TL072 circuit

Discussion in 'General Electronics Chat' started by crmccreary, Oct 20, 2009.

  1. crmccreary

    Thread Starter New Member

    Oct 19, 2009
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    I've searched the forums to no avail for the answer to this very basic circuit. I would like to amplify a low voltage (0-40mv), slowly varying signal. I happened to have a few TL072's from a previous project. To test the circuit, I have the following:
    5.5V - V_CC+
    Gnd - V_CC-
    1_IN+ - ~60 mv from a voltage divider circuit off of the same source as V_CC+
    1_IN- - shorted to 1_OUT

    I expected a unit gain output on 1_OUT (voltage follower) but the voltage is constant at ~4.5V. What am I missing? I should probably use a different op-amp, but these were handy and now it's a puzzle I need to solve.
     
    Last edited: Oct 20, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you post the circuit so we can see if you missed something?
     
  3. crmccreary

    Thread Starter New Member

    Oct 19, 2009
    2
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    Attached schematic to original post
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    I see it now.

    My spec sheet indicates the TL072 is a dual supply op amp, and is spec'ed at voltages from +/-9 volts to +/-15 volts. You might see what happens with a pair of 9 volt batteries supplying the op amp.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The TL072 is not suitable for use on a single 5V supply; all of its specs are stated while it is being operated on +15/-15V split supplies.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The TL072 can't get within about 1.5v of it's supply rails.

    So with ground and 4.5v as the rails, the lowest output would be 1.5v and the highest, 3v.

    [eta]
    I'm wondering if you may have a counterfeit TL072. The genuine TL072 opamps are supposed to have latch-up (aka phase inversion) free operation, but yours is definitely exhibiting symptoms of latch-up.

    With opamps that are prone to latch-up, if the input signal goes beyond a certain point, the output will suddenly go to nearly the voltage of one of the rails.

    See the attached for an example of a 741 opamp exhibiting latch-up.
     
    Last edited: Oct 20, 2009
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The minimum supply for a TL072 is 7V.
    It has the "Opamp Phase Inversion" problem: when an input gets within 2V to 3V from its negative supply (0V in this circuit) then its output suddenly goes high which is about +3.5V with a 5V supply.
     
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