OpAmp Buffer inputs not at same voltage

[Alemarius Nexus]

Hi,

I'm fairly new to electronics and trying to get a simple OpAmp-based circuit to work. It's a circuit to determine the ratio of two resistors, R2 and R3. Here's the circuit simulated in LTspice:



This is how I want it to work. The voltage at net1 should drop linearly as the resistance of R2 increases (and as R3 drops, because R2+R3 = 1kOhm), but only up to a certain threshold, 300mV in this case. It all looks great in the simulation, but I've built it up on a breadboard with an LM324 from TI and found that the lower voltage threshold is more like 800mV. I've measured it and found that the OpAmp output voltage is at 800mV, although the non-inverting input is at 300mV. How can that be? I thought OpAmps keps their inputs at the same voltage, so the output should be 300mV too in that case. It seems to have something to do with the circuitry behind the output: If I remove the connection from R2 to the positive terminal of the voltage source, I do get my 300mV. I thought the OpAmp output voltage is (relatively) independent of the load at the output pin? That's why I wanted to use an OpAmp in the first place.

Any ideas?

[Andy Watson]

Figure 11, page 9

http://www.ti.com/lit/ds/symlink/lm124-n.pdf

Suggests that is the correct operation of this amplifier. The output can only pull down to about 700mV. To get lower, the load must be less than 50 microamps.