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TL082 opamp input range?

Discussion in 'Electronic Design' started by D. G., Feb 19, 2006.

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  1. D. G.

    D. G. Guest

    When using a TL082 opamp as a simple voltage follower with a single supply,
    I noticed that, if I lower the input voltage near ground (about 1V and
    lower), the output suddenly slams to Vcc. Strange and dangerous behavior. I
    was assuming the output would, at least, stay as low as it is possible for
    the output stage configuration.

    What kind of general purpose with similar slew rate and input impedance
    would be available without this drawback?
    It must be able to buffer signals with sinus, triangle and square wave forms
    up to about 50KHz with 2Vp-p.

    Dan
     
  2. Working with a single dc supply?

    LM358 works well. http://cache.national.com/ds/LM/LM158.pdf
    1mhz bandwidth, Common-mode includes ground, should be fine.

    Martin
     
  3. You have found the phase reversal issue with the TL081/2/4 series. I
    didn't find 'phase reversal' in the TI data sheet but Analog Devices
    mentions it for the AD712 which is similar. Try a 5532.

    GG
     
  4. Genome

    Genome Guest

    You am not have read Data Sheet. It am full of data but cuold hurt head.

    See... In putt common mod thing iz not golfing term.

    I am recommend you use NZB30984T. Pleeze Google for data sheet befour
    askning for more questions.

    DNA
     
  5. Tim Wescott

    Tim Wescott Guest

    Ah, phase reversal is so fun. Somehow I've avoided it.

    Look for "common mode input range" on the data sheet of any candidate
    for replacement -- you want it going to ground.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  6. Have you checked if the non symmetrical slew rate and cross over
    distortion of the LM358 can handle 50kHz square waves at 2 Vp-p with
    any reasonable distortion?
     
  7. D. G. wrote:
    (snip)
    How fast must the rise and fall times of the 50kHz square wave be?
     
  8. And what is the power supply voltage?
     
  9. Fred Bloggs

    Fred Bloggs Guest

    When using a TL082 opamp as a simple voltage follower with a single supply,
    Maybe ac-coupling into the TL082 input biased at V+/2, you could still
    keep the input impedance fairly high and a relatively small capacitor
    required for reasonably low frequency range. Another method would be
    resistor divider between signal input and V+. Say RL goes from OA input
    to signal input and Ru goes from same OA input to V+. Then the quiescent
    OA input voltage is VDC=(VS*RU+(V+)*RL)/(RU+RL) where VS is signal DC
    bias. The signal ac-component attenuation is compensated for by adding
    gain to the configuration with feedback divider to (-) input.
     
  10. SioL

    SioL Guest

    TL072 does that, too. I use ne5532 whenever I need to avoid this behaviour.

    One question for everyone, does SMD version of ne5532 also come in the usually
    narrow so8 case? Can't seem to find it anywhere.
     
  11. Pooh Bear

    Pooh Bear Guest

    That's why an input common mode range is specified.

    Single supply users of op-amps need to be aware of these issues more than most.

    Graham
     
  12. Pooh Bear

    Pooh Bear Guest

    Hmmmm. The OP didn't mention why he was using an 082 but a 5532 has very different
    characteristics.

    Graham
     
  13. D. G.

    D. G. Guest

    Working with 9V supply. The signal is normally biased at 4.5V but can go
    lower by design to shift the DC level at output. I just checked with a LM358
    (got one in my stock :)
    According to datasheet, slew rate is about 0.2V/uS. A 50KHz cycle is only
    20uS so I feared it was too slow and... it is.
    The wave form looks more like triangular.
    May be I must stick with FET or MOSFET input to get a reasonnably higher
    slew rate (5V/uS min) and low input bias (<100nA)

    Dan
     
  14. There are lots of choices of fast CMOS rail-to-rail opamps for 5 volt
    supplys, but not for 9 volt.
     
  15. Fred Bloggs

    Fred Bloggs Guest

    May be I must stick with FET or MOSFET input to get a reasonnably higher
    Maybe you should just add a negative supply.
     
  16. Jamie

    Jamie Guest

    have you tested the OP-amp by tying the
    input to a common via a R to see if it does this?
    if memory serves i think this is a cmos input
    op-amp? is it possible your driving it with a
    bipolar type input and your dropping below the
    the cut off point thus maybe causing a floating
    input which to me would swing high ?
    or maybe the input of the op-amp has this in it
    internally ?
    just a thought..
     
  17. Guest

    I just remembered a primitive CMOS output, MOS input opamp. It is only
    about half as fast as the TL082, but its inputs and output work to the
    negative rail.
    http://www.intersil.com/cda/deviceinfo/0,1477,CA3260A,00.html#data
     
  18. SioL

    SioL Guest

    Not available in smd as well.

    Is there a cheap rail-to-rail dual opamp that'll do 15V min and be available in
    so8?

    Ups, some googling just turned up tlc272, this might do it.
     
  19. D. G.

    D. G. Guest

    TLC272 sounds interesting. Rail-to-rail output, input down to negative rail.
    But does it also "suffer" from phase reversal ?

    Dan
     
  20. SioL

    SioL Guest

    I think it only does phase reversal when input goes close to positive rail, which is
    in my case a lot easier to circumvent than the other extreem.

    Actually, TLC272 input should allow voltage lower than negative supply rail, if I understand this
    datasheet correctly. It looks like something you could use, its cheap too @0.28 euro / piece.
     
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