I'm trying to build a voltage-controlled current source which is supposed to work with a wide range of load resistances. Currently the plan is to use an NPN transisitor as voltage follower to do the heavy lifting, and control the current with an op amp:
simulate this circuit – Schematic created using CircuitLab
The load resistance RLoad does not change quickly (< 1 Hz), but can vary over a huge range (10 Ohm – 10k Ohm).
Since Q1 is a voltage follower, the op amp output voltage basically controls the high voltage on the load. Since the highest current I need is 50mA, the low voltage on the load is at most ~1.5V.
Now, this feedback loop for the op amp has some delay, which can cause the circuit to oscillate. The capacitor C1 is supposed to prevent this by adding lowpass behaviour.
This all works pretty well, but there is a catch: When I change the desired current by changing V_in, I want the controller to closely approach the new current in 1ms or less. However, the speed at which C1 is charged / discharged depends on RLoad with this circuit, so a large RLoad causes the output to settle very slowly.
This is because C1 is charged over R2, with a current that only depends on the difference between the actual shunt current and the desired shunt current. With a high RLoad, the op amp needs to output a higher voltage, so it needs to charge C1 to a higher voltage, which takes more time.
How can I modify this circuit so that the settling time becomes independend of the load resistance?
Best Answer
I've successfully used Vcc tied loads like this in constant current circuits: -
simulate this circuit – Schematic created using CircuitLab
The op-amp has negative feedback from the emitter and therefore the emitter voltage must be at the same potential as the input. This means the current through the BJT = \$\dfrac{V_{IN}}{R1}\$
You can also use a PNP BJT and an op-amp mirrored up to the positive supply rail for ground coupled loads.