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Discussion in 'Electronic Basics' started by [email protected], Oct 22, 2003.

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  1. Guest

    When you connect the load, it oscillates. What could you try to prevent it?


    |\
    -----|+\
    | >--------------------
    /\/\-----|-/ | |
    | |/ | load
    | | |
    | | -----
    ---\/\/\/---\/\/\/-- ---
    -


    Robin
     
  2. Put a cap from the neg op-amp input to the op-amp output. Place a small
    resister from the op-amp output to the load. Keep the FB resister
    connected to the load.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  3. John Larkin

    John Larkin Guest


    Use one of those cool National C-load opamps. A capacitive load just
    slows down the dominant pole.

    John
     
  4. John Larkin

    John Larkin Guest

    John
     
  5. Guest

    I tried putting a capacitor from ouput to -ve input and it made it
    worse! How come? More negative feedback should stop it?

    Confused Student
     
  6. Rich Grise

    Rich Grise Guest

    Put something on the noninverting input such that it doesn't act
    like a broadband receiving antenna, capacitively coupled to the
    opamp's own output. Also, bypass (decouple) the opamp's power
    supplies.

    Cheers!
    Rich
     
  7. Huh? You placed a capacitor from the output of the op-amp to where? What
    is a "-ve input"? Since it made the oscillation worse I assume you placed
    the capacitor from the output of the op-amp to the negative supply rail
    (sometimes labeled something like -Vee, or othertimes just V- in the
    datasheet). The negative supply rail isn't normally considered an "input"
    pin on the opamp. Rather it is a supply pin while the inverting and
    non-inverting input pins are referred to as the "input" pins. This type of
    arrangement is precisely the type of load most likely to cause op-amps to
    self oscillate for no apparent reason. A capacitor from the output to some
    low impedance connection (IE: ground, negative supply rail, positive supply
    rail) can cause problems. Normally a few tens of picofarads or less isn't a
    problem.

    For increased stability place the capacitor from the op-amp output to the
    op-amp's inverting input pin (IE: from pin 6 to pin 2 on the 741).
    Alternatively to, or in addition to this, place a small resistor (such as 47
    ohms) directly to the output pin to the load. Both of these circuit
    modifications will change the circuit's performance characterisitics,
    sometimes in undesireable ways.
     
  8. John Larkin

    John Larkin Guest


    In certain benighted parts of the world, "-ve" substitutes for the
    word "negative", and "+ve" means "positive." But these people also say
    "maths", so are clearly incurable.

    John
     
  9. Guest

    Of all the Americanisms
    Perhaps the worst of any:
    Radios have aerials
    Insects have antennae

    Robin
     
  10. Guest

    Yes, these things work ok. The oscillation stops but as you say the
    bandwidth is reduced.

    If I do this instead:-

    |\
    -----|+\
    | >--------------------
    /\/\-----|-/ | |
    | |/ | load
    | | |
    | | -----
    ---\/\/\/---\/\/\/-- ---
    | -
    /
    \
    /
    |
    |
    -----
    -----
    |
    |
    -------
    ---
    -

    and fiddle about with capacitor and resistor, I get better bandwidth.
    How does this work?


    Even more confused student
     
  11. Fred Abse

    Fred Abse Guest

    AFAIK, the agreed international term for a radiator or collector of
    electromagnetic radiation in the radio part of the spectrum is "antenna",
    plural "antennas".

    Cast in stone, I think, (correct me if I'm wrong), by the ITU many years
    ago.

    Probably enshrined in some ISO document, too.

    In German "antenne"
    In French "antenne"

    Don't know about Italian or Spanish, but I'm sure someone will enlighten
    me.

    "Aerial" is peculiarly British, and doesn't appear in any modern or recent
    engineering books or articles by British authors, either. It's deprecated
    in engineering circles.
     
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