Capacitors have three main functions:

      1. Energy storage: avoid the impact of power disturbance on load; "Emergency reserve" to avoid the fluctuation of power supply caused by load fluctuation (the load is "capacitive energy storage" nearby, and it is unnecessary to "ask the central government for instructions on everything" to request energy allocation)

      2. Filtering: What is filtering? Analogous examples in life: mask, filter element of water purifier, filter element of air purifier; The so-called filtering is to filter out clutter and retain the required useful signal.

      The principle of filtering: selective passing (selective passing of useful signals (a certain frequency band); Filter out useless noise and other useless frequency bands); Capacitance is sensitive to voltage fluctuation (i.e. AC) ("straight through AC"; noise and fluctuation are "AC", which can be passed through capacitance)

      3. Isolation: capacitors are connected in series in signal lines (audio input of audio circuit; signal input of serial communication circuit); Utilize the principle of capacitance "straight through AC" (only (useful) AC signals are allowed to pass through; Isolated DC signal (bias voltage that makes the circuit work normally)

Other functions of capacitor:

1. Three applications in power circuits:

(1) Energy storage: "golden section", maintaining stability

      Large energy demand: resist power input disturbance and restrain load fluctuation

      Major nodes: main power input circuit and branch circuit, main power output circuit and branch circuit

(2) Decoupling:

      Large capacity capacitors are "impure", such as 220uF electrolytic capacitors. When the input signal frequency is high, it is "inductive", especially for electrolytic capacitors. The "inductive" is particularly obvious at high frequencies;

      The large capacitance parallel small capacitance ceramic capacitor, such as 100nF, has a relatively higher frequency point of "inductance". It is used in parallel with the large capacitance to make up for its high frequency characteristics.

(3) Bypass: small capacitance near the chip power pin, such as 1nF

      Two functions:

      Meet the chip's current demand of "transient surge"

      Suppress the adverse effects of the fluctuation of the chip's transient current demand on other branches of the power supply.

      Take digital (logic gate circuit) as an example: to open the "gate", energy is required, and the bypass capacitance is nearby; Close the "door", release the energy, and absorb the bypass capacitor nearby; In a word, the so-called "bypass" capacitor, idle people and others walk away.

2. Power on reset circuit in digital circuit: (in series with resistor)

      Adjust the charging and discharging time of capacitor: delay the change of level; Provide sufficient power on reset time.

3. Phase compensation in negative feedback loop

4. The use of capacitors is complicated. Without capacitors, the circuit will lose its sense of security and be in a state of fear.