Is that correct, or have
    I misunderstood something? (I can't believe I didn't know -- or
    didn't understand -- this stuff from the get-go! A major glitch in my
    knowledge-base, that's for sure.)

    Many Thanks,

    -Bill

    *The mis-match created so as to optimize an LNA transistor's input for
    NF, or a PA transistor's output for P1dB, to name two common reasons.
     
  2. Joerg

    Joerg Guest

    That's pretty much the situation. But hey, it's not a glitch, we all
    learn. I recently had a similar comeuppance with laser diodes where I
    realized that the stuff engraved in my knowledge base was quite archaic
    and needed some serious updating. And, like you, I got some of those
    updates right here in the newsgroup.

    To push a LNA close to 50ohms is possible but I wouldn't torture myself
    too much. Above a few ten MHz the old trick of "noiseless feedback" via
    transformers becomes nasty because you'd almost need stripline
    techniques. Maybe you could compromise the BPF a bit instead.
     
  3. joseph2k

    joseph2k Guest

    OK. I am sure that someone has explained that the matched condition
    provides maximum _power_ transfer. This is not always the most desired
    property. It is frequent in audio work to have small impedances driving
    large impedances, this provides best broadband (3 decades of frequency)
    linearity and thus low distortion. Most audio power amplifier output
    impedances are below 1 ohm, intentionally, to keep reactive properties of
    the load from disrupting linearity.
    In RF work, power transfer (to reduce losses before radiating from the
    antenna) in the most important property. It also impacts operating economy
    as transmitters operate above P(out) over 100 W.
    For vacuum tube amplifiers the optimum matching impedance is the plate
    resistance. Three (tunable) L sections can transform impedances as high as
    50:1 for spot frequencies over the HF range (2 MHz to 30 MHz), i have seen
    it in operation and worked on such gear. The RF final was two 4CX600J in
    parallel each with a plate resistance of 2000 ohms.
    In earlier bipolar transistor RF outputs (about 15 years ago) when per
    transistor P(d) hit 300 watts but the operating voltage was not over 24 V
    the collector impedances became very low, about 2 ohms in this case. With
    modern mosfet output transistors operating at 100 V the characteristic
    drain impedance was about 33 ohms and much more reasonable matching
    requirements.
     
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