Hello. Please see the attached image first. 1. The textbook seems say resistor to + input in this image is necessary to provide a return path to ground. I completely don't understand even what it mean. Could you help me to clarify this? 2. Instead of understanding directly what the book said as above, I interpreted combination of capacitor and resistor on + input as high pass filter to block the DC. In high pass filter, what is the role that resistor take? Mathematical formula for high pass filter operation requires finite value of resistor to have desired cut-off frequency, but I would like to know physically the reason of presence of resistor. Thanks!