OpAmp Current Source Question

Discussion in 'General Electronics Chat' started by Nano001, Sep 25, 2015.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    0
    I have a very general question about using an OpAmp as a current source. For example, In the attached schematic I would like to supply a fixed 2mA through the 2 load resistors (the 4.4k will ultimately be slightly variable). If I use a 1V reference voltage, my series current through the resistors should be 2mA (1V/500 Ohm). However, I am only seeing 1 mA flowing through the resistor, and a voltage of 0.5 V at the V- terminal (across the 400 Ohm resistor). This device has a very low input bias current of 20 pA, so Vp should equal Vn = 1V. Could someone explain what I may be missing about this and/or what limitations the opamp choice has for this type of design?
    upload_2015-9-25_8-31-30.png
     
  2. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    2mA*4.4kOhm=8.8V>5V !!! Vout<Vcc.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,533
    1,251
    The clue is that the opamp output voltage is less than 50 mV different from its power rail. The opamp output is saturated. Increase Vcc to 12 V and rerun the simulation.

    ak
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
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    What are your design constraints? If you eliminate R4, circuit should work as expected.
     
  5. Nano001

    Thread Starter Active Member

    Jan 12, 2010
    88
    0
    Thanks for the suggestions. R4 will be a bridge sensor with nominal value of 4.4K +/- 1k. Sensor current should be a fixed 1.5-2 mA. This amp is only rated to 5 V, so looks like I need to choose a new part?
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    Yes; and, as a couple others have already mentioned, you'll need to use a higher supply voltage.
     
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