Capacitor Uses & Applications

It is particularly important to select the right capacitor or any given circuit design - understanding the key requirements for any given capacitor application or capacitor use will ensure the circuit operates correctly.

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Capacitors are used in virtually every area of electronics, and they perform a variety of different tasks. Although capacitors operate in the same way whatever the capacitor application or use, there are several different uses for capacitors in electronic circuit designs.

In order to select the right kind of capacitor it is necessary to have an understanding of the particular capacitor application so that its properties can be matched to the given use to which it is to be put.

Each form of capacitor has its own attributes and this means that it will perform well in a particular position in a circuit design or application.

Choosing the right capacitor for a given application is all part of the electronic circuit design process for a circuit. Using the wrong electronic component can easily mean that a circuit design will not work.

Capacitor uses, applications & electronic circuits

Capacitors can be used in a variety of different ways in different electronics circuits. Although their mode of operation remains exactly the same, the different forms of capacitor can be used to provide a variety of different functions in the electronic circuit.

Different electronic circuits will require capacitors with certain values and also to posses other attributes like current capability, value range, value accuracy, temperature stability and many other aspects.

Some of these electronic components will be available in different values, some capacitors types may have large value ranges, others smaller.

Other capacitors may have high current capabilities, others high levels of stability, and others still are available with very low values of temperature coefficient.

Understanding the different ways in which capacitors are used, helps to enable the selection of the best sort of capacitor for the given application.

By selecting the right capacitor for a given use or application, the electronic circuit circuit can be made to perform to its best.

Coupling capacitor

In this capacitor application or use, the electronic component allows only AC signals to pass from one section of a circuit to another while blocking any DC static voltage. This form of capacitor application is often required when connecting two stages of an amplifier together.

It is possible that a constant DC voltage will be present, say on the output of one stage, and only the alternating signal, audio frequency, radio frequency or whatever is required. If the DC components of the signal at the output of the first stage were present at the input of the second, then the bias and other operating conditions of the second stage would be altered.

Transistor electronic circuit with input and output coupling capacitors — Transistor circuit with input and output coupling capacitors

Even when using operational amplifiers where the circuit has been designed to provide small offset voltages, it is often wise to use coupling capacitors because of the high levels of DC gain present. Without a coupling capacitor, the high levels of DC gain could mean that the operational amplifier would run into saturation.

For capacitor applications of this nature it is necessary to ensure that the impedance of the capacitor is sufficiently low. Normally the output impedance of the preceding circuit is higher than the one it is driving, except for RF circuit designs, but more of that later. This means that the value of the capacitor is chosen to be the same as the impedance of the circuit, normally the input impedance of the second circuit. This gives a drop in response of 3dB at this frequency.

Important Parameters for Coupling Capacitor Uses
Parameter	Notes on capacitor use
Capacitor rated voltage	The rated voltage of the capacitor must be greater than the peak voltage that is likely to appear across it. Normally the capacitor must be selected with a rated voltage tat enables it to withstand the supply rail voltage with margin in hand to ensure reliability.
Capacitance value	High enough to pass lowest frequencies with little or no attenuation.
Tolerance	Wide tolerance capacitors can often be used because the exact value is not important, but the tolerance limits should be taken into consideration to ensure the lowest frequencies are passed even when the capacitor is on te lowest end of its tolerance.
Dielectric	Some capacitors, for example electrolytic capacitors have a limited frequency response, often only up to frequencies of around 100 kHz maximum. This should be taken into account. Also for high impedance applications, electrolytic capacitors should not be used as they have a relatively high level of leakage which may offset the operation of the second stage. Electrolytic capacitors also have a wide tolerance often -50% and +80%.

Decoupling capacitor use

In this application, the capacitor is used to remove any AC signals that may be on a DC bias point, power rail, or other node that needs to be free of a particular varying signal.

As the name of this capacitor use indicates, it used to decouple the node from the varying signal on it.

Transistor electronic circuit design with line and collector decoupling capacitors — Transistor circuit with line and collector decoupling capacitors

In this circuit there are two ways in which the capacitor is used for decoupling. C3 is used to decouple any signal that may be on the voltage rail. This type of capacitor must be able to withstand the supply voltage as well as supplying and absorbing the levels of current arising from noise on the rail. Also during switch-off, when the power is removed, large levels of current may be drawn from this capacitor dependent upon its value. Tantalum capacitors are not suitable for this position.

Decoupling is also provided by the combination of capacitor and resistor, C4, R5. This ensures that the collector signal does not leak through on the signal rail. The time constant of C4 and R5 is generally the dominating factor and the time constant should be chosen to be longer than the lowest frequency anticipated.

The type of decoupling used with C5 serves to isolate that particular stage well from any noise on the rail as well as preventing noise from the circuit passing onto the supply rail. During switch-off, current from the capacitor is limited by the resistor R5.

Important Parameters for Decoupling Capacitor Uses
Parameter	Notes on capacitor use
Capacitor rated voltage	Must be greater than the peak voltage across the capacitor. Normally the capacitor will be able to withstand the voltage of the node with some margin in hand to ensure reliability.
Capacitance value	High enough to pass lowest frequencies with little or no attenuation. This can sometimes result is relatively large values being required. However it is necessary to consider the frequencies being used. For low frequencies, large capacitance levels will typically be required and electrolytic capacitors are often used. If it is a low current circuit as in the case of C4, R5, in the circuit design above, a tantalum capacitor may also be appropriate, but typically isolated from the main voltage rail via a series resistor to prevent too much current being drawn as in the case of C4. For higher frequencies, ceramic capacitors may also be appropriate.
Tolerance	Wide tolerance capacitors can often be used because the exact value is not important.
Dielectric	Some capacitors like electrolytic capacitors have a relatively low upper frequency limit. Often to overcome this, a capacitor such as a ceramic capacitor with a smaller value may be used to provide the high frequency response, while a larger value electrolytic capacitor is used to pass the lower frequency components. The lower value ceramic or other capacitor still presents a low impedance at the higher frequencies because the reactive impedance is inversely proportional to the frequency.

RF coupling and decoupling applications

Within RF design and circuits, the coupling and decoupling follow the same basic rules as those needed for the ordinary coupling and decoupling capacitors. Circuits like the one shown for the standard coupling and decoupling are often used, and they perform in basically the same way.

However, when using capacitors for RF circuit design applications, it is necessary to consider their RF performance. This can be different to the performance at lower frequencies.

Normally electrolytic capacitors are not used - their performance falls with increasing frequency, and they are seldom used for applications above about 100 kHz. Ceramic capacitors are particularly popular as they possess a good RF performance, especially the surface mount MLCC capacitors.

The series inductance present in all capacitors to a greater or lesser degree makes itself felt at some frequencies, forming a resonant circuit wit the capacitance.

In general, ceramic capacitors have a high self resonant frequency, especially the surface mount capacitors that are very small and have no leads to introduce any inductance.

Some other types of capacitor could be used for RF circuit designs, but ceramic capacitors are most widely used in this application.

Smoothing capacitor applications

This is effectively the same as a decoupling capacitor, but the description of a smoothing capacitor is normally used in conjunction with a power supply circuits and systems.

When an incoming electrical supply or line signal is taken through a transformer and a rectifier, the resulting waveform is not smooth. It varies between zero and the peak voltage. If applied directly to an electronic circuit, this is most unlikely to operate as it is a series of half sine waves varying between zero and the peak voltage and instead circuits need a constant DC voltage.

To overcome this, a capacitor is used to decouple or smooth the output voltage in these circuit designs.

Electronic circuit with a rectifier and smoothing capacitor — Rectifier circuit with smoothing capacitor

In this use, the capacitor charges up when the peak voltage exceeds that of the output voltage, and supplies charge when the rectifier voltage falls below the capacitor voltage.

In this capacitor use, the electronic component decouples the rail and supplies charge where it is needed.

Normally relatively large values of capacitance are required to enable the required level of current to be supplied. As a result, the most widely used form of capacitor for this application is the electrolytic capacitor as these electronic components are often able to provide higher levels of ripple current.

Important Parameters for Smoothing Capacitor Uses
Parameter	Notes on capacitor use
Capacitor rated voltage	Must be greater than the peak voltage across the capacitor. The capacitor must be able to withstand the maximum peak rail voltage with some margin in hand to ensure reliability.
Capacitance value	Dependent upon the current required, but typically can be several thousand microfarads.
Tolerance	Wide tolerance capacitors can often be used because the exact value is not important.
Dielectric	Electrolytic capacitors are typically used because of the high values available. Tantalum capacitors, although they can come in reasonably high values, are not suitable because of the low level of ripple current they can tolerate. Ceramic capacitors are not available with the required level of capacitance.
Ripple current	In addition to the capacitor having sufficient capacitance to hold the required amount of charge, it must also be constructed in a way to be able to supply the current required. If the capacitor becomes too hot when delivering the current it may be damaged and fail. Ripple current ratings are particularly important on capacitors used for smoothing applications. Electrolytic capacitors are normally used, but even these must have their ripple current ratings checked for suitability.

Capacitor use as a timing element

In this application a capacitor can be used with a resistor or inductor in a resonant or time dependent circuit. In this function the capacitor may appear in a filter, oscillator tuned circuit, or in a timing element for an electronic circuit such as an a-stable, the time it takes to charge and discharge determining the operation of the circuit

LC or RC oscillators and filters are widely used in a host of circuits, and obviously one of the major elements is the capacitor.

In this particular capacitor use, one of the main requirements is for accuracy, and therefore the initial tolerance is important to ensure that the circuit operates on the required frequency. Temperature stability is also important to ensure that the performance of the circuit remains the same over the required temperature range.

Important Parameters for Timing Capacitor Uses
Parameter	Notes on capacitor use
Capacitor rated voltage	The actual peak voltage across the capacitor will vary according to the particular circuit and the rail voltage. It is necessary to assess each case on its own merits, noting that in some cases it may be higher than expected. In most cases it is unlikely to exceed the rail voltage.
Capacitance value	Dependent upon the frequencies used and the inductor or resistor needed to obtain the required operating frequency..
Tolerance	Close tolerance normally needed to ensure that the required operating frequency is obtained. In this application, capacitors with a good selection of values within each decade may be an advantage.
Dielectric	In many timing applications, the capacitor loss is important. High loss equates to low Q, and Q values should normally be as high as possible. There are many dielectrics that provide a suitable level of performance. Many ceramic capacitor dielectrics are able to provide high levels of stability these days. Also plastic film capacitors can offer high levels of performance. Silver mica capacitors are also used, especially in RF circuits. Although quite expensive, these silver mica capacitors offer high levels of performance: high Q; high stability; low loss; and close tolerance.
Temperature stability	The temperature stability of the capacitor should be high for these capacitor applications because the circuit will need to retain its frequency over the operating temperature range. If the value changes with temperature, even by a small amount, this can have a marked effect on the operation of the circuit.

Hold-up capacitor applications

In this particular capacitor application, the charge held by the capacitor is used to provide power for a circuit for a short while.

In the past small rechargeable batteries may have been used, but they often suffered from issues of memory effects and life limitation. Nowadays modern capacitor technology has improved to the degree that huge values of capacitance are available and therefore capacitors can offer a much better alternative.Super capacitors or supercaps offer are able to perform this function as they posses sufficiently large levels of capacitance to power electronic circuits during periods where the incoming power is unavailable. Offering capacitance levels of a Farad or more in some instances, they are relatively cheap and offer a great level of performance, although the maximum voltage is limited.

Important Parameters for Hold-up Capacitor Uses
Parameter	Notes on capacitor use
Capacitor rated voltage	Must be able to withstand the maximum operating voltage with a good margin for reliability.
Capacitance value	Can be up to several Farads.
Tolerance	Super capacitors widely used for this capacitor application have a wide tolerance. Fortunately this is not an issue as it primarily affects the time the hold-up can be maintained.

Super capacitor or supercap often used for battery hold up applications in many electronic circuit designs — Super capacitors are often used for battery hold up applications

Capacitor application choices

The choice of capacitor is often essential to the operation of a circuit. Knowing how a capacitor will be used and how its performance and parameters relate to the operation of the circuit, mean that some capacitors perform better than others in different applications. Selecting the right capacitor for any given application is an essential and very important part of the circuit design.