A capacitor is composed of two metal electrodes with an insulating dielectric material sandwiched between them. When a voltage is applied across the two metal electrodes, charges are stored on the electrodes, making the capacitor an energy storage component. The capacitance is defined by the formula C=Q/U (C = capacitance (measured in Farads, F), Q is the charge on one of the plates (measured in C), and U is the difference between the two plates (measured in Volts, V).
Capacitor Function—Filtering
Filtering is one of the most important functions of a capacitor. Almost all power supply circuits use capacitors for this purpose. Theoretically (assuming the capacitor is purely capacitive), the larger the capacitance, the smaller the impedance, and the higher the frequency that can pass through. However, in reality, most capacitors over 1µF are electrolytic capacitors, which have a significant inductive component. As the frequency increases, the impedance can also increase.
Sometimes you'll see a large electrolytic capacitor paired in parallel with a small capacitor. In this case, the large capacitor allows low frequencies to pass, while the small capacitor allows high frequencies to pass. The function of a capacitor is to pass high frequencies and block low frequencies. The larger the capacitance, the easier it is for low frequencies to pass; the smaller the capacitance, the easier it is for high frequencies to pass. Specifically in filtering applications, large capacitors filter low frequencies, while small capacitors filter out high frequencies.
Some people liken a filter capacitor to a "pond." Because the voltage across a capacitor does not change suddenly, it can be compared to a pond, where the water level does not change significantly with the addition or evaporation of a few drops of water. Similarly, the capacitor converts changes in voltage into changes in current, thereby smoothing out fluctuations in the output voltage. The filtering process involves charging and discharging, which helps stabilize the output voltage.
Capacitor Function—Bypass
As shown in the circuit diagram, capacitor C3 is a bypass capacitor. Due to the capacitor's ability to block DC and pass AC, C3 does not allow the DC component to pass through. However, when an AC signal passes through, C3 acts almost as a short circuit for the AC component, so the AC signal does not pass through R4 but is bypassed directly by C3. Capacitors like this are called bypass capacitors. Let’s analyze the function of a bypass capacitor
The main function of a bypass capacitor is to create an AC shunt. When a signal containing both high and low frequencies is amplified, and we want only the low-frequency signal to enter the next stage, a capacitor of appropriate size is placed at the input of that stage and connected to the ground. This allows the higher frequency signals to easily pass through the capacitor and be bypassed (because the capacitor has low impedance to high frequencies), while the low-frequency signals, due to the capacitor’s higher impedance to them, are sent to the next stage for amplification.
The Role of Capacitors - Decoupling
"Decoupling," also known as "isolation," involves the use of capacitors to reduce interference in electronic circuits. As shown in the diagram, bypass capacitors are connected at the signal input, while decoupling capacitors are connected at the signal output. Both types of capacitors serve to mitigate interference, but they have distinct roles in a circuit.
Decoupling capacitors function like batteries, providing the necessary current variations to the driving circuit and preventing mutual coupling interference. To better understand this, we can consider the relationship between bypass and decoupling capacitors. Bypass capacitors also perform decoupling; however, they typically refer to high-frequency bypassing, offering a low-resistance discharge path for high-frequency signals. Bypass capacitors are usually smaller in size, while decoupling capacitors tend to be larger, depending on the circuit's distributed parameters and the magnitude of the driving current changes.
The primary distinction lies in their filtering targets: bypass capacitors filter out interference in the input signal, whereas decoupling capacitors filter out interference in the output signal to prevent noise from returning to the power supply. This is the essence of their difference.
The Role of Capacitors - Energy Storage
When we talk about energy storage, the first thing that comes to mind is a battery. However, capacitors store electric charge, which is a physical reaction, while batteries involve chemical reactions. Common examples of energy storage using capacitors include capacitor discharge machines and capacitor welding machines, which operate in high-voltage and high-current situations. When using capacitors for energy storage, we generally use large capacitors or a group of several small capacitors connected in parallel. The specific capacity and voltage rating should be chosen based on the requirements.
The function of the capacitor - Coupling
Capacitor coupling is also called "electric field coupling." Coupling refers to the process of transferring a signal from the first stage to the second stage, which generally refers to AC coupling when not otherwise specified.
From the perspective of a circuit, it can always be distinguished between the driving power source and the driven load. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitor to complete the signal transition. When the rising edge is relatively steep, the current is relatively large. In this way, the driving current will draw a large power supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, which will cause bounce), this current is essentially a type of noise compared to normal conditions, which will affect the normal operation of the preceding stage. This is what is known as coupling.
The function of the capacitor - Resonance
Resonance uses the changes between voltage and current generated by capacitors and other passive components, which essentially leverage the charging and discharging characteristics of the capacitor. There are generally parallel resonance and series resonance with capacitors, and through series and parallel combinations of resonant capacitors, filters such as notch filters can be formed for various engineering applications.
The function of the capacitor - Time Constant
The time constant refers to a constant that represents the time process of a transient response. It indicates the time required for a physical quantity to decay from its maximum value to 1/e of its maximum value. In capacitors, the time constant is commonly seen in RC circuits. When the input signal voltage is applied to the input terminal, the voltage across the capacitor (C) gradually increases. Meanwhile, the charging current decreases as the voltage rises. The characteristics of the current passing through the resistor (R) and the capacitor (C) are described by the following formula: i = (V/R)e-(t/CR)
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