I thought as a first exercise in op amps, even before setting up a feedback circuit, I would determine the gain of the raw op amp.
I tried with an LM324 and an LM358 which should be essentially the same. I assume the input currents are really small, I believe when I glanced at the data sheet it was nA. So I used a 4 AA power supply with 6v (not fresh batteries). To get an input signal, I created a voltage divider from a 1M and 1K resistor (V0). I can measure .0056mV with the meter. Then I put a second divider of 1M and 1k across the 1k (labelled V1), which should give roughly 5.5 uV though I can't measure that.
For the LM324, pin 2 is V-, pin 3 is V+, and pin 1 is Vout. If I set pin 2 and 3 to 0V, essentially 0 comes out (some small noise as expected). If I set pin3 to Vcc, 4.8v comes out (roughly 1.2v drop through the amp).
But when I set pin 3 to the first stage voltage divider, 5.6mV, the output voltage does max out, but I notice the input voltage does change a bit. That one I didn't record, see below for more. Troubling, considering there should be essentially no effect from the op amp. Worse, once I put in the 5.5uA signal, I expected to see something like a gain of 850,000 or 900,000. I swear I did this before, and had planned a lab on it. But now I can't get this repeatably. When I tried again, this time replacing R3 and R4 by 100k each, V1 changed from 2.9mV to 3.2mV I don't see how that could possibly happen.
I tried again with positive feedback mode, I can get a gain of 101 using a 100k and 1k resistor. I used 1M and 1k, and then across the 1k, a second voltage divider with two 100k. Again, voltage V1 is 2.9mV but when plugged into the + side of the op amp it changes, only to 3.2mV, but I wouldn't have thought there would be any change at all.
I'd like to know whether I am simply using the op amp out of the range in which it behaves well, whether it should work in which case I will try to double check my power supply, my wiring. The voltage is still 6v, and the LM324N should be good down to 3V, so I can't believe it's the power. This whole experience is making me doubt my sanity.