I'm planning to build a current regulator for a power supply and I'm prototyping a number of different regulator circuits. First up, the op amp and transistor circuit shown below.
I felt I understood the principle well enough: negative feedback resulting in the opamp regulating the transistor in order to equalize its inputs to some given reference voltage. So, I pulled out whatever bits and bobs I had in the box and put the circuit together. I was aiming for a current limit of about 100mA and I wanted to keep the voltage drop across Rsense low. I had some low ohmic resistors from another project, so I used a .010 ohm resistor as the sense. Calculating the reference voltage to achieve 100mA gave me 1mV (100mA x .01R). The voltage divider shown on the left in the schematic below produces 1mV at the non-inverting pin. So far, so good... but then... many, many hours later and a lot of contemplation and confusion, I still don't understand it.
As you can see from the schematic below, the notes are the actual measurements taken using a dvm.
The opamp inputs are refusing to equalize. What's more, the opamp seems to be driving the transistor hard into saturation, the current almost completely owing to the 22 ohm (7W) load resistance. It's almost like the opamp is using positive feedback, rather than negative feedback. But, why so? The inverting pin is clearly reading 3mV. I'm stumped! Any help greatly appreciated.