Capacitors are among the most common and frequently used components in circuit design. They are one type of passive component. Simply put, active components are those that require an electrical power source, while passive components are those that do not. Capacitors also often play an important role in high-speed circuits.
Capacitors have a variety of functions and applications. For example, they are used for bypassing, decoupling, filtering, and energy storage; they are also used for oscillation, synchronization, and maintaining time constants…
Let's analyze this in detail below:
1. DC blocking: Its function is to prevent DC from passing through while allowing AC to pass through.
2. Bypass (decoupling): Provides a low-impedance path for certain parallel components in an AC circuit.
Bypass capacitor: Also known as a decoupling capacitor, a bypass capacitor is an energy storage device that provides energy to a specific component. It utilizes the frequency impedance characteristic of a capacitor (the impedance of an ideal capacitor decreases as frequency increases), acting like a pond to ensure a smooth output voltage and reduce load voltage fluctuations. Bypass capacitors should be placed as close as possible to the power supply and ground pins of the load component. This is an impedance requirement that requires careful attention when designing the PCB. Only by being close to a component can they suppress ground potential rise and noise caused by excessive voltage or other input signals. In short, they couple the AC component of the DC power supply to the power ground through the capacitor, effectively purifying the DC power supply. As shown in the figure, C1 is a bypass capacitor; it should be drawn as close as possible to IC1.
Figure C1
Decoupling capacitors: Decoupling capacitors filter out interference in the output signal. They act like batteries, using charging and discharging to prevent the amplified signal from being disturbed by sudden changes in current. Their capacitance depends on the signal frequency and the degree of ripple suppression. Essentially, decoupling capacitors function as a "battery," accommodating changes in the drive circuit current and preventing mutual coupling interference.
Bypass capacitors also serve as decoupling capacitors, but they generally refer to high-frequency bypass capacitors, providing a low-impedance path for high-frequency switching noise. High-frequency bypass capacitors are typically small, usually 0.1F or 0.01F depending on the resonant frequency; while decoupling capacitors are generally larger, possibly 10F or more, determined by the distributed parameters in the circuit and the magnitude of changes in the drive current. As shown in Figure C3, this is a decoupling capacitor.
Figure C3
The difference between them is that bypassing filters out interference in the input signal, while decoupling filters out interference in the output signal to prevent interference signals from returning to the power supply.
3. Coupling: As a connection between two circuits, it allows AC signals to pass through and be transmitted to the next stage of the circuit.
Using capacitors as coupling components is to transmit the signal from the previous stage to the next stage and to isolate the DC influence of the previous stage on the next stage, making circuit debugging simple and performance stable.
Without adding capacitors, the AC signal amplification will not change, but the operating points of each stage need to be redesigned. Due to the influence between the preceding and following stages, adjusting the operating points is very difficult, and it is almost impossible to achieve in multi-stage applications.
4. Filtering: This is very important for circuits; the capacitors behind the CPU are basically for this purpose.
That is, the higher the frequency f, the lower the impedance Z of the capacitor. At low frequencies, the useful signal can pass through the capacitor C smoothly because the impedance Z is relatively large; at high frequencies, the impedance Z of the capacitor C is very small, which is equivalent to short-circuiting the high-frequency noise to GND.
Filtering function: An ideal capacitor has a larger capacitance, lower impedance, and allows for higher frequencies to pass through. Electrolytic capacitors are generally larger than 1F, with a significant inductive component, so their impedance actually increases at higher frequencies. We often see a large electrolytic capacitor connected in parallel with a smaller capacitor. The larger capacitor passes low frequencies, and the smaller capacitor passes high frequencies, thus effectively filtering out both. The higher the frequency, the greater the capacitor's attenuation. A capacitor acts like a pond; a few drops of water are insufficient to cause a significant change. In other words, it buffers voltage fluctuations, as shown in Figure C2.
Figure C2
5. Temperature compensation: This compensates for the effects of insufficient temperature adaptability of other components, thereby improving the stability of the circuit.
Analysis: Since the capacitance of the timing capacitor determines the oscillation frequency of the horizontal oscillator, the capacitance must be highly stable and unaffected by changes in ambient humidity to ensure a stable oscillation frequency. Therefore, capacitors with positive and negative temperature coefficients are used for temperature complementarity.
As the operating temperature increases, the capacitance of C1 increases while the capacitance of C2 decreases. The total capacitance of the two capacitors connected in parallel is the sum of the capacitances of the two capacitors. Since one capacitance is increasing while the other is decreasing, the total capacitance remains essentially unchanged.
Similarly, as the temperature decreases, the capacitance of one capacitor decreases while that of the other increases, and the total capacitance remains basically unchanged, thus stabilizing the oscillation frequency and achieving the purpose of temperature compensation.
6. Timing: Capacitors and resistors are used together to determine the time constant of the circuit.
When the input signal transitions from low to high, it passes through buffer 1 and is then input to the RC circuit. Due to the charging characteristics of the capacitor, the signal at point B does not immediately follow the input signal; instead, it gradually increases. When it reaches a certain level, buffer 2 flips, resulting in a delayed low-to-high transition at the output.
Time constant: Taking a common RC series integrator circuit as an example, when an input signal voltage is applied to the input terminal, the voltage across the capacitor gradually increases. Meanwhile, the charging current decreases as the voltage increases. A resistor R and a capacitor C are connected in series to the input signal VI, and the output signal V0 is generated from capacitor C. When the value of RC(τ) and the width tW of the input square wave satisfy the condition: τ >> tW, this circuit is called an integrator circuit.
7. Tuning: Systematically tuning frequency-related circuits, such as mobile phones, radios, and televisions.
Tuning circuit of varactor diode
Because the resonant frequency of an lc-tuned oscillator circuit is a function of lc, we observe that the ratio of the maximum to the minimum resonant frequency of the oscillator circuit varies with the square root of the capacitance ratio. Here, the capacitance ratio refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Therefore, the circuit's tuning characteristic curve (bias voltage - resonant frequency) is essentially a parabola.
8. Rectification: Opening or closing a semi-closed conductor switching element at a predetermined time.
9. Energy storage: Store electrical energy for release when necessary.
For example, camera flashes, heating devices, etc. (Nowadays, the energy storage level of some capacitors is close to that of lithium batteries; the energy stored in one capacitor can power a mobile phone for a day.)
Energy Storage Function: Generally, electrolytic capacitors have an energy storage function. For capacitors specifically designed for energy storage, the energy storage mechanism is based on double-layer capacitance and Faraday capacitance, with supercapacitors being the primary form. Supercapacitors utilize the double-layer principle. When an external voltage is applied to the two plates of a supercapacitor, similar to a regular capacitor, the positive electrode stores positive charge, and the negative electrode stores negative charge. Under the influence of the electric field generated by the charges on the two plates of the supercapacitor, opposite charges are formed at the interface between the electrolyte and the electrodes to balance the internal electric field of the electrolyte. These positive and negative charges are arranged in opposite positions with extremely short gaps between them at the contact surface between the two different phases. This charge distribution layer is called the double layer, hence the very large capacitance.
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