What is a Coupling Capacitor : Construction & Its Working

Generally, the capacitor allows high-frequency signals like AC & blocks low-frequency signals like DC. The main function of the capacitor is to store an electrical charge. This is a reactive component and they react for low-frequency signals like DC. Whenever the capacitor is connected in series with the load, then the capacitor is known as a coupling capacitor. These capacitors are used where the o/p of the circuit requires high-frequency signals like AC like audio circuits. AC circuits use DC as input and AC as the output. The circuit output can be interfaced with a capacitor with a load called the coupling capacitor. But selecting suitable capacitance depending on the signal frequency is significant, but the resistance must connect in parallel with the capacitor.

What is a Coupling Capacitor?

Definition: A capacitor that is used to connect the AC signal of one circuit to another circuit is known as a coupling capacitor. The main function of this capacitor is to block the DC signal and allows the AC signal from one circuit to another. These capacitors are used in various circuits where AC signals are used to be output whereas DC signals are simply used to provide power to circuit components but they should not come out in the output.

For instance, normally this capacitor is used in audio circuits like a microphone circuit. For this, DC power is used to provide the supply. But, whenever a user talks through the microphone, then the speech is considered as an AC signal. When the AC signals supply from the microphone to the o/p device, then the DC signal cannot pass because this signal gives the power to the parts in the circuit. On the o/p end, we get the AC signal. So a coupling capacitor is placed between two circuits so that AC signals supplies while the DC signal is blocked.

Coupling Capacitor Construction

Generally, it is a parallel plate capacitor and its construction is extremely easy. In between the parallel plates of this capacitor, a dielectric material is used. So this capacitor plays a key role while getting final output like AC signals.

Coupling capacitors are mainly used in analog circuits whereas the decoupling capacitors are used in digital circuits. The connection of this capacitor can be done in series with the load for AC coupling.
A capacitor blocks low-frequency signals like DC and allows high-frequency signals like AC. In different ways, it reacts to different frequencies. For low-frequency signals, the resistance or impedance of this capacitor is very high. Similarly, it has less resistance or impedance for high-frequency signals to pass through the circuit easily.

Capacitors used in Coupling Applications

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

Tantalum capacitors offer high stability for high capacitance values. These capacitors are expensive and they have high ESR when compared with ceramic capacitors. These capacitors are used in coupling applications.

Ceramic capacitors are economical and are available in minute SMT packages. These capacitors are normally used in RF & audio applications.

Aluminum electrolytic capacitors are low-cost compare with tantalum capacitors. These capacitors include ESR characteristics and give stable capacitance related to tantalum. But, the size of these capacitors is large. These capacitors are extensively used in coupling applications for power amplifiers.

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

Coupling Capacitor Calculation

At high frequencies, a capacitor is shorted whereas, at low frequencies, it is open. The capacitance of this capacitor can be calculated by using the following formula.

Xc = 1/2πfc

From the above equation

‘Xc’ is the reactance of the capacitance

‘C’ capacitance

‘f’ frequency

C = 1/2πfXc

Applications

The coupling capacitor applications include the following.

• This capacitor is used in audio circuits
• This capacitor is used in many circuits where the AC signal is desired as output signal while DC signal is just used for certain components to provide power within the circuit however that should not come out like the output.
• This capacitor is used in substations in the power line communication.
• This capacitor is used in PLCC equipment to connect the carrier equipment as well as a transmission line.
• This capacitor is used in BJT for connecting two stages so that o/p of one stage is connected to the i/p of the next stage for amplification.

FAQs

1). What is the coupling capacitor?

A capacitor that is used to connect the AC signal from one circuit to another is known as a coupling capacitor.

2). What are the capacitors used in coupling applications?

They are aluminum electrolytic, tantalum, ceramic, polypropylene, and polyester.

3). How to select a coupling capacitor?

By measuring, calculating, and determining the minimum impedance value of the capacitor.

4). Can the capacitor allow DC?

It allows AC and blocks DC.

Thus, this is all about an overview of the coupling capacitor. The capacitor is a basic component in both the electronic circuits like analog & digital. These are used in a variety of applications like coupling, filtering, timing, and de-coupling. The coupling type allows AC components and blocks DC components. The circuit performance, reliability, and a lifetime can be determined by the capacitor. Here is a question for you, what is the function of a decoupling capacitor?