Operational amplifier

Discussion in 'Analog & Mixed-Signal Design' started by dk007, Jul 6, 2016.

  1. dk007

    Thread Starter New Member

    Jun 22, 2016
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    I want to make op apamp output to zero.When there is no input or zero input to opamp in voltage follower mode it showing me some voltage at its output pin 6.
    I have used OP07 op amp .
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,881
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    The MCP4725 has a typical offset error of 1mV.
    The input offset voltage of the opamp adds to this but for the OP07 this is negligible compared to the 1mV from the DAC.

    How much output voltage are you getting (from the DAC, and then from the opamp output).
     
  3. dk007

    Thread Starter New Member

    Jun 22, 2016
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    mr albert hall i get 1.1-1.2V at the op-amp output.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    What is the value of Vdd? What is the value of the sense resistor? Is the opamp using Vdd and GND for power? I'm thinking the emitter is probably at 0.3 to 0.4 Volts and the transistor is on and drawing current through the load. Your circuit is not configured as a follower because the base emitter junction is contained in the Feedback loop.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    If you are using Vdd and ground for op amp power then you need a rail-rail or single-supply op amp which the OP07 is not.
     
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    +1 crutschow.
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    Look at the data sheet for the OP07 and see what kind of pot you need for offset adjustment. Adjust for zero volts out of the circuit with zero setting at the input.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    That only works if the op amp is biased in its linear region with the proper input and supply voltages, but I don't think that's the case here.
     
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  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Thanks for the education. Why is that not the case here? I think I'm going to have to build this one and play with it.
    (edited to add ...)
    Ah. The output will never be less than the sense voltage. Maybe as an emitter follower this might work? I also didn't notice rail power to the op amp. Maybe an LM358 would be more appropriate? How low does the OP-07 operate close to the low rail?
    (edited again)
    It works better with +V and -V power rails with the output as an emitter follower.
     
    Last edited: Jul 8, 2016
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