Current Regulator Circuit

[JamesUWF]

Hey guys, I'm working on a circuit that is an adaptation of something I found online awhile back.  The first adaptation of the original circuit I made (shown below) draws a fixed amount of current from a positive voltage external supply.  The idea is that the amount of current flowing through the N-channel enhancement MOSFET and the 0.1 ohm resistor is regulated through negative feedback and equals 10x the value of V+ on the op-amp.  I have this version of the circuit working fine. 

What I'm trying to do now is build a similar circuit that will do the same thing with a negative voltage external supply.  The reference voltage being generated here is varied between -0.77 and 0 Vdc with the 250k potentiometer, and I'm using a P-channel enhancement MOSFET instead.  The problem I'm having is that for some reason, this configuration is driving the op-amp Vout into saturation (-12 V), which of course is driving the D-S resistance of the MOSFET down to almost nothing.  I'm not sure what I did wrong here, but I have a feeling I'm not setting up this op-amp properly for the new circuit.

Any thoughts?  Suggestions?

 

[JamesUWF]

Sorry guys, those came out blurry.  Here are the JPGs.

View attachment 40696

View attachment 40697

 

[audioguru2]

When you save a schematic as a JPG file type then it is very fuzzy. Save it as a PNG or GIF file type for a schematic to be very clear.

The negative current source circuit needs a negative input voltage. Invert the positive input with an inverting opamp then feed the negative input to your circuit that has the P-channel Mosfet.

View attachment 40698

 

[JamesUWF]

I did consider doing that with an inverting op-amp, but the control voltage I'm generating for the negative current regulator circuit is already taken from a -12V supply, thus already negative.  Wouldn't this be the same as inverting a positive control voltage?

 

[audioguru2]

Since you already have a negative control voltage then use it to control your negative current source.

 

[JamesUWF]

If push comes to shove, that's what I'll do, but I was hoping to make it pull the same current regardless of any voltage change on the supply under test,  using the feedback loop.