Capacitors for Coupling and Decoupling Applications

Capacitors are fundamental components in both analog and digital electronic circuits. These passive components play an important role in influencing the operational behavior of circuits. The characteristics of a capacitor vary mainly depending on the dielectric material used. The dielectric material determines the capacitance value, energy efficiency, and size of a capacitor. Fixed value capacitors can be broadly categorized into two: polar and non-polar capacitors. Non-polar capacitors include ceramic, film, and paper capacitors. Aluminium electrolytic capacitors and tantalum capacitors are polar components.

In circuits, capacitors are used for a wide range of applications including storing electrical charges, blocking DC components, bypassing AC components, filtering unwanted signals, and so on. The applications of a capacitor primarily depend on its characteristics. Key properties to consider when selecting a capacitor for a given application include capacitance value, voltage rating, frequency response characteristics, cost, and physical size. Other properties of a capacitor that can influence the performance of an electronic circuit include temperature characteristics, self-healing properties, aging, and flammability.

Coupling capacitors

Coupling capacitors are used in electronic circuits to pass the desired AC signal and block unwanted DC components. These unwanted DC signals come from electronic devices or preceding stages of an electronic circuit. In audio systems, DC components affect the quality of the desired signal by introducing noise. Furthermore, DC signals affect the performance of power amplifiers and increase distortion. In circuits, a coupling capacitor is connected in series with the signal path. Coupling capacitors are used in analog as well as digital electronic circuits. They find many applications in audio and radio frequency systems.

The reactive nature of a capacitor allows it to respond to different frequencies differently. In coupling applications, a capacitor blocks low frequency DC signals and allows high frequency AC signals to pass. To low frequency components, such as the DC signals, a capacitor exhibits high impedance, thereby blocking them. On the other hand, a capacitor exhibits low impedance to high frequency components. This allows high frequency signals such as AC components to pass.

In audio systems, DC sources are used to power audio circuits. However, since the audio signal is usually an AC signal, the DC component is unwanted on the output. To prevent the DC signal from appearing on the output device, a coupling capacitor is added in series with the load.

Coupling capacitors are essential components in amplifier circuits. They are used to prevent interference of a transistor’s bias voltage by AC signals. In most amplifier circuits, this is achieved by driving the signal to the base terminal of a transistor through a coupling capacitor. When a capacitor with the correct capacitance value is serially connected, the useful signal is allowed to pass while the DC component is blocked.

Presence of DC components on a transmission line can significantly affect the performance of a digital circuit. In communication systems, coupling capacitors are used to block unwanted DC components. Blocking the DC component helps to minimize energy loss and prevent accumulation of charge in digital circuits.

Types of capacitors for coupling applications ​

When selecting a capacitor for coupling/DC blocking applications, the key parameters to consider include impedance, equivalent series resistance, and series resonant frequency. The capacitance value primarily depends on the frequency range of the application and the load/source impedance. The types of capacitors that are commonly used for coupling applications include film, ceramic, tantalum, aluminium electrolytic, and aluminium organic/polymer electrolytic capacitors.

Tantalum capacitors offer high stability at high capacitance values, and they are available in different variants. As compared to ceramics, these capacitors have higher ESR and are more expensive. For coupling applications, tantalum capacitors are more popular than ceramic capacitors.
Aluminium electrolytic capacitors are cheaper than tantalum capacitors. They offer stable capacitance and have ESR characteristics similar to tantalum capacitors. However, these capacitors are relatively large in size and not recommended for circuits with limited circuit board space. Aluminium electrolytic capacitors are widely used for coupling applications in power amplifiers.

Ceramic capacitors are inexpensive and available in small SMT packages. These capacitors are cheaper compared to tantalum capacitors. Although ceramic capacitors are commonly used in audio and RF applications, they are generally unsuitable for applications that demand superior performance.

The large physical sizes of film capacitors limit their applications in AC coupling. If space is not an issue, polypropylene and polyester capacitors have characteristics that make them a good choices for coupling applications in pre-amp circuits.

Decoupling capacitors

Some electronic circuits are highly sensitive to voltage spikes, and rapid voltage changes can greatly affect their performance. Decoupling capacitors are used in electronic circuits to prevent quick voltage changes by acting as electrical energy reservoirs. In case of a sudden voltage drop, a decoupling capacitor provides the electrical energy required to maintain a stable voltage supply. On the other hand, if there is a sudden voltage spike, the capacitor stabilizes voltage by absorbing the excess energy.

Apart from stabilizing voltage in electronic circuits, decoupling capacitors are also used to allow DC components to pass while shorting AC components to ground. Capacitors that are used for bypassing AC noise in electronic circuits are also commonly known as bypass capacitors. Bypass capacitors absorb AC noise to produce a cleaner DC signal.

To remove AC noise, a bypass capacitor is placed in parallel with a resistor. A capacitor offers high resistance to low frequency signals and less resistance to high frequency signals. As such, low frequency DC components use the resistor path while high frequency AC components are shunted to ground through the bypass capacitor. This yields a clean DC signal that is free from AC components.

Types of capacitors for decoupling applications ​

When selecting a capacitor for decoupling applications, it is critical to consider the electrical requirements of the design. The key parameters to consider when selecting a bypass capacitor include the lowest frequency of the AC signal and resistance value of the resistor. In most cases, the lowest frequency is 50 Hz.

Although different types of capacitors are available for decoupling/bypassing applications, their characteristics vary markedly depending on the dielectric material used and structure. The two determine temperature stability, linearity, voltage rating, physical size and cost. The types of capacitors that are commonly used for decoupling applications include ceramic, tantalum, and aluminium electrolytic capacitors.

The performance and cost of ceramic capacitors make them a popular option for decoupling applications. These capacitors have low equivalent series resistance (ESR) and equivalent series inductance (ESL). In addition, multi-layer ceramic capacitors (MLCCs) are available in a wide range of packages and capacitance values. Ceramic capacitors are an excellent option for decoupling applications in HF circuits.

Switching type aluminium electrolytic capacitors are commonly used for decoupling applications in low frequency and medium frequency electronic circuits. These capacitors are inexpensive,available in a wide range of capacitance values, and have high capacitance-to-volume ratio. However, aluminium electrolytic capacitors exhibit temperature related wear out and have high ESR at low temperatures. These capacitors are widely used for decoupling applications in consumer products.

Solid tantalum capacitors have high CV, and they are less susceptible to wear out. Furthermore, they exhibit impressive stability at low temperatures. As compared to aluminium electrolytic capacitors, tantalum capacitors have higher capacitance-to-volume ratios and lower ESR. On the flip side, tantalum capacitors are expensive and limited to low voltage applications, usually up to 50 V. These capacitors are commonly used in higher reliability applications.

Film capacitors such as polyester, polypropylene, Teflon, and polystyrene capacitors have limited decoupling applications. Although these capacitors are suitable for high voltage applications and are less susceptible to wear out, the cost of producing them is relatively high. Nevertheless, the characteristics of these capacitors make them suitable options for high voltage, high current, and audio decoupling applications.

Source: Capacitor Faks blog
Simon Ndiritu from General Dielectrics explains some basic capacitor selection guide for coupling and decoupling applications.