I'm trying to build a 0 to 2A variable current source following the Linear app note.
Here's the schematic:
The circuit is a difference amplifier (LT1995) providing gain to the voltage developed across a sense resistor. The amplifier output voltage is fed into a inverting integrator (LTI1880) with a RC network (R = 1k, C = 10 nF). The output from the integrator is fed into a low pass RC filter (R = 100, C = 10 nF) and that voltage is placed on the gate of a P Power MOSFET (IRF9530).
The source of the IRF9530 is tied to the "low side" of the sense resistor (terminal going into the input P1 on the LTI1995 in figure). The drain of the FET I have tied to ground for testing but I plan to have an inductive coil load in the future.
The output current in my measurements does not scale as \$\dfrac{V_{in}}{5 \times R_s}\$. This is my goal for this circuit.
With no load attached (drain left floating), I provide voltage on the input (Vin) pin the amplifier output will develop a positive output as it should but the integrator will quickly integrate down to the negative rail (-15V). The IRF9530 is operating with a very large (-15V) gate to source voltage in this case.
When the drain was tied to ground, and the voltage at the input was tied to ground the the FET gate voltage was -2.2V.
Can someone provide a better picture of the operation of the circuit and places where perhaps I'm assembling this wrong. I have prototyped the circuit on the breadboard.
Datasheets LTI1995: http://cds.linear.com/docs/en/datasheet/1995fb.pdf