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How to distinguish between the filter capacitor, decoupling capacitor, a bypass capacitor
Source:Original | Author:Calio Huang of C&T RF Antennas Inc | Publish time: 2019-08-02 | 195 Views | Share:

The types of capacitors are complicated, but no matter how they are classified, the basic principle is to use capacitors to make the alternating signal low impedance. 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 to 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 direct the noise to ground. The ripple interference of the voltage after the addition of the decoupling capacitor is significantly reduced; the filter capacitor is often used in the filter circuit.
For an ideal capacitor, regardless of the effects of parasitic inductance and resistance, there is no concern about the capacitor design. The larger the value of the capacitor, the better. However, the actual situation is quite different. It is not that the larger the capacitor, the better the high-speed circuit, but the small capacitor can be applied to the high frequency.
The filter capacitor is used in the power rectifier circuit to filter out the AC components and make the output DC smoother. The decoupling capacitor is used in the amplifier circuit where no AC is needed to eliminate self-excitation and stabilize the amplifier. The bypass capacitor is used when there is a resistor connection and is connected to both ends of the resistor to make the AC signal pass smoothly.
1. Understanding the decoupling capacitor energy storage
(1) The decoupling capacitor is mainly to remove the interference of high frequency such as RF signal, and the way of entering the interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of energy storage, which is the second. You can think of the total power supply as a reservoir. Every household in our building needs water. At this time, the water is not directly from the reservoir. It is too far away. When the water comes, we are already thirsty. The actual water is from the water tower on the top of the building. The water tower is actually a buffer. If microscopically, when the high-frequency device is working, its current is discontinuous and the frequency is high, and the device VCC has a distance to the total power supply, even if the distance is not long, at a 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 supplied in time when current is needed. Decoupling capacitors can make up for this deficiency. This is one of the reasons why many boards place small capacitors on the VCC pin of the high-frequency device (a decoupling capacitor is usually connected in parallel with the Vcc pin so that the AC component is grounded from this capacitor.