Introduction to Coupling Capacitor

Catalog

I. Working Principle

A coupling capacitor is one of the capacitors, and its function can be simply understood as coupling. In fact, the coupling capacitor is also known as electrostatic coupling, which is a coupling method due to the existence of distributed capacitance. So what about the coupling capacitor principle? Let's take a look together.

The capacitor is connected to an AC circuit, and the voltage of the circuit connected to a pin gradually increases, and gradually accumulates charge on the plate where it is located. When the voltage of the circuit connected to the pin drops, the charge accumulated when the potential is high returns to the circuit. The same goes for the other end. The capacitor is insulated, and no current flows through the entire capacitor, but the phenomenon that it accumulates and releases charges as the potential rises and falls, which makes people mistakenly think that there is current passing.

Coupling refers to the process of signal transfer from the first stage to the second stage. Capacitors are often used in the coupling of AC signals because they can cut off DC and prevent DC signals from passing, and the DC biases between the various levels are irrelevant. The "AC" characteristic of the capacitor allows the AC signal to flow through the capacitor to the next stage.

The coupling of the capacitor is based on the "traffic blocking" characteristic of the capacitor. The input frequency is 1kHz and the amplitude is 20mV sinusoidal signal in the picture; 2N2218 is a triode when the circuit provides suitable DC voltage to the 3 pins of the triode, it can be amplified The input sine signal.

Coupling capacitors make the two systems of strong and weak currents coupled and isolated by capacitors, provide high-frequency signal paths, prevent power frequency current from entering the weak current system, and ensure personal safety. In addition to the above functions, the coupling capacitor with voltage extraction device can also extract power frequency voltage for protection and reclosing use, and play the role of a voltage transformer. There are many coupling methods. The capacitor is connected to the AC circuit, and the voltage of the circuit connected to a pin gradually increases, and the charge is gradually accumulated on the plate where it is located. When the voltage of the circuit connected to the pin drops, the potential is increased. When the accumulated charge returns to the circuit.

In fact, as far as the principle of coupling capacitor is concerned, it can isolate the DC, and the AC signal is coupled in the form of increasing and decreasing the potential at both ends and transmitted to the following circuit components. The function is to cut off the current of the previous amplifier circuit so that the AC signal can pass normally.

II. Features

1. Construction

It is a parallel plate capacitor in general, and its construction is extremely simple. A dielectric material is used in between the parallel plates of this capacitor. So while getting final output like AC signals, this capacitor plays a key role.

Coupling capacitors are mainly used in analog circuits, while digital circuits use decoupling capacitors. The connection of this capacitor with the load for AC coupling can be done in series. 

Low-frequency signals like DC are blocked by a capacitor and high-frequency signals like AC are allowed. It reacts to various frequencies in various ways. This capacitor's resistance or impedance is very high for low-frequency signals. Likewise, high-frequency signals, it has less resistance or impedance to easily pass through the circuit.

2. Calculation

A capacitor is short at high frequencies, whereas it is open at low frequencies. By using the following formula, the capacitance of this capacitor can be calculated.

Xc = 1/2πfc

From the above equation:

‘Xc’ is the reactance of the capacitance

‘C’ capacitance

‘f’ frequency

C = 1/2πfXc

III. Applications

The following are included in the coupling capacitor applications.:

In audio circuits, this capacitor is used in 

This capacitor is used in many circuits where the AC signal is required as an output signal, while the DC signal is only used to supply power within the circuit for certain components, but it should not come out like the output. 

This capacitor is used in power line communication substations. 

In PLCC equipment, this capacitor is used for connecting the carrier equipment as well as the transmission line. 

In BJT, this capacitor is used to connect two stages in order to connect one stage's o/p to the next stage's i/p for amplification.

Capacitors used in Coupling Applications:

There are some key parameters that need to be considered whenever a capacitor is selected for coupling applications, such as series resonant frequency, impedance, and equivalent series resistance. The capacitance value mainly depends on the application frequency range & the load or source impedance. There are various types of capacitors, such as ceramic, film, tantalum, polymer electrolytic or organic aluminum, and aluminum electrolytic capacitors, used in coupling applications.

For elevated capacitance values, tantalum capacitors offer high stability. These capacitors are costly and, when compared to ceramic capacitors, they have high ESR. In coupling applications, these capacitors are used. 

Ceramic capacitors are cost-effective and available in minute packages of SMT. In RF & audio applications, these capacitors are normally used. 

Compared with tantalum capacitors, aluminum electrolytic capacitors are inexpensive. These capacitors include ESR characteristics and give tantalum-related stable capacitance. The size of these capacitors, however, is great. In coupling applications for power amplifiers, these capacitors are used extensively.

Polyester & polypropylene capacitors in pre-amp circuits for coupling applications are fine choices.