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Half-wave rectifier opamp in single supply

Discussion in 'Electronic Basics' started by MRW, Jun 22, 2007.

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  1. MRW

    MRW Guest

    Good day! I have the following circuit:

    Just looking at the configuration on paper, the voltage at R18's
    terminal next to D2 would have given me V2 if Vin < V2. I'm not quite
    sure why my spice simulation is showing 1.683V.

    Any insights? Thanks!
  2. Eeyore

    Eeyore Guest

    Yes. The op-amps 2 input terminals aren't at the same voltage so for some reason
    the feedback's not working.

    Is it a rail-to-rail output op-amp ?

  3. MRW

    MRW Guest

    Hi Graham, according to TI, it's supposed to be rail-to-rail:

    I decided to change the opamp, and I'm getting similar results on
    paper. Thanks!
  4. Is V2 defined as 2.5 volts, somewhere?

    The output should go positive enough to supply current
    through D1 and R14, to ring the right end of R14 up to 2.5
    volts. However, that takes (2.5-1.1)/1k= 700uA from the
    opamp output.

    Take a look at the data sheet:
    page 27, figure 18.

    The output current limits with more than the whole supply
    dropped across the pull up fet before the current gets to
    700 mA. Change the 2k resistors to 20k, and it should start
  5. That's 700 uA.
  6. MRW

    MRW Guest

    Thanks, John! I appreciate the additional insight.
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