What Do Capacitive Coupling and Decoupling Mean?

Capacitors may be the most component in a circuit. SMD or Plug-in capacitors are densely placed on PCB! The role of the capacitor is filtering, bypass, coupling, decoupling, storing energy and so on. " Let the AC signal pass and block the DC signal" is the most basic function of capacitor. Today Shenzhen PCBA manufacturing factory SysPCB are going to talk about the functions of "coupling" and "decoupling" that are difficult to understand. Let's talk about the functions of these two.

1. Coupling

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.

What is used here is capacitive coupling. In this way, the DC operating points of the front-end and back-end do not affect each other! If C2 and C3 are directly connected, this is called direct coupling. Adding a capacitor is capacitive coupling. That is, the input AC signal is sent to the post-amplifier circuit through the capacitor for amplification, and the DC signal is isolated.

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.


2. Decoupling

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 some reasons, at the load end, there will also be some majeure factors that cause voltage fluctuations. This voltage fluctuation will in turn affect the voltage of the power chip. This output capacitor decouples the voltage fluctuation coupled from 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.