Electrical elements are conceptual abstractions representing idealized electrical components, such as resistors, capacitors, and inductors, used in the analysis of electrical networks. All electrical networks can be analyzed as multiple electrical elements interconnected by wires. Where the elements roughly correspond to real components the representation can be in the form of a schematic diagram or circuit diagram. This is called a lumped element circuit model. In other cases infinitesimal elements are used to model the network in a distributed element model.
These ideal electrical elements represent real, physical electrical or electronic components but they do not exist physically and they are assumed to have ideal properties, while actual electrical components have less than ideal properties, a degree of uncertainty in their values and some degree of nonlinearity. To model the nonideal behavior of a real circuit component may require a combination of multiple ideal electrical elements in order to approximate its function. For example, an inductor circuit element is assumed to have inductance but no resistance or capacitance, while a real inductor, a coil of wire, has some resistance in addition to its inductance. This may be modeled by an ideal inductance element in series with a resistance.
Circuit analysis using electric elements is useful for understanding many practical electrical networks using components. By analyzing the way a network is affected by its individual elements it is possible to estimate how a real network will behave.
Circuit elements can be classified into different categories. One is how many terminals they have to connect them to other components:
- One-port elements – these represent the simplest components, that have only two terminals to connect to. Examples are resistances, capacitances, inductances, and diodes.
- Multiport elements – these have more than two terminals. They connect to the external circuit through multiple pairs of terminals called ports. For example, a transformer with three separate windings has six terminals and could be idealized as a three-port element; the ends of each winding are connected to a pair of terminals which represent a port.
- Two-port elements – these are the most common multiport elements, which have four terminals consisting of two ports.
Elements can also be divided into active and passive:
- Active elements or sources – these are elements which can source electrical power; examples are voltage sources and current sources. They can be used to represent ideal batteries and power supplies.
- Passive elements – These are elements which do not have a source of energy, examples are diodes, resistances, capacitances, and inductances.
Another distinction is between linear and nonlinear:
- Linear elements – these are elements in which the constituent relation, the relation between voltage and current, is a linear function. They obey the superposition principle. Examples of linear elements are resistances, capacitances, inductances, and linear dependent sources. Circuits with only linear elements, linear circuits, do not cause intermodulation distortion, and can be easily analysed with powerful mathematical techniques such as the Laplace transform.
- Nonlinear elements – these are elements in which the relation between voltage and current is a nonlinear function. An example is a diode, in which the current is an exponential function of the voltage. Circuits with nonlinear elements are harder to analyse and design, often requiring circuit simulation computer programs such as SPICE.
Most electrical components and circuits can be modeled using nine standard elements, five passive and four active. Each element is defined by a relation between the state variables of the network: current, ; voltage, , charge, ; and magnetic flux, .
- Two sources:
- in this relationship does not necessarily represent anything physically meaningful. In the case of the current generator, , the time integral of current, represents the quantity of electric charge physically delivered by the generator. Here is the time integral of voltage but whether or not that represents a physical quantity depends on the nature of the voltage source. For a voltage generated by magnetic induction it is meaningful, but for an electrochemical source, or a voltage that is the output of another circuit, no physical meaning is attached to it.
- Both these elements are necessarily non-linear elements. See #Non-linear elements below.
- Three passive elements: (In 2008 a fourth passive element, the memristor, was created in a lab, but it is not yet found in most circuits.)
- Resistors, with resistance measured in ohms – produces a voltage proportional to the current flowing through the element. Relates voltage and current according to the relation .
- Capacitors, with capacitance measured in farads – produces a current proportional to the rate of change of voltage across the element. Relates charge and voltage according to the relation .
- Inductors, with inductance measured in henries – produces the magnetic flux proportional to the rate of change of current through the element. Relates flux and current according to the relation .
- Four two-port.active elements:
- Voltage-controlled voltage sources (VCVS) – Generates a voltage based on another voltage with respect to a specified gain. (has infinite input impedance and zero output impedance).
- Voltage-controlled current sources (VCCS) – Generates a current based on a voltage elsewhere in the circuit, with respect to a specified gain, used to model field-effect transistors and vacuum tubes (has infinite input impedance and infinite output impedance). The gain is characterised by a transfer conductance which will have units of siemens.
- Current-controlled voltage sources (CCVS) – Generates a voltage based on an input current elsewhere in the circuit with respect to a specified gain. (has zero input impedance and zero output impedance). The gain is characterised by a transfer impedance which will have units of ohms.
- Current-controlled current sources (CCCS) – Generates a current based on an input current and a specified gain. Used to model bipolar junction transistors. (Has zero input impedance and infinite output impedance).
Linear approximations and non-linear elements
A nonlinear element or in a circuit does not have a linear relationship between its circuit variables. Examples include diodes, are transistors and other semiconductor devices, vacuum tubes, and iron core inductors and transformers when operated above their saturation current. Independent voltage and independent current sources can be considered non-linear resistors.
While linear circuits are easy to model and analyze, their linearity is an approximation that only holds over a certain range of input. Elements that operate linearly at low signal levels often show nonlinearity at higher levels. For instance in many audio systems, turning the volume up can make amplifying elements operate nonlinearly, distorting the sound.
In the more general case: resistance is some function of voltage and current; capacitance some function of voltage and charge; and inductance some function of current and flux. Following this pattern, memristance was hypothesized to be a physical property that was some function of flux and charge.
|Property||General relation||Linear approximation|
The memristor was proposed by Leon Chua in a 1971 paper, and its definition remains somewhat controversial. A physical component demonstrating memristance was first created in 2008, by a team at HP Labs led by scientist R. Stanley Williams.
There are also two special non-linear elements, the nullator and norator, which are sometimes used in analysis but are not the ideal counterpart of any real component. These are sometimes used in models of components with more than two terminals, such as transistors.
- Nullator: defined as
- Norator: defined as an element which places no restrictions on voltage and current whatsoever.
Linearizing a nonlinear element
Nonlinear elements can be made to operate linearly if the signal in them is limited to a low level. If the input of a non-linear device such as a transistor only varies in a small range around a fixed value, then the input/output relation is linearized around this fixed value (usually called the quiescent point, Q-point, or bias point). This is called a small signal model.
Other two-port elements
All the above are two-terminal, or one-port, elements with the exception of the dependent sources. There are two lossless, passive, linear two-port elements that are normally introduced into network analysis. Their constitutive relations in matrix notation are;
The transformer maps a voltage at one port to a voltage at the other in a ratio of n. The current between the same two port is mapped by 1/n. The gyrator, on the other hand, maps a voltage at one port to a current at the other. Likewise, currents are mapped to voltages. The quantity r in the matrix is in units of resistance. The gyrator is a necessary element in analysis because it is not reciprocal. Networks built from the basic linear elements only are obliged to be reciprocal and so cannot be used by themselves to represent a non-reciprocal system. It is not essential, however, to have both the transformer and gyrator. Two gyrators in cascade are equivalent to a transformer but the transformer is usually retained for convenience. Introduction of the gyrator also makes either capacitance or inductance non-essential since a gyrator terminated with one of these at port 2 will be equivalent to the other at port 1. However, transformer, capacitance and inductance are normally retained in analysis because they are the ideal properties of the basic physical components transformer, inductor and capacitor whereas a practical gyrator must be constructed as an active circuit.
The following are examples of representation of components by way of electrical elements.
- On a first degree of approximation, a battery is represented by a voltage source. A more refined model also includes a resistance in series with the voltage source, to represent the battery's internal resistance (which results in the battery heating and the voltage dropping when in use). A current source in parallel may be added to represent its leakage (which discharges the battery over a long period of time).
- On a first degree of approximation, a resistor is represented by a resistance. A more refined model also includes a series inductance, to represent the effects of its lead inductance (resistors constructed as a spiral have more significant inductance). A capacitance in parallel may be added to represent the capacitive effect of the proximity of the resistor leads to each other. A wire can be represented as a low-value resistor
- Current sources are more often used when representing semiconductors. For example, on a first degree of approximation, a bipolar transistor may be represented by a variable current source that is controlled by the input current.
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- Strukov, Dmitri B; Snider, Gregory S; Stewart, Duncan R; Williams, Stanley R (2008), "The missing memristor found", Nature, 453 (7191): 80–83, Bibcode:2008Natur.453...80S, doi:10.1038/nature06932, PMID 18451858
- EETimes, 30 April 2008, 'Missing link' memristor created, EETimes, 30 April 2008
- Engineers find 'missing link' of electronics – 30 April 2008
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