# Operational amplifier used as a current source

As a laboratory task at my practical engineering school we were asked to make the simple circuit shown below, prove that it acts as a constant current source regardless of its load, and answer some questions.

One of these questions is: what limits the amplifier? Why would its output stay at a maximum value (clamp?) above a certain load resistance?
Another one is: Why is it necessary to supply the amplifier with a dual-supply? Wouldn't single-supply voltage work just as well?

Theoretically it should be able to supply constant current on R, which is the same current on the load, therefore the voltage should not be limited. I couldn't find an answer other than the fact that the amplifier has a power limit it cannot go beyond, but I am unsure as to whether that is the answer they are looking for.

Vcc=15v, -Vcc=-15v, R1=R1=5KΩ, R=1kΩ, RL=1kΩ
LM324 datasheet: http://www.ti.com/lit/gpn/lm324

• Hint: a real op-amp is not the same as an ideal op-amp. Aside from the inputs and outpus, the other terminals on your drawing are related to the limiting behavior. Commented Dec 8, 2014 at 22:48
• I told my teacher that it cannot output anything beyond Vcc (when R=RL), but he said that wasn't the only answer. Commented Dec 8, 2014 at 22:57
• Could the opamp output, say, 10 Amps? Commented Dec 8, 2014 at 22:59
• It probably couldn't. But the case here is a low constant current that is flowing through loads with different resistances - the current isn't supposed to go to such values, only the out-voltage is. Commented Dec 8, 2014 at 23:07
• Have you tried to interpret the datasheet ? You might want to link the one you are using in your question. The answers to both your questions require a datasheet. Commented Dec 9, 2014 at 2:06