Capacitors

Capacitors are a simple passive device that can store an electrical charge on their plates when connected to a voltage source, much like a rechargeable battery. There are many different kinds of capacitors available, from very small capacitor beads used in resonance circuits to large power factor correction capacitors, but they all do the same thing, they story charge.

Types of Capacitors

The capacitor is a fundamental component for influencing the behavior of electronic circuits. It has applications in analog and digital circuits and at voltages from less than one volt to several thousand volts.

Designers have a variety of capacitor technologies at their disposal, differentiated primarily by the type of dielectric. This influences key aspects such as the capacitance value and size (volume) of the device, as well as energy efficiency. When combined with critical dimensions including the thickness of the dielectric, the capacitor's maximum voltage rating, and insulating capability can also be affected.

 

TypesOfCapacitors
Figure 1: Overview of Capacitor Technologies

Major Capacitor Applications

Decoupling

The most prevalent use of capacitors is to decouple a system, such as an integrated circuit, from sudden changes, including transfers of energy, within the power supply. Connecting a decoupling capacitor between supply voltage connection and ground in an Integrated Circuit (IC), close to the IC itself, helps maintain a stable voltage level and supply fast power to the IC when needed.

Filtering

The second most common capacitor application is in filtering to remove signals at unwanted frequencies.

  • By exploiting the capacitor's inherently high impedance at low frequencies and reducing impedance at higher frequencies, a resistor-capacitor (RC) filter can be designed to remove high-frequency interference such as noise in analog signals or to protect circuitry unwanted AC voltages applications, such as AM radio, cellar telephone, or IC switching noise applications.
  • By combining capacitive, resistive, and inductive elements (inductors display increasing resistance at higher frequencies), analog filters can be built with low-pass, high-pass, band-pass or band-stop properties with various cut-off frequencies and selectivity characteristics.

Coupling

Because capacitors do not conduct DC currents, they are often used to separate different voltage levels from each other by blocking DC voltages. Since AC signals can pass through capacitors, this property is used in circuits such as multi-stage amplifiers. Signals are allowed to pass through but to separate voltage levels.

Timing and Waveshaping

The manner in which a capacitor charges and discharges through a resistor can be exploited for various timing purposes, such as introducing a delay or changing the shape of a waveform.

Oscillators

Together with inductive components, capacitors can be used to build oscillators for generating accurate sinusoidal signals.

Different Materials and Applications of Capacitors

The differing properties of various capacitor technologies allow designers to select the optimum device type of any given application based on criteria such as operating voltage, required capacitance, device size, and frequency response. Other characteristics, such as aging (drying of wet electrolyte) causing loss of capacitance, maximum recommended operating temperature, flammability, and self-healing properties, are also important factors influencing device selection. In some cases, special low ESR devices that feature minimal parasitic resistance may be required, for example, to minimize power losses in high current applications.