How does the bias circuit work in an amplifier IC?

As the amplifier or op-amp positive and negative inputs in closed loop operation, always maintain the same voltage with the output, generally if the output to maintain a 0v DC voltage, must be used + - dual power supply. IC input is a differential pair circuit, in order to make it work in a single power supply amplification state, the output voltage for the power supply voltage 1/2, so that in the closed-loop state to achieve the purpose, positive and negative inputs should also be worked in the 1/2 voltage level, R1, 2 equivalent voltage divider to produce 1/2 voltage added to the IC first pin 1, R3 resistor because it is far from the input resistance. In the 1/2 voltage level, R1, 2 equal voltage divider produces 1/2 voltage in the IC first 1 foot, R3 resistance due to far less than the input impedance, its voltage drop is negligible, resulting in the output voltage work in the 1/2 voltage level, the IC second 2 feet connected to the output foot through the R5 as a reverse DC feedback, so that the positive and negative input foot potential and the output foot is the same.C1 is just a bypass capacitance, to avoid the divider circuit cascading into the AC interference, C1 is just a bypass capacitor, to avoid the divider C1 is just a bypass capacitor to avoid the voltage divider circuit into the AC interference, C3 is the role of DC so that R4, 5 AC gain control, without affecting the DC feedback. In summary, R1, 2 voltage divider circuit is to IC outputs work in a single supply midpoint voltage and the bias reference voltage applied.