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? RE transistor biasing

Discussion in 'Electronic Design' started by Dave, Sep 28, 2004.

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  1. Dave

    Dave Guest

    I am trying to design and build a simply RF amplifier to give me better
    reception on standard television (ch 2 is the main thing I want. If I can
    get that, the rest will fall in place.) Problem is, I can't remember
    anything about transistor biasing. Do you simply need a .6 volt difference
    between the different junctions? I am wanting to use two transistors, the
    first in common-base configuration and the second in common collector
    configuration, for the benefits of impedance matching I understand this will
    provide. I understand the requirements of forward/reverse biasing, but
    can't seem to dig out the actual voltages needed to do the job. Can anyone
    offer help, or parhaps a website that explains the intricacies? Currently
    using my old tradeschool textbooks (Grob Basic Electronics, and Transistor
    Electronics by I-can't-remember-who.) Also, what frequencies cover the
    "standard" TV spectrum? I mean Ch. 2 through 69, or whatever it is. Can't
    seem to find that. *Think* it's roughly 50 Mhz to 450 Mhz, or something
    like that. Is this close?

    Thanks much for any help.

    Ignorantly yours...

  2. Tim Wescott

    Tim Wescott Guest

    Unless you have a long run of cable from your antenna to your TV a
    preamp isn't going to do you much good -- and if you use one you'd want
    it out on the mast at the antenna, not at the TV.

    You can find TV frequencies on the web; channel is on 56MHz and the
    nominal spacing is 6MHz, but I can't remember where the gap is for FM radio.

    If you can't remember how to bias a transistor then you probably can't
    design a good TV preamp from scratch. I'd look around and see if
    there's any kits -- Down East Microwave would be good but I doubt they
    have anything wide band enough.
  3. Joerg

    Joerg Guest

    Hi Tim, Hi Dave
    Another option is to go to a hardware store and buy one. I must confess
    that's what I did. It cost around $20 with a mast mounted amp and a
    power supply that sends filtered DC up the coax. The upside is you
    instantly get something in a fairly good weatherproof housing. No busted
    knuckles, cuts, scrapes or messes in the shop. The downside is that the
    folks who designed it didn't know much about dynamic range. When an
    aircraft comes in at our local airstrip and the pilot announces turning
    left base there is a moiree pattern on the TV screen. I could have done
    better but TV ain't important enough.

    Regards, Joerg
  4. Jim Thompson

    Jim Thompson Guest

    Ain't that the truth? PBS isn't even fit to watch anymore.

    ...Jim Thompson
  5. Joerg

    Joerg Guest

    Hi Jim,
    Sometimes it's ok. If you filter out some of the political agenda, that
    is ;-)

    Regards, Joerg
  6. Jim Thompson

    Jim Thompson Guest

    The best times to watch PBS are during what I call "gimme week".
    That's the only time they run the good stuff.

    ...Jim Thompson
  7. Dave

    Dave Guest

    Well, the old Rat Shack 10db amp in combination with the RF amp built into
    the old VCR used to do the job just fine, with both of them inside of the
    house, so I am willing to gamble some time and spare parts on a DIY project.
    And yes, I do have a long run of coax out to the antenna, which is the only
    way to get decent reception without cable.

    Thanks anyway, I'll figure it out via Google. Just thought I'd ask.

  8. Jeff

    Jeff Guest

    I don't watch TV much anymore. Maybe 0.5 to 3 hours a week max, in addition
    to a movie or two. TV is too full of time wasting garbage programs,
    propaganda, advertisements, and other misc junk. 30 minutes of TV often
    consists of 20 minutes of content, and 10 minutes of advertisements, with
    advertisements sometimes heavily working their way into the content.
  9. Tim Wescott

    Tim Wescott Guest

    Now, if you want to do it so you'll know how in the end that's a
    different story. Google, by all means, but dig out your old copy of the
    ARRL handbook, and maybe go get a new one. You'll want to get whichever
    of their books applies to VHF these days as well -- the Handbook keeps
    getting bigger and bigger, but the useful stuff keeps migrating out of
    it, somehow.
  10. Jim Thompson

    Jim Thompson Guest

    My daily schedule:

    8AM-10AM Barry Young, Right Wing Talk Radio
    10AM-12NOON Al Franken, Left Wing Air America
    12NOON-2PM Bob Mohan, Libertarian
    2PM-5PM Fox TV News
    5PM-6:30PM Local TV News + ABC
    6:30PM+ Pull a movie from my collection of over 300 DVDs
    10PM-10:30PM Local News (sometimes)
    10:30PM+ Leno (sometimes)

    ...Jim Thompson
  11. TV channel 2 is 54 - 60 MHz. FM broadcast is 88 - 108 MHz. Aero and
    land mobile communications go up to 174 MHZ, so Channel 7 is 174-180,
    and channel 13 ends at 216 MHz - then there's a big gap before the UHF
    channels (14-up) start.
  12. Here is a chart of the US TV channels:
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