Hello. I need to confess that I'm noob for electronics and studying right now. I'm in OP-AMP study and found some sentence which is hardly understood for me. "If the signal source is ac-coupled, you must provide a return to ground for the (very small) input current, as in Figure 4.6." (Please see the attached image) Why does AC signal require additional path to ground? What make difference this from DC which don't requires this path, according to textbook. In addition, OP amp requires supply voltage of +15 V and -15 V, from textbook. How can I get negative voltage? I guess both numbers are with respect to ground voltage which is typically thought as zero voltage.