Coupling and decoupling function of capacitor

Capacitor is the commonly used component in a circuit. The role of the capacitor is filtering, bypass, coupling, decoupling, energy storage and so on. "Isolating DC and pass AC" is the most basic function of the capacitor. Today we are talking about two functions that are difficult to understand: "coupling" and "decoupling". Today, Shenzhen China PCBA manufacturer will talk about these two functions

First. Coupling

Capacitors have a coupling effect. Send the output signal of the previous circuit to the subsequent circuit, which is called coupling. It is a process of isolating DC and coupling AC. C2 and C3 in the picture below are coupling capacitors.

Capacitive coupling amplifier circuit

What is used here is capacitive coupling. In this way, the DC operating points of the front and rear circuit 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 rear 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 direction of the negative terminal of polar electrolytic capacitor connected to ground. 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 capacitor to filter out the noise of the input signal. R1, C1 form a resistance-capacitance filter circuit.

Second, Decoupling

What does decoupling mean? As mentioned earlier, coupling means that the signal is sent from the previous circuit to the subsequent circuit, which is called coupling. 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 the decoupling capacitor. The value of this capacitance depends on the size of the interference signal coupled from the subsequent load end.

Because of 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 supply chip. The 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 moment of high current to small current conversion, the power chip will not have time to respond. This decoupling capacitor works, and stabilizes the voltage fluctuations coupled from the load. The power chip is not affected.