Decoupling capacitors and coupling capacitors

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.

What is a decoupling capacitor?

A decoupling capacitor acts as a local electrical energy reservoir. Capacitors, like batteries, need time to charge and discharge. When used as decoupling capacitors, they oppose quick changes of voltage. If the input voltage suddenly drops, the capacitor provides the energy to keep the voltage stable. Similarly, if there is a voltage spike, the capacitor absorbs the excess energy. Decoupling capacitors are used to filter out voltage spikes and pass through only the DC component of the signal. The idea is to use a capacitor in such a way that it shunts, or absorbs the noise making the DC signal as smooth as possible. Because of this, decoupling capacitors are also called bypass capacitors, since they bypass the power source when needed. They can be regarded as small uninterruptible power supplies dedicated to a single circuit board, or even a single component on a board. It is not uncommon to have a single capacitor for each integrated circuit used. As a matter of fact, in digital systems, almost all capacitors on the board may be used for decoupling.

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.

Power supply decoupling

Decoupling capacitors are often used to decouple the circuit from the power supply. Some components require a strictly regulated power source in order to function properly. A good example are microcontrollers and microprocessors. If there is a voltage spike, the program loaded into the processor could skip instructions and start behaving unpredictably. Digital logic circuits are also sensitive to power supply voltage. Therefore it must be well regulated for stable operation. For this reason, decoupling capacitors are added to the circuit in order to smooth out the power supply voltage. A good rule of thumb for digital circuits is to use a single 100nF ceramic capacitor for each logic integrated circuit, as well as a single larger (up to a few hundred µF) electrolytic capacitor per board or circuit segment. The larger electrolytic capacitor stores most of the energy in the circuit, and decouples lower frequencies. However, electrolytic capacitors have poor high-frequency characteristics, and logic gates can operate at very high frequencies - computer processors may have operating frequencies in the gigahertz range. At these higher frequencies, ceramic capacitors provide better decoupling. For the best results, the decoupling capacitor should be placed as close as possible to the chip. The following schematic illustrates the use of decoupling capacitors with a 7400 NAND logic gate (pin 14 is used for the positive supply voltage, while pin 7 is connected to ground):

Transient load decoupling

In digital circuits, the power source may be “contaminated with noise coming from logic circuits or other devices. Logic circuits are made of millions of logic gates which constantly change their output states between ON and OFF, which means that many transistors are switched on and off countless times in a second. With each switch, these transistors generate what is called a transient load. As a result, the current drawn by the device fluctuates, generating noise which propagates back to the power source. When capacitors are used for power supply decoupling, they serve two roles: protecting the power source from electrical noise generated within the circuit, and protecting the circuit from electrical noise generated by other devices connected to the same power source.


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.

Coupling capacitors

While decoupling capacitors are connected in parallel to the signal path and are used to filter out the AC component, coupling capacitors, on the other hand, are connected in series to the signal path and are used to filter out the DC component of a signal. They are used in both analog and digital circuit applications.

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.