JamesUWF Posted November 14, 2009 Report Share Posted November 14, 2009 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?[img width=680 height=408] Quote Link to comment Share on other sites More sharing options...
JamesUWF Posted November 14, 2009 Author Report Share Posted November 14, 2009 Sorry guys, those came out blurry. Here are the JPGs. Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 14, 2009 Report Share Posted November 14, 2009 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. Quote Link to comment Share on other sites More sharing options...
JamesUWF Posted November 14, 2009 Author Report Share Posted November 14, 2009 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? Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 15, 2009 Report Share Posted November 15, 2009 Since you already have a negative control voltage then use it to control your negative current source. Quote Link to comment Share on other sites More sharing options...
JamesUWF Posted November 15, 2009 Author Report Share Posted November 15, 2009 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. Quote Link to comment Share on other sites More sharing options...
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