Capacitor FAQ

1. What kind of circuit can a series or parallel capacitor achieve the coupling effect? What is the difference between using a capacitor and not using a capacitor?

In the AC multi-stage amplifier circuit, due to the different gain and power of each level, the DC working offset value of each level is different. If the direct coupling between the stages will make the working offset value of each level mixed and cannot work normally. The AC and DC isolation feature not only solves the coupling of the inter-stage communication, but also isolates the inter-stage bias value from mixing.

2. What is the role of capacitors in RC coupled amplifier circuits?

The DC signal is isolated, so that the static operating points of adjacent amplifier circuits are independent of each other and do not affect each other.

3. Basic amplifier circuit coupling capacitors, can the coupling capacitors be non-polar?

In the basic amplifier circuit, the coupling capacitor depends on the frequency. When the frequency is high, the electrodeless capacitor is required. Its biggest use is that it can pass alternating current and cut off direct current, and is widely used in high-frequency alternating current paths, bypasses, resonance circuits and other circuits, which are simply understood as high-frequency paths.

When the frequency is low, the capacitive reactance of the electrodeless capacitor is relatively increased due to the low capacity, so it is necessary to use a polar electrolytic capacitor. Since the electrolyte is added inside, the capacity can be made large, allowing the 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, and it is mostly used in circuits such as low-frequency AC paths, filtering, decoupling, and bypassing, which are simply understood as low-frequency paths.

4. What is the role of the coupling capacitor?

In the amplifying circuit, the high-frequency AC signal can pass through the circuit smoothly and be amplified step by step by using the coupling capacitor to block AC and DC, while the DC quantity is blocked inside each stage.

5. In a circuit powered by a battery, why does the capacitor charge and discharge?

Capacitors accumulate charges. During the charging process, the voltage rises slowly. On the contrary, when discharging, you only need to detect the voltage across the capacitor to realize the delay. For example, when charging, the voltage across the capacitor is zero at the beginning, and as the charging time prolongs, the voltage gradually rises to the voltage you set to control the switch of the circuit. Of course, the discharge can also be used in reverse. The delay time is related to capacitor capacity, capacitor leakage, charging resistance, and voltage, and sometimes the load resistance is also taken into account.

6. How to distinguish between coupling capacitor and bypass capacitor in amplifier 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.

7. How to choose capacitive coupling for the multi-stage AC amplifier circuit of the op amp?

Generally, ceramic capacitors can be used. If the effect is good, tantalum capacitors can be used. According to the frequency range of your input signal, high-frequency capacitors with 103 or 104 capacitance can be used. For low-frequency AC signals, electrolytic capacitors of about 22uF can be used.

8. The amplifier circuit adopts direct coupling, and the feedback network is pure resistance network. Why is the circuit only possible to generate high frequency oscillation?

The phase shift of the oscillation originating from the closed loop reaches 180 degrees and the loop gain at this time is greater than zero. Using a pure resistance network as the feedback network will definitely not introduce phase shift, so all the phase shift comes from the open loop of the amplifier. circuit. Using a directly coupled open-loop amplifier, there will be no capacitive element between stages to cause phase shift, so it is the capacitance inside the transistor or MOS tube that can cause the phase shift. These capacitances are fF, the maximum pF level capacitance, The resonant frequency of the circuit formed by these capacitors and the equivalent circuit resistance 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.

9. How to use bypass capacitors, filter capacitors, and decoupling capacitors?

• Filter capacitor: a capacitor usually used 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 capacitor is generally an electrolytic capacitor, and the capacity is large, at the microfarad level.
• Bypass capacitor: filter out the high-frequency components in the input signal, mainly used to filter out high-frequency clutter, usually ceramic capacitors with small capacity.
• Decoupling capacitor: The interference of the output signal is used as the filtering object. The decoupling capacitor is equivalent to a battery. It is charged and discharged so that the amplified signal will not be disturbed by the sudden change of the current. Its capacity depends on the frequency of the signal and the degree of ripple suppression.

10. How does the capacitor realize the functions of charging, discharging, rectifying and filtering?

The charging, discharging, rectifying and filtering of the capacitor and even its phase shifting, reactance and other functions are all in the storage function of the capacitor. The reason why the capacitor can store the charge is to use the strong mutual attraction between positive and negative charges. When charging the capacitor, people introduce positive charge into the positive plate through the power supply, and negative charge into the negative plate of the capacitor. But the positive and negative charges can't get together because there is a layer of insulating mode blocking them. The larger and thinner the diaphragm, the greater the gravitational force. The more charge is stored. Positive and negative charges are attracted between the ten plates, but if you provide it with an external circuit, they will combine with this external circuit, that is, discharge. They are one high and one low after all. The image of a capacitor is like a reservoir. It can vividly illustrate the role of its rectifier filter.

11. What is the function of the capacitor?

• Capacitors are mainly used in AC circuits and pulse circuits. In DC circuits, capacitors generally play the role of blocking DC.
• Capacitors neither generate nor consume energy, and are energy storage elements.
• Capacitors are important devices for improving power factor in power systems. They are the main components for obtaining oscillation, filtering, phase shifting, bypassing, coupling and other functions in electronic circuits.
• Because the load used in the industry is mainly the inductive load of the motor, the capacitive load must be paralleled to balance the power grid.

12. How does a capacitor function as a filter in a circuit?

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 is filtered out, set different capacitance values. In this way, the unnecessary current can be drawn to the ground, and the filtering is completed. For the current of the required frequency, the capacitor is a channel or the impedance is small. When the alternating current passes through, it is a process of repeated charging and discharging.

13. What is the principle of capacitive decoupling?

If the DC circuit enters the AC signal or the self-excited feedback of the AC amplifier circuit, it will have adverse consequences. In order to prevent the AC component from being coupled and amplified step by step, a capacitor is set between the stages to make it return to the ground.

14. How to distinguish whether a capacitor in an electronic circuit is a filter capacitor or a bypass capacitor?

• Filter capacitor: Bypass or filter out the pulsating current component and play the role of charging and discharging.
• Bypass Capacitor: Filter out or bypass high frequency or low frequency components in the circuit.