What Is a Coupling Capacitor?

Coupling capacitance, also known as electric field coupling or electrostatic coupling, is a coupling method due to the presence of distributed capacitance. Coupling capacitors are used to couple and isolate the strong and weak current systems through capacitors, provide a high-frequency signal path, prevent low-frequency currents from entering the weak current system, and ensure personal safety. In addition to the above functions, the coupling capacitor with a voltage extraction device can also extract the power frequency voltage for protection and reclosing, and functions as a voltage transformer.

Coupling capacitance, also known as electric field coupling or electrostatic coupling, is a coupling method due to the presence of distributed capacitance. Coupling capacitors are used to couple and isolate the strong and weak current systems through capacitors, provide a high-frequency signal path, prevent low-frequency currents from entering the weak current system, and ensure personal safety. In addition to the above functions, the coupling capacitor with a voltage extraction device can also extract the power frequency voltage for protection and reclosing, and functions as a voltage transformer.

Decoupling refers to taking further filtering measures on the power supply to remove the influence of the interference between the signals between the two stages through the power supply. Coupling constant refers to the time constant corresponding to the product of the coupling capacitance value and the input impedance value of the second stage.

The interference signal generated by the interference source is controlled by a certain coupling channel.

The capacitor is connected to the AC circuit, the voltage of the circuit connected to one foot gradually increases, and the charge is gradually accumulated on the electrode plate. When the voltage of the circuit connected to the foot drops, the accumulated charge is returned to the circuit when the potential is high. The same is true at the other end. Capacitors are insulated. There is no current flowing through the entire capacitor, but the phenomenon of accumulation and discharge of charges as the potential rises and falls makes people mistakenly believe that a current is flowing. Therefore, it can isolate DC, while AC signal is coupled to increase and decrease the potential at both ends, and is transmitted to the following circuit components. Capacitors have the characteristics of blocking AC and DC. As a coupling capacitor, its function is to allow AC signals to pass normally, and to block the DC current of the amplifier circuit of the previous stage so that it will not affect the operating point of the amplifier circuit of the next stage. Why can a capacitor pass AC current and DC cannot flow? The two plates of the capacitor can store electric charges but do not form a loop. DC current can charge the capacitor, but when the voltage across the capacitor is the same as the power supply voltage, the circuit stabilizes, so no current flows; the positive half of the AC current gives the capacitor When charging, the capacitor is first discharged during the negative half cycle. Such constant charging and discharging is equivalent to the current flowing through the capacitor and forming a path.

The role of capacitive coupling is to pass the AC signal from the previous stage to the next stage. Coupling methods include direct coupling and transformer coupling. The direct coupling has the highest efficiency and the signal is not distorted. However, the adjustment of the two-stage working points before and after is more complicated and involves each other. In order to make the working point of the next level not affected by the previous level, it is necessary to separate the previous level and the next level in terms of DC. At the same time, the AC signal can be smoothly transmitted from the previous level to the next level. The method that can accomplish this task is to use capacitor transmission or transformer transmission. They can both pass AC signals and cut off DC, so that the working points of the front and back stages are not involved. But the difference is that when transmitting with a capacitor, the phase of the signal is delayed, and when transmitting with a transformer, the high-frequency component of the signal is lost. In general, a capacitor is often used as a coupling element for small signal transmission, and a transformer is often used as a coupling element for large signal or strong signal transmission.