The concept and application of bypass capacitors

Capacitors that can bypass high-frequency components in alternating current mixed with high-frequency currents and low-frequency currents are called "bypass capacitors."

For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object, filtering out the high-frequency clutter carried by the previous stage, and decoupling (decoupling, also known as decoupling) The capacitor is to filter out the interference of the output signal.

Definition: A capacitor that can bypass and filter the high-frequency components of an AC signal mixed with high-frequency current and low-frequency current is called a "bypass capacitor".　　

The main function of the bypass capacitor is to generate an AC shunt to eliminate unnecessary energy that enters the susceptible area. That is, when the signal mixed with high frequency and low frequency is amplified by the amplifier, it is only allowed to pass through a certain stage. Low-frequency signals are input to the next stage, and high-frequency signals are not required to enter, so add an appropriate size grounding capacitor at the input of this stage, so that higher-frequency signals can easily be bypassed through this capacitor (this is because The capacitor has a small impedance to high frequency), and the low frequency signal is transmitted to the next stage of amplification due to the large impedance of the capacitor to it. For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object, and filters the high-frequency clutter carried by the previous stage, while the decoupling (also called decoupling) capacitor is the output signal The interference of the filter is the object of filtering.　　

Bypass capacitor is not a theoretical concept, but a practical method that is often used. Electron tubes or transistors need to be biased, which is the DC power supply condition that determines the operating point. For example, the grid of an 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 plate current is used to form the positive potential of the cathode to the ground, and the grid is DC grounded. This biasing technique is called "self-biasing", but for (AC) signals, it is also a negative feedback. In order to eliminate this effect, a sufficiently large capacitor is connected in parallel with this resistor, which is called bypass capacitance. Generally speaking, uf-class capacitors, like electrolytic capacitors or tantalum capacitors, have larger inductance and smaller resonant frequency, which can pass low-frequency signals better, but show strong inductance for high-frequency signals. The impedance is large, and at the same time, the large capacitor can also act as a local charge pool, which can reduce local interference through the power supply coupling; the capacitance of 0.001~0.1uf capacitors, generally ceramic capacitors or mica capacitors, small inductance, resonance frequency High, low impedance to high-frequency signals, can provide a bypass for high-frequency interference signals, reduce the external coupling interference to this part.

Bypass is to filter out high-frequency clutter or signals carried by the pre-stage or power supply; decoupling is a "small pond" set up to ensure stable output at the output end (mainly for the operation of the device), and work in other high currents Ensure that the fluctuation range of the power supply will not affect the operation of the circuit; one additional point is the so-called coupling: it is the high frequency generated by the active components of the active devices that transmit signals between the front and rear stages without affecting each other's static operating points. The switching noise 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. From the circuit, there is always a source of driving and a load being driven. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with normal conditions, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is coupling.

The decoupling capacitor acts as a battery to meet the change of the drive circuit current and avoid mutual coupling interference. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to high-frequency bypass, that is, to improve a low-impedance bleeder way for high-frequency switching noise. High-frequency bypass capacitors are generally small. According to the resonant frequency, they are generally 0.1u, 0.01u, etc., while decoupling capacitors are generally larger, 10u or greater, depending on the distribution parameters in the circuit and the magnitude of the drive current change.