Bypass capacitors, decoupling capacitors, filter capacitors, etc., no matter what they are called, their principles are the same, that is, they use the characteristics of low impedance to AC signals, which can be seen from the equivalent impedance formula of capacitors: Xc= 1/2лfC , the higher the operating frequency, the larger the capacitance value, the smaller the impedance of the capacitor.
In the circuit, if the main function of the capacitor is to provide a low-impedance path for the AC signal, it is called a bypass capacitor ;
If the main purpose is to increase the AC coupling between the power supply and the ground and reduce the influence of the AC signal on the power supply, it can be called a decoupling capacitor ; the decoupling capacitor acts as a battery to meet the current changes of the drive circuit and avoid mutual Coupling interference.
If it is used in a filter circuit, it can also be called a filter capacitor ; in addition, for DC voltage, the capacitor can also be used as a circuit energy storage, using the charge and discharge to play the role of a battery.
In electronic circuits, both decoupling capacitors and bypass capacitors play an anti-interference role. The location of the capacitor is different, and the name is different. For the same circuit, the bypass capacitor is used to filter the high-frequency noise in the input signal, and the high-frequency clutter carried by the front stage is filtered, and the decoupling capacitor is also called decoupling. Capacitance is to filter out the interference of the output signal. Decoupling capacitors are used in amplifier circuits where AC is not required to eliminate self-excitation and make the amplifier work stably. From a circuit, there is always a source to drive and a load to be driven. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitor to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so the driving current will absorb a large power supply current. Inductance and resistance (especially the inductance on the chip pins, which will cause rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the front stage. This is coupling.
The decoupling capacitor acts as a battery to meet the change of the current of the drive circuit and avoid mutual coupling interference.
Both decoupling and bypassing can be thought of as filtering. The decoupling capacitor is equivalent to a battery to avoid voltage drop due to sudden changes in current, which is equivalent to filtering ripples. The specific capacitance value can be calculated according to the size of the current, the expected ripple size, and the size of the action time. Decoupling capacitors are generally very large and are basically ineffective for higher frequency noise. The bypass capacitor is for high frequency, that is, the frequency impedance characteristic of the capacitor is used. Capacitors can generally be seen as an RLC series model. At a certain frequency, resonance occurs and the impedance of the capacitor is equal to its ESR . If you look at the frequency impedance curve of the capacitor, you will find that it is generally a V -shaped curve. The specific curve is related to the medium of the capacitor, so the medium of the capacitor should also be considered when choosing a bypass capacitor. A safer method is to add several capacitors.
The decoupling capacitor is mainly to remove 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 supply as Miyun Reservoir, every household in our building needs water supply,
At this time, the water does not come directly from the reservoir, so the distance is too far,
By the time the water came, we were already thirsty.
The actual water comes from the water tower on the top of the building , and the water tower is actually a buffer .
From a microscopic point of view, when a high-frequency device is working, its current is discontinuous and the frequency is very high.
The device VCC has a certain distance from the total power supply, even if the distance is not long, in the case of high frequency,
Impedance Z=i*wL+R , the influence of the inductance of the line will be very large,
As a result, the device cannot be supplied in time when current is required.
Decoupling capacitors can make up for this deficiency.
This is one of the reasons why many boards place small capacitors at the VCC pins of high frequency devices
( A decoupling capacitor is usually placed in parallel with the vcc pin so that the AC component is grounded from this capacitor.)
High-frequency switching noise from active devices switching will propagate along the power lines. The main function of the decoupling capacitor is to provide a local DC power supply to the active devices to reduce the propagation of switching noise on the board and to guide the noise to the ground. The decoupling capacitor has two functions between the integrated circuit power supply and the ground: On the one hand, it is the energy storage capacitor of the integrated circuit, and on the other hand, it bypasses the high-frequency noise of the device. A typical decoupling capacitor value in a digital circuit is determined by the 0.1μF cloth parameter and the change in drive current.
the difference
The bypass capacitor is not a theoretical concept, but a practical method that is often used. The valve or transistor needs to be biased, which is the DC power supply condition that determines the operating point. For example, the grid of the electron tube often requires a negative voltage relative to the cathode. In order to work under a DC power supply, a resistor is connected in series between the cathode and the ground, and the positive potential of the cathode to the ground is formed by the plate flow, and the grid is DC grounded. This bias technique is called "self-bias", but for the (AC) signal, it is also a negative feedback. In order to eliminate this effect, a large enough capacitor is connected in parallel with this resistor, which is called bypass capacitance. Generally speaking, capacitors with a capacity of uf level, such as electrolytic capacitors or tantalum capacitors, have large inductance and small resonant frequency, so they can pass low-frequency signals better, while high-frequency signals show strong inductance. The impedance is large, and at the same time, the large capacitor can also play the role of a local charge battery, which can reduce local interference and couple out through the power supply; capacitors with a capacity of 0.001~0.1uf are generally ceramic capacitors or mica capacitors, with small inductance and high resonant frequency. High, the impedance to high-frequency signals is small, which can provide a bypass for high-frequency interference signals to reduce external coupling interference to the local area. The bypass is to filter out high-frequency clutter or signals carried by the front stage or power supply;
Decoupling is a "small pond" set up to ensure the stable output of the output (mainly for the work of the device), and to ensure that the fluctuation range of the power supply will not affect the work of the circuit when other large currents are working; a supplement is the so-called Coupling: It is a component that transmits signals between the front and rear stages without affecting each other at the static operating point of each level. 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 devices to reduce the propagation of switching noise on the board and to direct the noise to ground. From a circuit, there is always a source to drive and a load to be driven. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitor to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so the driving current will absorb a large power supply current. Inductance and resistance (especially the inductance on the chip pins, which will cause rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the front stage. This is coupling. The decoupling capacitor acts as a battery to meet the change of the current of the drive circuit and avoid mutual coupling interference. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to the high-frequency bypass, which is to improve a low-impedance leakage prevention way for the high-frequency switching noise. The high-frequency bypass capacitor is generally small, according to the resonant frequency, it is generally 0.1u , 0.01u , etc., while the decoupling capacitor is generally large, 10u or more, which is determined according to the distribution parameters in the circuit and the change of the driving current.