When designing a circuit, many novice engineers and hobbyists take a stable and well regulated power supply for granted, only to find out that their circuits don’t perform as expected during testing, or after the assembly is already complete. Analog circuits such as audio amplifiers or radios may produce a strange hum or a crackling noise audible in the background, and digital circuits such as microcontrollers may become unstable and unpredictable. The reason for this underperformance often lies in the fact that the input voltage is rarely stable in practice. Instead, when viewed with an oscilloscope, a DC power supply often shows many glitches, voltage spikes and AC voltage components.
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.
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):
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.
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.
In analog circuits, coupling capacitors are extensively used in amplifiers. The voltage bias of a transistor is crucial for normal operation of the amplifier. The role of coupling capacitors is to prevent the incoming AC signal from interfering with the bias voltage applied to the base of a transistor. In such applications, the signal is driven to the base of a transistor through a serially connected coupling capacitor. The capacitance value must be chosen so as to allow the useful signal, for example voice, to propagate freely, while blocking out the DC component.
In digital circuits, especially in communications systems, coupling capacitors are used to block the DC signal on the transmission line. The presence of a DC signal across a transmission line means that some energy is wasted as heat dissipated on the transmission line’s resistance. It could also cause other problems, such as grounding problems or charge accumulation problems between two distant connected circuits.