Regarding the AC channel in the amplifier circuit, why is the DC power supply of the AC signal grounded? Please elaborate.

Have you ever learned the principle of superposition? There, when there are several power sources in a circuit, when analyzing the circuit, let one of the power sources work, and the rest are treated as follows: short circuit the voltage source (but keep the internal resistance) and disconnect the current source (but keep the internal resistance).

In the amplifier circuit, there are both DC power supply and AC power supply. In this way, when we calculate DC (that is, find the static operating point), we will short-circuit the AC voltage source, that is, short-circuit the input terminal (because there is an AC input signal voltage source at the input terminal); Also short-circuit the output terminal (because the output terminal has an AC output power supply).

Similarly, when we analyze the AC state and draw the AC path, we must short-circuit the DC voltage source. Because one end of the DC power supply in the circuit is grounded, of course, the DC power supply is also regarded as grounding. The internal resistance of the DC power supply we use is very small and is considered equal to zero.