I'm trying to implement the following circuit in a project:
VCC coming in is 18v. The op-amp is supplied with 3.3v. There is a waveform voltage input from 0-2 v that can oscillate at up to 100hz, or not at all. I need to convert this into 0-2 mA, so I used 1K RSENSE. I obviously need constant current with a variable RL. I only know enough about electronics to be dangerous, but this seems like a fairly straightforward circuit, and I got it working on a simulator, so I tried prototyping it. When I hooked up my ammeter, it gave a range of 0.17-0.19 mA for an input of 0-2 v. I used an an LMC6482 and a 2N3904. When it didn't work, I switched the BJT with a MOSFET (2N7000G) and got the same result. I'm assuming that there are some parameters for the transistor that is causing the unexpected behavior. Am I on the right track?
Why is the circuit currently acting the way it is? What should I do to get working as described? Thanks
I scoped all inputs and outputs and there were no oscillations. In fact, touching my scope to the anode of the load resistor gave me the perfect waveform I was looking for. I then touched it to the anode of my multimeter and the same signal was there, yet my multimeter oscillated between 0.15-0.19 mA... does this mean my multimeter is shot? It was connected in series right before the load resistance.