The difference and application of filter capacitor, decoupling capacitor and bypass capacitor

There are many types of capacitors, but no matter how they are classified, the basic principle is to use capacitors to show low resistance to alternating signals. The higher the frequency f of the alternating current, the lower the impedance of the capacitor. The main function of the bypass capacitor is to provide a low-impedance path for the AC signal; the main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise to the ground. After adding decoupling capacitors, the voltage ripple interference will be significantly reduced; filter capacitors are often used in filter circuits. (read more about filter capacitors)

For an ideal capacitor, without considering the influence of parasitic inductance and resistance, there is no concern in the design of the capacitor. The larger the value of the capacitor, the better. But the actual situation is very different. It is not that the larger the capacitance, the better the high-speed circuit, but the small capacitance can be applied to high frequency.

The filter capacitor is used in the power rectifier circuit to filter out AC components and make the output DC smoother. The decoupling capacitor is used in the amplifying circuit where no AC is needed to eliminate self-excitation and make the amplifier work stably. The bypass capacitor is used when a resistor is connected, and it is connected to both ends of the resistor to allow the AC signal to pass smoothly.

1. Understanding of the energy storage effect of decoupling capacitors

(1) The decoupling capacitor mainly removes the interference of high frequency such as RF signal, and the way of interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of storing energy, which is second. You can think of the main power source as a reservoir. Every household in our building needs water. At this time, the water does not come directly from the reservoir. The distance is too far. When the water comes, we are already thirsty. The actual water comes from the water tower on the top of the building, which is actually a buffer. From a microscopic point of view, the current of a high-frequency device is discontinuous when it is working, and the frequency is very high, and there is a distance between the device VCC and the total power supply, even if the distance is not long, in the case of high frequency, the impedance Z =i*wL+R, the inductance of the line will also be very large, which will cause the device to be unable to be supplied in time when it needs current. The decoupling capacitor can make up for this deficiency. This is one of the reasons why many circuit boards place small capacitors at the VCC pin of high-frequency devices (a decoupling capacitor is usually connected in parallel to the Vcc pin, so that the AC component is grounded from this capacitor.

(2) The high-frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and to guide the noise to the ground.

2. The difference between bypass capacitor and decoupling capacitor

Decoupling: Remove the RF energy that enters the power distribution network from high-frequency devices during device switching. The decoupling capacitor can also provide a localized DC voltage source for the device, which is particularly useful in reducing the surge current across the board.

Bypass: Transfer unwanted common-mode RF energy from components or cables. This is mainly by generating an AC bypass to eliminate unintentional energy from entering the sensitive part. In addition, it can also provide baseband filtering (limited bandwidth).

We can often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: one is the energy storage capacitor of the integrated circuit; the other is to filter the high-frequency noise generated by the device and cut it off. The path for propagation through the power supply loop; the third is to prevent the noise carried by the power supply from interfering with the circuit.