Capacitor has a coupling effect. We send the output signal of the front-end circuit to the back-end circuit, which is called coupling. Capacitance is a process of isolating DC and coupling AC. C2 and C3 in the picture below are capacitive coupling.
When the frequency is low, such as audio, the capacitor here may be replaced with a polar electrolytic capacitor. The negative pole is connected to the ground direction. Why use electrolytic capacitors? Because the capacity of electrolytic capacitors is larger, the capacity of non-polar capacitors is generally smaller.
By the way, C1 in the figure is a filter to filter out the noise of the input signal. R1, C1 form a resistance-capacitance filter.
What does decoupling mean? As mentioned earlier, coupling means that the signal is sent from the front-end to the back-end circuit. And decoupling, as the name implies, is to remove the useless signals that are coupled. Decoupling capacitors are generally placed at the output terminal of the power supply. As shown in the figure below, C27 is a decoupling capacitor. The value of its capacitance depends on the size of the interference signal coupled from the back-end circuit, that is, the load end.
For example, when the load is running at full load, the current is very large. Sometimes there is no need to run at full load, the current will be small. At the conversion moment of high current and small current, the power chip will not have time to respond. And this decoupling capacitor works, and stabilizes the voltage fluctuations coupled from the load. Thus the power chip is not affected.