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?

 

2mA*4.4kOhm=8.8V>5V !!! Vout<Vcc.

 

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

 

What are your design constraints? If you eliminate R4, circuit should work as expected.

 

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?

 

This amp is only rated to 5 V, so looks like I need to choose a new part?

Yes; and, as a couple others have already mentioned, you'll need to use a higher supply voltage.