I am having a play about with the differential amplifier using a LM324 and have shorted both inputs and set them to 3V sine wave at 100Hz. i understand that in a ideal op-amp the output should be 0, how ever i am unsure of what is happening at the output..... i increase the frequency every 1Khz and record the voltage on the output which is in the mV range. when i plot the results i can see that the gain slowly increases then all of a sudden the gain increase rapidly then as you further increase the frequency it rapidly decreases. The same type of repression you get with a band pass filter. my only explanation i can see from reading books is that it is to do with the compensation capacitor in the device. could some one shed some light on the subject?