# Introduction to 20 Uncertainties about Capacitor Uses

A capacitor is a passive electronic component that stores and releases energy. Its unique characteristic is blocking direct current while allowing alternating current to pass. The main functions of capacitors are based on these characteristics. The use of capacitors is also based on this. Here are collecting 20 questions about capacitor use in electronics. Let you have a more comprehensive understanding of the use of capacitors. #### 1) What is the function of a capacitor connected in parallel with the positive and negative terminals of the voltage source?

Answer: When a cap is used in a rectifier circuit, it has a good filtering effect. When the voltage is alternating, the voltage at both ends cannot be changed suddenly due to the charging effect of the capacitor, which ensures the stability of the voltage. When the cap is used as a battery power supply, it is equivalent to short-circuiting the battery's AC signal, avoiding the increase in battery internal resistance and parasitic oscillation of the circuit due to the battery voltage drop.

#### 2) A capacitor in series or in parallel can achieve the effect of coupling in the circuit. What is the difference between whether there is a capacitor in the circuit?

Answer: In the AC multi-stage amplifying circuit, because of the different gains and powers of each stage, the DC working offset values of each stage are different. If the levels are directly coupled, it will cause the bias values of all levels to be mixed and unable to work normally. The “Pass AC, Block DC” characteristic not only solves the coupling of inter-stage exchanges but also isolates the inter-stage biased value intermixing.

#### 3) The two coupling capacitors in the basic amplifying circuit, the positive pole of the capacitor and the DC positive pole are connected to pass the AC and block the DC. Can the reverse connection also have this function?

Answer: 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?

Answer: Block 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) Can the analogue circuit amplifier do not have a coupling capacitor? The theory in the amplifier circuit adds a coupling capacitor between the transformer secondary coil and the transistor. Turn the output of the former stage into the input of the later stage, so that the two stages do not affect. The former stage is alternating current, so does the latter stage, so there is no mutual influence.

Answer: The former stage is indeed alternating current, but the latter stage is alternating current superimposed direct current. The transistor needs a DC bias. If there is no capacitor to block the DC, the coil of the transformer will bypass the DC bias of the transistor (because the inductor passes DC).

#### 6) In the basic amplifier circuit, can the coupling capacitor be non-polar?

Answer: In the basic amplifying circuit, the coupling capacitor depends on the frequency. When the frequency is high, a non-polar capacitor is needed. It is characterized by relatively stable, high withstand voltage, small size and capacity. Its biggest use is to block direct current and pass alternating current. A coupling capacitor is widely used in high-frequency alternating-current paths, bypass, resonance and other circuits. (high-pass)

When the frequency is low since the capacitance of the non-polar capacitor is relatively low, the capacitive reactance is relatively increased, 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. However, because of the organic medium between the internal two poles, the withstand voltage is limited. The non-polar capacitor is mostly used in circuits such as low-frequency AC paths, filtering, decoupling, and bypassing. (low-pass)

#### 7) In a battery-powered circuit, why does the capacitor charging and discharging have the delay effect?

Answer: Capacitors accumulate electric charge. During the charging process, the voltage rises slowly, while discharging vice versa. During charging, at the beginning, the voltage across the capacitor is zero, as time goes by, the voltage gradually rises to the voltage you set to control the switching of the circuit. Of course, the discharging process can also be used to achieve this. The delay time is related to capacitor capacity, capacitor leakage, charging resistance, voltage, and sometimes the load resistance is also taken into consideration.

#### 8) The resistance-capacitance coupling amplifier circuit can only amplify AC signals, but cannot amplify DC signals?

Answer: Capacitor is an electronic component that blocks DC and AC. Therefore, the resistance-capacitance coupling amplifier circuit can only amplify AC signals. A direct coupling amplifier circuit is used to amplify DC signals.

#### 9) How to tell the coupling capacitor and the bypass capacitor in the amplifying circuit?

Answer: The negative pole of the coupling capacitor is not grounded but is connected to the input of the next stage, and the negative pole of the bypass capacitor is grounded.

#### 10) How to choose a coupling capacitor for the multi-stage AC amplifier circuit?

Answer: Generally ceramic capacitors can be done, and tantalum capacitors can be used if the performance is good. According to the frequency range of your input signal, the capacitance of 103,104 can be used for high frequencies, and also electrolytic capacitors of about 22uF can be used for lower frequency AC signals. #### 11) The amplifying circuit adopts direct coupling, and the feedback network is a pure resistor network. Why is the circuit only possible to produce high-frequency oscillation?

Answer: The oscillation comes from the phase shift of the closed-loop reaching 180 degrees and the loop gain at this time is greater than zero. Using a pure resistor network as a feedback network will definitely not have phase shift, which comes from the open-loop circuit of the amplifier only. Using a direct-coupled open-loop amplifier, there will be no capacitive elements between stages that will cause phase shifts, but the capacitor inside the transistor or MOS tube will cause it. These capacitances are all fF, the maximum is pF. 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, with a pure resistor feedback network, which can only produce high-frequency oscillations.

#### 12) How to estimate the output resistance of the first-stage amplifier and the input resistance of the second-stage amplifier? When the amplitude of the signal source is too large, what will happen at the output of the two-stage amplifier? Shake the input end of the amplifier and observe the output end to see what appears? why?

Answer: A. The input resistance of the second stage amplifier is the output resistance of the first stage amplifier.

B. Have distortion.

C. Cause clutter due to human body induction.

#### 13) How to use the charge and discharge of capacitors to understand filtering, decoupling and bypassing?

Answer: Capacitors block DC and pass AC. Blocking DC is easy to understand, but passing AC is not easy to understand. As long as you understand it, you can understand filtering, decoupling and bypass.

Capacitors are charging and discharging, but the direction of alternating current changes alternately. The magnitude of the amplitude also changes periodically. The entire changing image is a sine curve.

The capacitor is connected to the AC circuit, and due to the periodic change of the AC voltage, it is also periodically changing. There is charging and discharging current in the line. This charging and discharging current have 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.

The alternating current passing through the resistance consumes electric 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 here 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 charge either. Understand that the capacitor is connected to AC, then the AC component is bypassed to the ground, and also the filtering is completed.

#### 14) How to use bypass capacitor, filter capacitor and decoupling capacitor respectively?

Answer: These three types of capacitors are actually used for filtering, but they are used in different circuits, so their names and usages are different.

Filter capacitor, this is the capacitor we usually use after power rectification. It is a capacitor that rectifies the AC of the rectifier circuit into a pulsating DC and smoothes it by charging and discharging. This type of capacitor is generally an electrolytic capacitor with a large capacity.

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

The decoupling capacitor takes the interference of the output signal as the filtering object. It 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.

#### 15) Filtering capacitor?

Answer: 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, there is an alternating current of the same frequency in the circuit, which is the passing characteristic of the capacitor.

When the frequency is appropriate, the capacitor can be regarded as a path to the circuit, and the AC output of the previous stage can be transmitted to the subsequent circuit through the capacitor.

For direct current, 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 front and rear AC signals, it is coupling, and when acting on filtering out fluctuation components and useless AC components, it is filtering.

#### 16) 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 to the non-pass frequency signal, such as a bypass capacitor. So which point is used when analyzing capacitor filtering?

Answer: The theoretical explanation of using capacitor characteristics is more general, and the theory using capacitive reactance is more in-depth. The role of the capacitor is to use its charge and discharge characteristics, depending on what components you want to filter out. Use large capacitors to filter low frequencies and a small capacitor for high frequency. In theory, the filtering in the low-frequency rectifier circuit and the bypassing in the high-frequency circuit are the same, and the difference is the capacitive reactance.

#### 17) After the filter capacitor is fully charged, it will discharge the back circuit and then in cycle?

Answer: Such a working process in the circuit, the capacitor 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 used as a filter, it filters out a certain frequency signal to the ground. For example, the capacitors at the front end of the chip power supply are decoupling. The phenomenon you mentioned is like the filter capacitor before the voltage regulator is turned off and the filter capacitor of the switching power supply.

#### 18) What is the specific coupling of capacitors? Is there any difference compared with filtering?

Answer: 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 on 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:

A.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.

B.When the large signal is working, the circuit's demand for the power supply increases, causing the power supply fluctuations, here decoupling reduces the impact of power fluctuations on the input stage/high voltage gain stage during large signals.

C.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 noise on the board and to guide it to the ground.

#### 19) How to distinguish whether the capacitor in the circuit is a filter capacitor or a bypass capacitor?

Answer: The filter capacitor is in the power circuit; the bypass capacitor is in the signal circuit.

In fact, their function is basically the same.

The filter capacitor: Bypasses or filters out the pulsating current components and plays the role of charging and discharging.

Bypass capacitor: Filter or bypass high frequency or low-frequency components in the circuit.

#### 20) Is the coupling capacitor a decoupling capacitor?

Answer: It is completely different. The coupling capacitor is for signal transmission, and the decoupling capacitor is for reducing interference.

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