Why the small capacitance is the high operating frequency

The capacitor has the function of taking the intersection of the direct separation, when the high-frequency signal is half a cycle, it is charged to the plate of the capacitor through the load, and when the electrical signal is half a cycle, the positive charge on the primary plate is charged when the negative half cycle arrives.

Through the load release, the negative half cycle is reversed to the other end of the capacitor to charge, the negative half cycle passes, the positive half cycle arrives, the positive half cycle is recharged to the capacitor, and the cycle begins, the current is generated in the circuit, in fact, the current does not pass through the capacitor, which is the reason why the AC signal can pass through the capacitor and flow directly. The frequency of the flow through the capacitor is related to the size of the capacitor, the smaller the capacitor, the shorter the charging time, and the higher the frequency that can pass through.

Point contact diode is to place a metal wire (generally with gold as material) as the electrode (positive electrode) on the germanium (silicon) plane, because the contact surface of this diode is very small, so the formation of junction capacitance is also very small (the capacity of the capacitor depends on the relative area and relative distance of the capacitor plate, the larger the relative area, the larger the capacitance, the smaller the relative distance, the larger the capacitance). The junction capacitance of the diode has little effect when the diode operates at a low frequency, but at a high operating frequency, the effect is very prominent (the higher the frequency of the capacitance, the smaller the capacitive reactance), and the rectification effect will be lost to the high-frequency signal. Capacitive reactance is the resistance of a capacitor to alternating current.

Point contact diodes are generally used as frequency modulation detection circuits, such as the detection level of radios, the monitoring frequency of TV signals from intermediate frequency signals to audio signals, etc.

Since the contact surface of the point contact diode is very small, the point contact diode cannot pass large currents, so it cannot be used in rectifier circuits with large currents.

The smaller the capacitance, the higher the operating frequency

The capacitor has the function of direct blocking and crossing, when the high-frequency signal is half a week when the load is charged to the plate of the capacitor, the capacitor will be charged after being energized, and the reverse discharge will be discharged after being charged, for the capacitor its characteristics are high resistance and low resistance, the higher the capacitance, the easier it is to reverse conduction, thus affecting the unidirectional conductivity of the diode! reduce the operating frequency; On the contrary, the smaller the junction capacitance, the more conducive to unidirectional conduction, so that the operating frequency will be increased.

Have you learned the principle of IC resonance?

For example, the working principle of a parallel resonant circuit is like charging a capacitor with an external AC signal, then the capacitor charging the inductor, and then the inductor charging the capacitor. The capacitor then charges the inductor, so that the cycle repeats, and the lost energy is replenished in time by the AC signal. In this way, the natural frequency of a charge-discharge process is related to the product of l c.

The larger the capacitance, the longer it will take for the inductor to be fully charged; The larger the inductance, the longer it will take for the capacitor to be charged to the inductance. Therefore, the higher the signal frequency, the smaller the parameter value of the resonant circuit.

The closer the external AC signal is to the natural frequency of the resonant circuit, the less power is added to the resonant circuit. In other words, the more efficient the external signal is to replenish the resonant circuit. In terms of device impedance, the greater the resistance between the two ends of the LC circuit, the smaller the impedance at both ends of the LC resonant state.

The signal of other frequencies is either early or late to replenish the circuit, and the lack of energy replenishment will naturally reduce the impedance at both ends. The AC voltage at both ends of the LC will be lower.

In the past, when it came to this problem physically, it was to make a Maxwell roll to explain the problem, and you searched and searched, and then asked if you still didn't understand it.

The nature of the series resonant circuit is exactly the same as that of the parallel resonant circuit, but the principle is slightly different. No more talking.

First, the capacitive reactance is XC=1 (2 Fc), the capacitance is C, the frequency is F, and the capacitive reactance is C. From the formula, it can be seen that frequency is inversely proportional to capacitance, and capacitive reactance is inversely proportional to capacitance. Therefore, the larger the capacitance of the capacitor, the higher the AC frequency, and the smaller the capacitive reactance.

Alternating current can pass through the capacitor, but when the capacitor is connected to the AC circuit, the charge carried on the capacitor plate has a hindering effect on the directionally moving charge, which is called capacitive reactance in physics, and is represented by the letter XC.

Therefore, the capacitor still has an obstructive effect on the alternating current, and the alternating current is easy to pass through the capacitor, indicating that the capacitance is large and the obstructive effect of the capacitor is small; The frequency of alternating current is high, and the alternating current is easy to pass through the capacitor, indicating that the frequency is high and the impediment effect of the capacitor is small.

Capacitance is not always better.

Intuitively, it seems that the larger the energy storage capacitance, the stronger the ability to provide current compensation for the IC. For this reason, many people prefer to use capacitors with large capacitances. Actually, this is a wrong concept.

Due to the existence of parasitic inductance on the capacitor, the capacitor discharge circuit will resonate at a certain frequency point, and at the resonance point, the impedance of the capacitor is small, so the impedance of the discharge circuit is the smallest, and the effect of supplementing energy is also the best.

However, when the frequency exceeds the resonant point, the impedance of the discharge loop begins to increase, which means that the capacitor's ability to deliver current begins to decrease. The larger the capacitance of the capacitor, the lower the resonant frequency, and the smaller the frequency range in which the capacitor can effectively compensate for the current.

Therefore, in order to ensure the capacitor's ability to provide high-frequency current, the capacitor is not as large as possible. The larger the capacitor capacity, the greater the amount of charge the capacitor can carry. If we think of a capacitor as a battery, each charge and discharge of the capacitor can bring a larger load.

It is true that large capacitors can bring loads that can have greater tremor, but with this, the time for capacitors to charge and discharge will also increase, thereby reducing the high-frequency performance of the capacitors, and at the same time, large capacitors tend to have greater parasitic inductance, thereby reducing the filtering effect and affecting the stability of the circuit. Therefore, the capacitance capacity should be allocated according to the demand in order to achieve the best performance of the appliance.

The use of capacitors does not necessarily mean that large capacity is good, it mainly depends on where it is used, the large capacity is large, the small capacity is small, and the appropriate is important.

The smaller the capacitance, the shorter the charging time, the faster the discharge, and the faster the discharge, it can keep up with the changes in the high-frequency frequency, so it can only filter out the high-frequency alternating current in this frequency band. The filtering of each frequency band must use the corresponding capacitance capacitance.

Capacitors, usually referred to as capacitors, are represented by the letter C, which is a container for charging electricity and a device that holds charges. Capacitors are one of the electronic components widely used in electronic equipment, which are widely used in circuits such as direct and direct crossing, coupling, bypass, filtering, tuning loop, energy conversion, control, etc.

Because the junction capacitance is small, it is equivalent to a PN junction connected in parallel with a capacitor, and the current cut-off is slow. Low frequencies are ok. It is not suitable for use in high-frequency circuits, and if it is used in high-frequency circuits, it will quickly be damaged due to severe heating of the PN junction.

On the other hand, a point-contact diode with a small junction capacitance has a fast current cut-off, which will cause less heat generation and will not be damaged. It can be used in medium and high frequency circuits.

When the applied forward voltage increases, the PN junction narrows, the space charge region narrows, and the amount of space charge in the junction decreases, which is equivalent to capacitive discharge. In the same way, when the forward voltage decreases, the pn junction becomes wider, the space charge region widens, and the amount of space charge in the junction increases, which is equivalent to capacitor charging.

When the reverse voltage increases, on the one hand, it widens the depletion area, which is also equivalent to charging the capacitor. When the reverse voltage is reduced, the holes in the P region and the electrons in the N region flow to the depletion region, narrowing the depletion region, which is equivalent to a discharge.