In DC circuit, capacitor is equivalent to open circuit. Capacitor is a kind of component that can store electric charge, and it is also one of the most commonly used electronic components. This starts with the structure of the capacitor. The simplest capacitors consist of polar plates at both ends and an insulating dielectric (including air) in the middle. When energized, the plates are charged, creating a voltage (potential difference),but because of the insulating material in the middle, the whole capacitor is non-conductive. However, this case is under the precondition that the critical voltage (breakdown voltage) of the capacitor is not exceeded. As we know, any substance is relatively insulated. When the voltage across a substance increases to a certain extent, all substances can conduct electricity, which is called breakdown voltage. Capacitors are no exception. After capacitors are broken down, they are not insulators. However, in the middle school stage, such voltages are not seen in the circuit, so they all work below the breakdown voltage and can be regarded as insulators. However, in AC circuits, the direction of current changes as a function of time. The process of charging and discharging capacitors has time. At this time, the changing electric field is formed between the electrodes, and this electric field is also a function of changing with time. In fact, current passes between capacitors in the form of electric field.

Function of capacitor：

●Coupling: The capacitor used in coupling circuit is called coupling capacitor, which is widely used in resistance-capacitance coupling amplifier and other capacitive coupling circuits, and plays the role of isolating DC and passing AC.

●Filtering: Capacitors used in filter circuits are called filter capacitors, which are used in power filter and various filter circuits. Filter capacitors remove signals within a certain frequency band from the total signal.

●Decoupling: Capacitors used in decoupling circuits are called decoupling capacitors, which are used in DC voltage supply circuits of multistage amplifiers. Decoupling capacitors eliminate harmful low-frequency cross-connections between each stage amplifier.

●High frequency vibration elimination: The capacitor used in the high frequency vibration elimination circuit is called high frequency vibration elimination capacitor. In the audio negative feedback amplifier, in order to eliminate the high frequency self-excitation that may occur, this capacitor circuit is used to eliminate the high frequency howling that may occur in the amplifier.

●Resonance: Capacitors used in LC resonant circuits are called resonant capacitors, which are required in LC parallel and series resonant circuits

●Bypass: The capacitor used in the bypass circuit is called the bypass capacitor. If the signal in a certain frequency band needs to be removed from the signal in the circuit, the bypass capacitor circuit can be used. According to the frequency of the removed signal, there are full frequency domain (all AC signals) bypass capacitor circuit and high frequency bypass capacitor circuit

  ●Neutralization: Capacitors used in neutralization circuits are called neutralization capacitors. In radio high frequency and intermediate frequency amplifiers and television high frequency amplifiers, this neutralization capacitor circuit is used to eliminate self-excitation.

●Timing: Capacitors used in timing circuits are called timing capacitors. Timing capacitor circuit is used in the circuit that needs to control the time by charging and discharging capacitors, and capacitors play the role of controlling the time constant.

●Integration: Capacitors used in integration circuits are called integration capacitors. In the synchronous separation circuit of electric potential field scanning, the field synchronous signal can be extracted from the field compound synchronous signal by using this integral capacitor circuit

●Differential: Capacitors used in differential circuits are called differential capacitors. In order to obtain the spike trigger signal in the flip-flop circuit, the differential capacitor circuit is used to obtain the spike pulse trigger signal from various signals (mainly rectangular pulse).

●Compensation: The capacitor used in the compensation circuit is called compensation capacitor. In the bass compensation circuit of the card holder, this low-frequency compensation capacitor circuit is used to improve the low-frequency signal in the playback signal. In addition, there is a high frequency compensation capacitor circuit.

●Bootstrap: The capacitor used in the bootstrap circuit is called bootstrap capacitor, which is commonly used in the output stage circuit of OTL power amplifier to increase the positive half-cycle amplitude of the signal by positive feedback.

●Frequency division: The capacitor in the frequency division circuit is called frequency division capacitor. In the loudspeaker frequency division circuit of the sound box, the frequency division capacitor circuit is used to make the high-frequency loudspeaker work in the high-frequency band, the medium-frequency loudspeaker work in the medium-frequency band and the low-frequency loudspeaker work in the low-frequency band.

●Load capacitance: refers to the effective external capacitance that determines the resonant frequency of the load together with the quartz crystal resonator. Common standard values for load capacitors are 16pF, 20pF, 30pF, 50pF, and 100pF. The load capacitance can be adjusted according to the specific situation, and the working frequency of the resonator can be adjusted to the nominal value by adjusting it.

At present, the film capacitor industry is entering a period of stable development from a
period of rapid growth, and the new and old kinetic energy of the industry is in the
transition stage.