What is the function and use of a capacitor?


  Capacitor is the most commonly used equipment in circuit design, and it is one of the passive components. Simply put, an active device is a device that needs an energy (electricity) source called an active device, and a device that does not require a power source is a passive device. Capacitors also often play an important role in high-speed circuits.

  In general, capacitors have many functions and uses. For example: in the role of bypass, decoupling, filtering, energy storage, etc.; in the role of oscillation, synchronization and time constant...

  Let's analyze it in detail:

  1. Isolate DC: The function is to prevent the passage of DC and allow the passage of AC.

  Bypass (decoupling): Provides a low-impedance path for some parallel components in an AC circuit.

  Bypass capacitor: bypass capacitor, also known as decoupling capacitor, is an energy storage device that provides energy for a certain device. It utilizes the frequency impedance characteristic of the capacitor (the frequency characteristic of the ideal capacitor decreases with the increase of the frequency), like a pond, which can uniformly output the output voltage and reduce the fluctuation of the load voltage. Bypass capacitors should be placed as close as possible to the power and ground pins of the load device, which is the impedance requirement. Special attention should be paid to the PCB, and the ground potential rise and noise caused by excessive voltage or other transmission signals can only be suppressed when it is close to a certain component. To put it bluntly, it is to couple the AC weight in the DC power supply to the power ground through the capacitor to purify the DC power supply. As shown in the figure, C1 is a bypass capacitor, and it should be as close as possible to IC1 when drawing.

  Decoupling capacitor: The decoupling capacitor takes the interference of the output signal as the filtering object. A decoupling capacitor is equivalent to a battery. Using its charge and discharge, the amplified signal will not be disturbed by the sudden change of current. Its capacity depends on the frequency of the signal and how well it suppresses ripple. 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 decoupling, but the bypass capacitor generally refers to a high-frequency bypass, that is, a low-impedance leakage method that improves high-frequency switching noise. The high-frequency bypass capacitor is generally relatively small, and generally adopts 0.1F, 0.01F, etc. according to the resonance frequency; while the capacity of the decoupling capacitor is generally large, maybe 10F or more, according to the distribution parameters in the circuit and the change of the driving current to make sure.

  Difference: bypass is to filter the interference in the input signal, and decoupling is to filter the interference of the output signal to prevent the interference signal from returning to the power supply.

  3. Coupling: As a connection between two circuits, it allows the AC signal to pass and be transmitted to the next circuit.

  Capacitors are used as coupling elements to transmit the signal from the previous stage to the subsequent stage, and isolate the influence of the direct current of the previous stage on the latter stage, so that the circuit debugging is simple and the performance is stable.

  If the capacitive AC signal is not amplified, it will not change, but the operating points of all levels need to be redesigned. Due to the influence of the front and back stages, it is very difficult to debug the working point, and it is almost impossible to realize it in multiple stages.

  4. Filtering: This is very important to the circuit, and the capacitors behind the CPU are basically like this.

  That is, the greater the frequency f, the smaller the impedance Z of the capacitor. At low frequencies, the capacitor C can pass useful signals because the impedance Z is very small, which is equivalent to short-circuiting the high-frequency noise GND.

  Filtering effect: the larger the capacitance, the smaller the impedance and the higher the passing frequency. The electrolytic capacitor generally exceeds 1F, and the inductance component in it is very large, so the impedance will increase when the frequency is high. We often see, sometimes we see an electrolytic capacitor with a large capacitance connected in parallel with a small capacitor. In fact, large capacitors pass low frequencies and small capacitors pass high frequencies, thereby adequately filtering high and low frequencies. The higher the frequency of the capacitor, the greater the attenuation. A capacitor is like a pond. A few drops of water are not enough to cause a big change, that is, the voltage can buffer when the voltage fluctuation is not large.

  5. Temperature compensation: Compensate the influence of other components on insufficient temperature adaptability, and improve circuit stability.

  Analysis: Since the capacity of the timing capacitor determines the oscillation frequency of the oscillator, it is required that the capacity of the timing capacitor is very stable and will not change with changes in the ambient humidity, thereby stabilizing the oscillation frequency of the oscillator. Therefore, capacitive release connections with positive and negative temperature coefficients are used to supplement the temperature.

  When the working temperature rises, the capacity of Cl is increasing, while the capacity of C2 is decreasing. The total capacity of the two capacitors in parallel is the sum of the two capacitors. Because one capacity is increasing and the other capacity is decreasing, the total capacity Basically unchanged.

  Similarly, when the temperature drops, the capacity of one capacitor decreases, the capacity of the other capacitor increases, and the total capacity remains basically unchanged, stabilizing the oscillation frequency and achieving the purpose of temperature compensation.

  6. Timing: Capacitors are used with resistors to determine the time constant of the circuit.

  When the input signal transitions from low to high, enter the RC circuit after buffering 1. The characteristic of capacitor charging makes the signal at point B not jump immediately with the input signal, but has a process of gradual expansion. When it gets large enough, buffer 2 flips, making a low-to-high latency jump on the output.

  Time constant: Take the common RC connected in series to form an integrating circuit as an example. When the input signal voltage is added to the input, the voltage across the capacitor gradually rises. As the voltage increases, the resistor R and the capacitor C are connected in series to the input signal VI, and the signal is output from the capacitor C to V0. When RC (τ) is input into the square wave, the value and the width tW satisfy: τ》》tW, this integrating circuit

  7. Tuning: The system tunes circuits related to frequency, such as mobile phones, radios, TVs, etc.

  diode tuning circuit pole tube

  Because the resonant frequency of an lc-tuned oscillatory circuit is a function of lc, we find that the ratio of the maximum to minimum resonant frequency of the oscillatory circuit varies with the square root of the capacitance ratio. Here, the capacitance ratio refers to the ratio of the capacitance when the reverse bias voltage is the smallest to the capacitance when the reverse bias voltage is the largest. Thus, the tuning characteristic curve (bias voltage-resonant frequency) of the circuit is basically a parabola.

  8. Rectification: Turn on or off the conductor switching element at a predetermined time.

  9. Energy storage: Release stored energy when necessary.

  Such as camera flashes, heating equipment, etc. (The energy storage level of some capacitors is now close to the level of lithium batteries, and the energy stored in a capacitor can be used for a mobile phone for a day.

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