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OPAMP trouble

Discussion in 'Electronic Basics' started by jw, Aug 22, 2005.

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  1. jw

    jw Guest


    My opamp looks like it isn't amplifying and I need some help. I have a
    MCP606 opamp in a DIP package.

    I am inputting a very fast (almost 1GHz) sine wave into this OPAMP on the +
    input, but this is a very low voltage signal usually between 10 to 30

    Then want to amplify this signal at least 1000-fold, so I connected the
    output to the - input with a 1000 ohm resistor and further, a 1 ohm resistor
    connecting this same - input to ground, to get the desired amplification.

    But when I connect the Oscope to the output of the OPAMP, it does not show
    any amplification of the input signal.

    In fact, the magniture of the output is still in the 10-30 MV range - the
    same as the input.

    I forgot to mention that I have also connected a 0.01 F cap between GND and
    Vdd, as recommended.

    Can anone tell me why this is not amplifying? Is the signal too fast for
    this amp?

    --thx, jw
  2. Ingvar Esk

    Ingvar Esk Guest

    You are far beyond what the OPamp can deliver. If you look at the datasheet
    its gain*bandwidth=155kHz. That means that at 155kHz you don't have more
    that 1 time amplification. (At 15kHz you can have 10 times.)

  3. jw

    jw Guest

    Thanks for the help.

    Does anyone know an opamp that can operate at these high speeds?

  4. John Larkin

    John Larkin Guest

    Are you sure you need this much gain at 1 GHz?

    Your spec of 30 mv in and a gain above 1000 implies over 30 volts out.
    That's very serious power at this sort of frequency.

    And anything with that much gain at a GHz will need very serious
    shielding between the input and output to keep from oscillating; so
    much so that any single gain stage, if it existed (and it doesn't)
    would oscillate like a bandit.

    Only a few opamps will give a gain of over 1 at 1 GHz, and then not
    much over 1. InGaAs MMICS are usually used for gain up here, maybe 20
    dB or so (10:1 voltage) per stage.

    What's this for?

  5. jw

    jw Guest

    Oh - here it looks like I would need, say, a gain of 30-fold, so this would
    give me output of about 1 V or so, from the amplifier.

    I am using this to amplify a signal I am receiving on an antenna, as an
    experiment for learning about wireless circuits, no more than that.

  6. John Larkin

    John Larkin Guest

    Maybe look at the Mini-Circuits ERA-series of mmic amps. They have
    gains to 10:1 or so at mucho GHz, and they're cheap and easy to use.
    To play with these, read the appnotes carefully. Breadboard by
    soldering everything flat and low on an piece of unetched copperclad
    PCB material.

    At these speeds, everything must be zero length, if not shorter, and
    the input must be far away from the output... obviously an interesting

  7. Jasen Betts

    Jasen Betts Guest

    you'll be lucky to find any op-amp thjat works that fast.
    yes, try a broadband amplifier chip instead, you may need a few in series
    for that gain.

  8. jw

    jw Guest

    Thanks for these suggestions - I'm learning a lot already. --jw
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