This is a response to @agaelema
The op amp circuit at the top shows a differential amplifier working. The inputs are connected to two generators 180 degrees out of phase. Thus the amplifier is able to see this as a wave that it can amplify.
This is the kind of wave that we see from an AM modulated TX or from humbucking guitar pickups
The example at the bottom is the same configuration. However, here the signals are NOT 180 degrees out of phase, which is the kind of common mode noise that the op amp in this configuration can reject and it does so well! The output here is shown as the DC green line on the scope trace above.
Cheers
