Summing Junction

The Summing Junction, labeled SJ, is the node on which the feedback path in an operational amplifier is closed on.

Due to the feedback action in a properly configured op-amp, the currents at this point are equal and cancel out. This means that the input current through the input resistor and the feedback current through the feedback resistors are equal when the amplifier is working in the linear range, and their algebric sum is zero. Thanks to this, the SJ is also termed a Virtual Ground point.