# How much do you know about the basic common sense of capacitors in circuits?

When used as a battery power supply, it has the function of an AC path, which is equivalent to short-circuiting the battery’s AC signal, avoiding the increase in battery internal resistance due to the battery voltage drop, and the parasitic oscillation of the circuit.

1. The positive and negative ends of the voltage source are connected with a capacitor in parallel with the circuit. When used in a rectifier circuit, it has a good filtering effect. When the voltage alternates, due to the charging effect of the capacitor, the voltage at both ends cannot change suddenly, which ensures the stability of the voltage.

When used as a battery power supply, it has the function of an AC path, which is equivalent to short-circuiting the battery’s AC signal, avoiding the increase in battery internal resistance due to the battery voltage drop, and the parasitic oscillation of the circuit.

2. For example, in what kind of circuit, a series or parallel capacitor can achieve the coupling effect, and what is the difference between a non-discharged capacitor and a discharge capacitor?

In an AC multi-stage amplifying circuit, the DC operating bias values ​​of each stage are different due to the different gains and powers of each stage. If the stages are directly coupled, it will make the working biases of all levels to be mixed and unable to work normally. The use of the capacitor’s pass-through and direct-current blocking characteristics not only solves the coupling of inter-stage exchanges, but also isolates the inter-stage partial value intermixing, achieving two goals with one stone.

3. The two coupling capacitors in the basic amplifying circuit, the capacitor + pole and the DC + pole are connected, which play a role in the communication and isolation. , Why do you want to pick it up! If the connection is reversed, the electrolytic capacitor will leak, which will change the DC operating point of the circuit and make the amplifying circuit abnormal or unable to work.

4. What is the role of the capacitor in the resistance-capacitance coupling amplifier circuit? Isolate the DC signal so that the static operating points of adjacent amplifying circuits are independent of each other and do not affect each other.

5. Does the analog circuit amplifier do not need coupling capacitors, and can still amplify? The amplifier in the book adds a coupling capacitor between the transformer secondary coil and the triode. The explanation is that the output of the previous stage is changed to the input of the next stage through the AC resistance to the DC, so that the front and rear stages do not affect it. The first stage is alternating current, and the latter stage is also alternating current. How can they affect each other? I really can’t figure out that adding a capacitor is not a superfluous act.

You made a mistake. The former stage is indeed alternating current, but the latter stage is alternating current superimposed on direct current. The transistor needs a DC bias. If there is no capacitor to block the direct current, the transformer’s coil will bias the transistor’s DC bias to the bypass. Because the inductance is direct current.

6. The basic amplifier circuit coupling capacitor, can the coupling capacitor be non-polar? In the basic amplifying circuit, the coupling capacitor depends on the video frequency. When the frequency is high, a non-polar capacitor is needed. It is characterized by relatively stable, withstand voltage can be made relatively high, relatively small in size, but not large in capacity. Its biggest use is to cut off direct current through alternating current. It is widely used in high-frequency alternating current paths, bypass, resonance and other circuits. It is simply understood as high-frequency paths.

When the frequency is low, the non-polar capacitor has a relatively high capacitance due to its low capacity, so it is necessary to use a polar electrolytic capacitor. Because of the electrolyte inside, the capacity can be made large, allowing low-frequency alternating current to pass through. Cut off direct current. However, due to the organic medium between the internal two poles, the withstand voltage is limited. It is mostly used in circuits such as low-frequency AC paths, filtering, decoupling, and bypass, and is simply understood as low-frequency paths.

7. What role does the coupling capacitor play? In the amplifying circuit, the use of the coupling capacitor to pass the DC blocking function allows the high-frequency AC signal to pass through the circuit smoothly and be amplified one by one, while the DC quantity is blocked in each stage.

8. In a battery-powered circuit, why does the capacitor charge and discharge to delay the function? Capacitors accumulate electric charge. You can think of it as a water cup. Charging and discharging means charging and discharging water. During the charging process, the voltage rises slowly; when discharging, you only need to detect the voltage across the capacitor to achieve a delay. .

For example, at the beginning of charging, the voltage across the capacitor is zero. As the charging time increases, the voltage gradually rises to the voltage you set to control the switching of the circuit. Of course, the discharge can also be used in reverse. The delay time is related to the capacity of the capacitor, the leakage of the capacitor, the charging resistance and the voltage, and sometimes the load resistance is also taken into consideration.

9. Resistance-capacitance coupling is to use the capacitor’s pass-through and DC-blocking characteristics to prevent the DC component between the front and back stages from causing crosstalk and causing instability of the operating point.

10. The resistance-capacitance coupling amplifier circuit can only amplify AC signals, not DC signals, right or wrong? Yes, a capacitor is an Electronic component that blocks DC and AC. Therefore, the resistance-capacitance coupling amplifier circuit can only amplify the AC signal, and the direct coupling amplifier circuit is used to amplify the DC signal.

11. How to distinguish the coupling capacitor and the bypass capacitor in the amplifying circuit? The negative pole of the coupling capacitor is not grounded, but is connected to the input terminal of the next stage, and the negative pole of the bypass capacitor is grounded.

12. How to choose capacitive coupling for the multi-stage AC amplifier circuit of the op amp? In fact, it is very simple, ordinary ceramic capacitors can be done! For good results, you can choose tantalum capacitors. According to the frequency range of your input signal, you can choose capacitors with capacitance values ​​of 103 and 104 for high frequency. For lower frequency AC signals, you can choose electrolytic capacitors of about 22uF.

13. The amplifying circuit adopts direct coupling, and the feedback network is a pure resistance network. Why is the circuit only possible to produce high-frequency oscillation? Oscillation comes from the phase shift of the closed loop reaching 180 degrees and the loop gain at this time is greater than zero. The use of a pure resistance network as a feedback network will definitely not introduce phase shift, so all the phase shift comes from the amplifier’s opening Ring circuit.

Using a direct-coupled open-loop amplifier, there will be no capacitive elements between stages that will cause a phase shift. What can cause a phase shift is the capacitance inside the transistor or MOS tube. These capacitances are all fF, the maximum pF capacitance. The resonant frequency of the circuit composed of these capacitors and the equivalent resistance of the circuit is quite high. Therefore, the amplifier adopts direct coupling, and the feedback network is a pure resistance network, which can only produce high-frequency oscillation.

14. The bandwidth of the resistance-capacitance coupling amplifier circuit refers to (the difference between the upper limit cut-off frequency and the lower limit cut-off frequency) the upper cut-off frequency of the resistance-capacitance coupling amplifier circuit refers to (as the frequency increases, the amplification factor decreases to 0.707 times, That is, the frequency at -3dB) The lower cut-off frequency of the resistance-capacitance coupling amplifier circuit refers to (as the frequency decreases, the amplification factor drops to 0.707 times the original, that is, the frequency at -3dB).

The upper cut-off frequency of the RC coupling amplifier circuit is mainly affected by the (transistor junction capacitance, the distributed capacitance of the circuit), and the lower cut-off frequency of the RC coupling amplifier circuit is mainly affected by the capacitor (blocking capacitor and bypass).

15. In the multi-stage amplifier circuit, how does the electrolytic capacitor couple to the next stage? Isn’t the characteristic in the capacitor blocking the DC? How is it transmitted? And because the capacitor needs to be connected through the collector of the transistor, why can’t the emitter be connected?

Electrolytic capacitors work in AC amplifiers, and the direction of AC current changes periodically, can the transistor be turned on normally? And isn’t the collector of the NPN transistor from C to B? How does its current flow to the base of the next-stage transistor?

The amplifiers that use electrolytic capacitors as coupling are all AC amplifiers. The electrolytic capacitors are used here as “passing and blocking”. Which pole of the transistor outputs is a matter of circuit form. Both have.

16. How to estimate the output resistance of the first-stage amplifier and the input resistance of the second-stage amplifier? The input resistance of the second-stage amplifier is the output resistance of the first-stage amplifier.

When the amplitude of the signal source is too large, what will happen to the output of the two-stage amplifier? distortion.

Shake the input end of the amplifier with your hand and observe the output end of the amplifier to see if something appears? what is the reason? Clutter, human body induction.

17. Capacitors are charging and discharging. Then how to use the charge and discharge of the capacitor to understand filtering, decoupling, bypass… Capacitor blocking DC and AC, DC blocking is easy to understand, and AC is not easy to understand. As long as you understand AC, you can understand filtering, decoupling and bypass.

The capacitor is charging and discharging, which is good, but the direction of the alternating current alternates between forward and reverse, and the amplitude of the amplitude also changes periodically. The entire image of the change is a sinusoidal curve. The capacitor is connected to the AC circuit, and due to the periodic change of the AC voltage, it is also periodically changing in charge and discharge. There are charging and discharging currents in the circuit. This charging and discharging current has the same shape as the voltage, except that the phase is 90 degrees ahead of the voltage, which is equivalent to the AC passing through the capacitor.

It is different from alternating current passing through a resistor. When alternating current passes through a resistor, it consumes electrical energy (heating) on ​​the resistor. However, the capacitor only exchanges energy with the power supply. The power supply sends energy to the capacitor when charging, and the capacitor returns the electrical energy to the power supply when discharging. Therefore, the power generated by multiplying the voltage by the current is called reactive power.

What needs to be clear is that when the capacitor is connected to an AC circuit, the flowing electrons (current) do not really rush through the insulating layer, but generate current in the circuit. This is because in the circuit, reverse discharge and forward charge are in the same direction. The forward discharge and reverse charging are in the same direction, just like a relay race. One team runs the positive half-cycle of the AC, and the other team takes the baton and continues to run the negative half-cycle of the AC.

Understand that the capacitor is connected to AC, then the AC component is bypassed to the ground and the filtering is completed.

18. How to use bypass capacitor, filter capacitor and decoupling capacitor respectively? Can cite some examples. These three types of capacitors are actually used for filtering, but they are used in different circuits, and their names and usages are different.

Filter capacitor: This is the capacitor we usually use after the power supply is rectified. It is a capacitor that rectifies the rectifier circuit AC into a pulsating DC and smoothes it through charging and discharging. This type of capacitor is generally an electrolytic capacitor with a large capacity. Law level.

Bypass capacitor: It filters out the high-frequency components in the input signal. It is mainly used to filter out high-frequency clutter. Usually, ceramic capacitors and polyester capacitors are used. The capacity is small, in the picofarad level.

Decoupling capacitor: It takes the interference of the output signal as the filtering object. The decoupling capacitor is equivalent to the battery, and uses its charge and discharge so that the amplified signal will not be interfered by the sudden change of the current. Its capacity depends on the frequency of the signal and the degree of ripple suppression.

19. What are coupling capacitors and decoupling capacitors, and what are their characteristics and functions? Coupling capacitors transmit AC signals and are connected to the line. Decoupling capacitors remove unnecessary AC signals. One section is connected to the line and one end is grounded.

20. Regarding the functions of capacitors, under what circumstances is capacitive coupling and under what circumstances is capacitive filtering? In the final analysis, the eighteen martial arts of the capacitor in the circuit are two: charging charge and discharging charge. Its characteristic is to pass AC and block DC. After the alternating voltage is applied to the two ends of the capacitor, it will continue to charge and discharge with the alternating frequency of the current. At this time, the alternating current of the same frequency passes through the circuit, which is the pass characteristic of the capacitor.

In the case of the right frequency, the capacitor can be regarded as a path to the circuit. The AC output of the previous stage can be transmitted to the subsequent circuit through the capacitor. For DC, it is isolated. Because when the voltage at both ends is charged to be equal to the circuit voltage, there will be no more charging current. When acting on the transmission of the front and rear AC signals, it is coupling. When it acts to filter out fluctuation components and useless AC components, it is filtering.

21. Everyone knows that the capacitor filter of the rectifier circuit uses its charge and discharge; but sometimes the filter uses the capacitor to have a different capacitive reactance for the non-pass frequency signal, such as a bypass capacitor, so which angle is used when analyzing the capacitor filter?

In fact, no matter what kind of statement is the same, the theory of using charge and discharge is more general, and the theory of using capacitive reactance is more in-depth. The role of capacitor is to use its charge and discharge characteristics, depending on what components you want to filter out. A large capacitor is used to filter low frequencies, and a small capacitor is used to filter high frequencies. In theory, the filtering in the low-frequency rectifier circuit and the bypass in the high-frequency are the same, and the difference in capacitive reactance is used.

22. How does the capacitor realize the functions of charging and discharging, rectification, and filtering? The charging, discharging, rectification and filtering of the capacitor, and even its phase shifting, reactance and other functions, are all the storage functions of the capacitor at work.

The reason why the capacitor can store charge is realized by the strong mutual attraction between positive and negative charges. When charging a capacitor, people introduce positive charges into the positive plate through the power supply, and negative charges into the negative plate of the capacitor. But the positive and negative charges can’t get together. This is because there is a layer of insulating mold blocking them. The larger and thinner the mold, the greater the attractive force and the more stored charge. The positive and negative charges are attracted between the ten plates, but if you provide it with an external circuit, they will be able to combine with each other through this external circuit, that is, discharge. After all, they are one high and one low. Visually speaking, the capacitor is like a reservoir. It can graphically illustrate the role of its rectifying wave.

23. After the filter capacitor is fully charged, then discharge the back circuit and then in the charge and discharge cycle? Is the Zener diode breakdown or non-breakdown?

In fact, what you said is quite right. It is such a working process in the circuit, but it is related to the frequency of the signal. First of all, it depends on what you want to put the capacitor in the circuit. When it is used as a filter, it must The frequency signal is filtered to the ground, such as the capacitor at the front end of the chip power supply, and some are decoupling. The phenomenon you said is like the filter capacitor before the voltage regulator is turned off and the filter capacitor of the switching power supply output.

Regarding the Zener tube, let me give you an example. If there is a 5V Zener tube, when the voltage is less than 5V, the voltage is equal to its own voltage. When the voltage is higher than 5V, the Zener tube will stabilize the voltage to 5V. The excess voltage has broken the Zener tube up the channel.

24. What is the specific meaning of capacitive coupling? Is it different from filtering? Coupling refers to the process of signal transmission from the first stage to the second stage, and usually refers to AC coupling when it is not specified. Decoupling refers to taking further filtering measures to the power supply to remove the influence of mutual interference between the two levels of signals through the power supply. The coupling constant refers to the time constant corresponding to the product of the coupling capacitance value and the second-stage input impedance value.

Decoupling has three purposes:

Remove the high-frequency ripple in the power supply, and cut off the high-frequency signal of the multi-stage amplifier through the crosstalk path of the power supply;
When working with a large signal, the circuit’s demand for power will increase, causing power fluctuations. Decoupling reduces the impact of power fluctuations on the input stage/high voltage gain stage when the signal is large;
Form a floating ground or floating power supply, and complete the coordination of various parts of the ground or power supply in a complex system.

The high frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise arrived.

25. Capacitors are mainly used in AC circuits and pulse circuits. In DC circuits, capacitors generally function to block direct current.
The capacitor neither generates nor consumes energy, and is an energy storage element.

Capacitors are important devices to improve power factor in power systems; in electronic circuits, they are the main components that obtain functions such as oscillation, filtering, phase shift, bypass, and coupling.

Because the loads used in industry are mainly motor inductive loads, the capacitive load must be paralleled to balance the power grid.

On the grounding line, why some have to pass a capacitor before being grounded? In the DC circuit, it is anti-interference. The interference pulse is grounded through the capacitor. This time, the role is to block the direct current-the potential relationship in the circuit; in the AC circuit, there is also the grounding through the capacitor. Generally, the capacity is small, and it is also anti-interference and anti-interference. Potential isolation effect.

For inductance circuits with coils such as motors and transformers, because the current through the inductance cannot change suddenly, it is the opposite of capacitors. A voltage needs to be established at both ends of the coil before there is current. When there is no resistance and capacitance in the inductance current loop, Called pure inductance circuit, the current of pure inductance circuit lags the voltage by 90 degrees. Since power is voltage multiplied by current, when voltage and current are different, for example: when the voltage on the capacitor is the maximum, the charge is full and the current is 0; when there is voltage on the Inductor first, the inductor current is also 0, so, The product (power) obtained is also 0! This is reactive power. Then, the relationship between the voltage and current of the capacitor is just the opposite of the relationship between the voltage and current of the inductor, and the capacitor is used to compensate the reactive power generated by the inductor. This is the principle of reactive power compensation.

26. How does the capacitor play a filtering role in the circuit? The capacitor is open circuit, is it charging the capacitor when the alternating current passes through it? Are the capacitors in parallel or in series?

The capacitive reactance of the capacitor changes with the frequency of the alternating current applied at both ends, Z=1/2*3.14*FC, according to which frequency current needs to be filtered, set different capacitance values. In this way, the unnecessary current can be led to the ground, and the filtering is completed. For the current of the required frequency, the capacitor has a path or the impedance is small. When the alternating current passes through, it is a process of repeated charging and discharging.

27. What are the functions of decoupling capacitors, filter capacitors, and bypass capacitors? What is the difference and connection between them?

For example, the emitter of a transistor amplifier has a self-biased resistor, which at the same time causes the signal to generate a voltage drop and feed it back to the input to form an input and output signal coupling. This resistor is the element that produces the coupling. If a capacitor is connected in parallel across the resistor, Since a capacitor with an appropriate capacity has a small impedance to the AC signal, which reduces the coupling effect generated by the resistance, this capacitor is called a decoupling capacitor.

Electron tubes or transistors need to be biased, which is the DC power supply condition that determines the operating point. For example, the gate of an electron tube often requires a negative voltage relative to the cathode. In order to work under a DC power supply, a resistor is connected in series between the cathode and the ground. , The use of plate current to form the positive potential of the cathode to the ground, while the grid is DC grounded, this bias technique is called “self-biasing”, but for (AC) signals, this is also a negative feedback, in order to eliminate this effect , Just connect a sufficiently large point capacitance in parallel with this resistor, which is called a bypass capacitor. Later, some materials extended it to similar situations.

The filter capacitor is better understood. The capacitor has the effect of passing AC and blocking DC. Filtering means that I can filter out the AC signal of a certain frequency by selecting different filter capacitors, leaving the desired frequency signal.

28. Is the coupling capacitor a decoupling capacitor? It is completely different. The coupling capacitor is for signal transmission, and the decoupling capacitor is for reducing interference.

29. What is the principle of capacitive decoupling? Intrusion into the AC signal from the DC circuit or the self-excited feedback of the AC amplifier circuit will produce undesirable consequences. In order to prevent the AC component from coupling and amplifying step by step, a capacitor is set between the stages to make it flow back to the ground. This capacitor is a decoupling capacitor.

30. What is the difference between coupling and decoupling? What are the roles of coupling capacitors and decoupling capacitors, how to place them in the circuit, and what are the principles?

The function of the coupling capacitor is to transmit the AC signal of the previous stage to the next stage.
The position of the coupling capacitor is connected across the output of the previous stage and the input of the latter stage.
The function of the decoupling capacitor is to short-circuit the unprofitable AC signal that is inter-staged between amplifier stages to the ground.
The location of the decoupling capacitor is between the ground of a certain input stage.

31. How to distinguish whether the capacitor in the electronic circuit is a filter capacitor or a bypass capacitor? The filter capacitor in the power circuit, the bypass capacitor in the signal circuit, in fact, the function is basically the same, filter capacitor: bypass or filter out the pulsating current components and play a role in charging and discharging, bypass capacitor: in the circuit The high-frequency or low-frequency components of the filter are filtered or bypassed.

32. What is the difference between a decoupling capacitor and a bypass capacitor? Bypass capacitor is not a theoretical concept, but a practical method that is often used. Electron tubes or transistors need to be biased, which is to determine the DC power supply conditions of the operating point.

For example, the grid of an electron tube often requires a negative voltage relative to the cathode. In order to work under a DC power supply, a resistor is connected in series between the cathode and the ground, and the plate current is used to form the positive potential of the cathode to the ground, and the grid is DC grounded. This biasing technique is called “self-biasing”, but for (AC) signals, it is also a negative feedback. In order to eliminate this effect, a sufficiently large point capacitance is connected in parallel to this resistor, which is called side-by-side. Road capacitance.

The decoupling capacitor has two functions between the power supply of the integrated circuit and the ground: on the one hand, it is the energy storage capacitor of the integrated circuit, and on the other hand, it bypasses the high frequency noise of the device. The typical value of the decoupling capacitor in the digital circuit It is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH.

The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz. That is to say, it has a better decoupling effect for noise below 10MHz, and it has little effect on noise above 40MHz. 1μF, 10μF capacitors, the parallel resonance frequency is above 20MHz, the effect of removing high frequency noise is better, every 10 or so integrated circuits need to add a charge and discharge capacitor, or an energy storage capacitor, you can choose about 10μF.

It is best not to use electrolytic capacitors. Electrolytic capacitors are rolled up with two layers of film. This rolled structure is inductance at high frequencies. Tantalum capacitors or polycarbonate capacitors should be used. The selection of decoupling capacitors is not strict. Press C=1/F, that is, 0.1μF for 10MHz and 0.01μF for 100MHz.

Generally speaking, uf-level capacitors, such as electrolytic capacitors or tantalum capacitors, have larger inductance and smaller resonance frequency, which can pass low-frequency signals better, but show strong inductance for high-frequency signals. The impedance is large, and at the same time, the large capacitor can also play the role of a local charge pool, which can reduce the coupling of local interference through the power supply.

Capacitors with a capacity of 0.001~0.1uf, generally ceramic capacitors or mica capacitors, have small inductance, high resonance frequency, and low impedance to high-frequency signals. They can provide a bypass for high-frequency interference signals to reduce the external Coupling interference.

Bypass is to filter out high-frequency clutter or signals carried by the pre-stage or power supply; decoupling is a “small pond” set up to ensure stable output at the output end (mainly for the operation of the device), and work in other high currents Make sure that the fluctuation range of the power supply will not affect the operation of the circuit; one additional point is the so-called coupling: it is the components that transmit signals between the front and rear stages without affecting each other’s static operating points.

The high frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise arrived.

From the circuit point of view, there are always driving sources and driven loads. If the load capacitance is relatively large, the driving circuit must charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large. .

In this way, the driving current will absorb a large power supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, it will bounce), this current is actually a kind of noise compared to normal conditions. , It will affect the normal work of the previous stage. This is coupling.

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 decoupled, but the bypass capacitor generally refers to high-frequency bypass, that is, to improve a low-impedance leakage prevention method for high-frequency switching noise.

High-frequency bypass capacitors are generally relatively small. According to the resonance frequency, they are generally 0.1u, 0.01u, etc., while decoupling capacitors are generally larger, 10u or greater, depending on the distribution parameters in the circuit and the magnitude of the drive current change.

33. How do diodes, transistors, and capacitors work in a circuit?

The diode plays a role of unidirectional conduction.

The triode plays an amplifying role in an analog circuit and a switch in a digital circuit.

In general, capacitors play a role in blocking AC and DC, such as filter capacitors, coupling capacitors, etc. The fundamental purpose is to “pass DC and block them.”

34. What role does the filter capacitor play in the circuit? Low-frequency filter capacitors are mainly used for mains filtering or filtering after transformer rectification, and their working frequency is the same as that of mains at 50Hz; while high-frequency filter capacitors are mainly used for filtering after switching power supply rectification, and their working frequency is several thousand Hz to several thousand Hz. Ten thousand Hz.

When we use low-frequency filter capacitors in high-frequency circuits, due to the poor high-frequency characteristics of the low-frequency filter capacitors, it has large internal resistance and high equivalent inductance during high-frequency charging and discharging. Therefore, in use, the frequent polarization of the electrolyte will generate greater heat, and the higher temperature will vaporize the electrolyte inside the capacitor, and the pressure in the capacitor will increase, which will eventually lead to the bulging and bursting of the capacitor.