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DC coupling or DC biasing? It is needed to operate the amplifier in a specific Q-point where you can determine the saturation and cutoff regions for the amplifier.
AC coupling is just passing the input signal and getting the output (all acs) without any attenuation, interference, distortion or blocking. Also the input and output impedances of the amplifier is affected by ac coupling-the value of capacitance and inductance you are using...That's all what I knew...let me know if it helped you or then rather you help me out about it!!!
The best example is the standard oscilloscope input. You can switch it or to DC or AC coupled mode. If you want to see full signal, sey rectified voltage which consists from DC component plus AC ripple, you should switch to DC couplung. If you want to measure the ripple voltage only, then switch to AC mode and increase sensitivity.