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Buffer Amp IC suggestions plse

Discussion in 'Electronic Design' started by Jeff, Sep 21, 2005.

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  1. Jeff

    Jeff Guest

    Hi,
    Ive been trying to locate a suitable unity gain buffer amplifier IC that can
    drive a 50 load.
    Application is to buffer a VCO output at 5Mhz.
    Needs to be 8pin DIP and operate on a single-ended supply.
    I had found what I wanted in the CLC109, but cant seem to find a supplier.
    Any suggestions greatly appreciated regarding type and supplier that is user
    friendly to international countries.
    JEFF
    ZL3JK
    New Zealand
     
  2. Jeff wrote...
    LTC's LT1206 comes in TO220, miniDIP and soic packages,
    and Digi-Key has them in stock for $6.25 each...
    http://rocky.digikey.com/WebLib/Linear Tech/Web Data/LT1206.pdf
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=329191
    Oops, +/-5V supply. What's your signal voltage? And did
    you want your output to have a 50-ohm source impedance?
     
  3. Jeff

    Jeff Guest

    Needs to drive into 50 ohms. input level approx 150mV at 5Mhz
     
  4. An AD817 can deliver +-50 mA and has a gain bandwidth product of 50 MHz.
    http://www.analog.com/UploadedFiles/Data_Sheets/289066266ad817.pdf
    <4$ from Digikey.
    Good for either a gain 1 buffer or a gain 2 with series 50 ohm
    resistor, 50 ohm source.
     
  5. Jeff wrote...
    Unless you source with 50 ohms (2x signal plus 50-ohm resistor),
    and terminate with 50-ohms, the issue becomes how much cable
    capacitance? Either the load looks resistive (100 ohms in the
    former case) or capacitive.
     
  6. Jeff

    Jeff Guest

    Thanks. The AD817 looks good, and I can source it locally from Australia.
    Thanks all for your help.
    JEFF
     
  7. Mac

    Mac Guest

    What you want is a video buffer or video op-amp. Linear Technologies,
    National Semiconductor, and Analog Devices are among the many companies
    that make such things.

    Note that some of these devices get a bit squirrelly if they see a
    capacitive load. So I recommend that you do something like this: (use
    courier or other constant-width font for ASCII art schematic)



    |\ 50
    in--|+\__+____/\/\____/> to 50 Ohm load
    +--|-/ |
    | |/ |
    | |
    +--/\/\-+
    | Rf
    \
    /Ra
    \
    /
    |
    -----
    GND

    Rf and Ra would be the same value. Use a value recommended by the
    datasheet, or 1k if the datasheet doesn't recommend a specific value.
    Follow any other suggestions in the datasheet as well. For example, they
    may suggest a small capacitor directly from the op-amp output to inverting
    input.

    You probably want to terminate the input with a 50 Ohm resistor, too, but
    I don't know, so I'll leave it out.

    It sounds like you may need AC-coupling, so you can put blocking
    capacitors on the input and/or output. Oh, if you put a blocking cap on
    the input, be sure to also provide a DC (resistive) path to GND or to the
    mid-rail point if you are going single-supply.

    Just chose a capacitor big enough so that 1/(2*pi*R*C) is much lower than
    5 MHz, your signal of interest. (R in this case is 50 Ohms) There is no
    problem putting a capacitor like this in the signal path: Since it looks
    like a short circuit at higher frequencies, it does not capacitively load
    the amp at frequencies where the amp might be unstable. Also, the series
    50 Ohm resistor keeps the overall impedance resistive.

    HTH

    --Mac
     
  8. Mac

    Mac Guest

    Maybe an LT1191?

    digikey #: LT1191CN8-ND (24 in stock at USD 3.25)
    http://rocky.digikey.com/WebLib/Linear Tech/Web Data/LT1191.pdf

    --Mac
     
  9. The LT1010 comes to my mind.
    A untiy gain buffer with 20MHz BW, delivers up to +-150mA from
    a 4-40V supply, 75V/us slewrate 3.30$ in single unit quantities.
    TO220-5 case

    Rene
     
  10. Phil Hobbs

    Phil Hobbs Guest

    Since the swing is so small (only 3 mA to the load), why not use a
    normal amplifier and a voltage divider on the output? Lots of parts can
    drive 1k ohm to 3V, for example.

    Cheers,

    Phil Hobbs
     
  11. Fred Bloggs

    Fred Bloggs Guest

    Well- why not your super-duper 74HCU04 linear amp or whatever it was?-
    is that available in Tiny Logic?
     
  12. Jeff

    Jeff Guest

    Thanks for those ideas. I was considering putting a low pass filter after
    the amp. The 2 shunt caps work out to be 560pF each.
    JEFF
     
  13. Ross Herbert

    Ross Herbert Guest

    Here's a dual op-amp in 8 pin DIP which should do what you want
    http://www.analog.com/UploadedFiles/Data_Sheets/242276322AD8397_0.pdf
     
  14. johan aeq

    johan aeq Guest

    Try a SL560
     
  15. Chris Jones

    Chris Jones Guest

    I would say it depends on the cable you choose. If there is a 50 Ohm load
    and you connect it to your buffer amplifier with 50 Ohm coaxial cable, then
    the buffer amplifier will see a 50 Ohm resistive load, not capacitive at
    all.

    It is probably still wise to use the gain-of-two amplifier and 50 Ohm source
    resistor in case the load happens to be a poor match. If you really don't
    need this feature then I would be tempted to use an emitter follower with
    just one transistor. (Do they make transistors in DIP-8 packages?)

    Chris
     
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