Let's take C1 as an example, to talk about this problem, R1 and R2 are the partial voltage bias resistance of the transistor, the partial voltage voltage value of these two resistors at point A must be greater than 0.7V, that is, when the circuit is energized (do not send the input signal) C1 right plate is established by the circuit voltage, at this time the left plate is suspended, there is no current loop, Therefore, the capacitor is not charged (only when it is an empty wire head with voltage to the ground), when an AC signal is sent, that is, when the left end of C1 has an alternating current loop, the charge on the right plate of the capacitor will change with the change of the AC signal (current, voltage), and the \"changing charge\" (voltage, voltage, voltage) will change. Current) is provided by the power supply via R1 and R2.
The positive and negative polarity of the charging of the left and right plates on the capacitor is relative, even if the amplifier has only one bias resistance (that is, A fixed bias circuit), the voltage at point A will change with the change of the input signal, and the current that changes is of course provided by the power supply through the bias resistance. C2 charges and discharges in the same way.
When the circuit is working, the charge and discharge of the coupling capacitor is far more complex than this, and the said is only the basic charge and discharge circuit, along with this idea to look at the circuit, pro will understand.
Capacitor \"through\" AC is only an eight-strand lecture, which is dismissive of this practical school, because the high degree of insulation between the capacitor plates simply cannot \"through\" the current (whether it is AC or DC).
fyi