Choosing the right opamp

Discussion in 'The Projects Forum' started by dmag0006, Oct 17, 2012.

  1. dmag0006

    Thread Starter New Member

    Dec 8, 2010
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    Hi, I need an op amp which is good when having very high gains (approx 2mV amplified to 0.5V) and also good at filtering frequencies which are out of the audible range. (I'm designing an amplifier and a filter). Ideally it would be a Voltage Feedback Amp and as cheap as possible (as this is a project and costs will be assessed).

    So far, I tried using the LM324 opamp and it has made my output clipped at the end of each half-wave for low voltages which I later found out to be due to push-pull amplifiers inside the op-amp.

    Any solutions please?Thanks :)
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. wayneh

    Expert

    Sep 9, 2010
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    That's a gain of only 250 (not "very high", in the op-amp world) and should be achievable by almost any op-amp. Not a search criterion
    Filtering is usually done outside of the op-amp, rather than relying on the slowness of the op-amp to provide de facto filtering. So this is not really a search criterion either.
    I understand cheap, and you'll be able to find something well under $1. Not sure I understand what you mean by VFA.
    I'm skeptical. You may have been asking the amp for output too close to the rail voltage, but it's not clear. What was your input and what is the power supply to the amp? The LM324 is too slow for hi-fi audio.

    FWIW, I just bought a couple TLV272IP dual op-amps because they're fast rail-to-rail op-amps. About $1 at Mouser.
     
    Last edited: Oct 17, 2012
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    NE5532

    Also maybe TLO82, LF356
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Please post schematic. That does not sound right.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The lousy old LM324 quad and its sister the LM358 dual are old low power opamps. They have low power because their output stage is missing the biasing that would make it linear, therefore it produces crossover distortion.

    Their slew rate is also very low, giving trouble above 2kHz.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,054
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    Not that simple. ;) A gain of 250 is not high for a DC signal but it requires an op amp with a minimum of a 5MHz gain-bandwidth for a 20kHz sinewave.

    The TLV272IP only has a 3MHz gain-bandwidth and would start to roll off at about 12kHz with a closed-loop gain of 250.

    An NE5532 does have a 5MHz gain-bandwidth and so should work.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Good point. Since the OP was using the LM324, I assumed he wasn't interested in that high a frequency.
     
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