Electrical element

One-port elements

Only nine types of element (memristor not included), five passive and four active, are required to model any electrical component or circuit. Each element is defined by a relation between the state variables of the network: current, ${\displaystyle I}$; voltage, ${\displaystyle V}$; charge, ${\displaystyle Q}$; and magnetic flux, ${\displaystyle \mathrm{\Phi }}$.

• Two sources:
  • Current source, measured in amperes – produces a current in a conductor. Affects charge according to the relation ${\displaystyle dQ=-Idt}$.
  • Voltage source, measured in volts – produces a potential difference between two points. Affects magnetic flux according to the relation ${\displaystyle d\mathrm{\Phi }=Vdt}$.

${\displaystyle \mathrm{\Phi }}$ in this relationship does not necessarily represent anything physically meaningful. In the case of the current generator, ${\displaystyle Q}$, the time integral of current represents the quantity of electric charge physically delivered by the generator. Here ${\displaystyle \mathrm{\Phi }}$ 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:
  • Resistance ${\displaystyle R}$, measured in ohms – produces a voltage proportional to the current flowing through the element. Relates voltage and current according to the relation ${\displaystyle dV=RdI}$.
  • Capacitance ${\displaystyle C}$, 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 ${\displaystyle dQ=CdV}$.
  • Inductance ${\displaystyle L}$, 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 ${\displaystyle d\mathrm{\Phi }=LdI}$.
• Four abstract active elements:
  • Voltage-controlled voltage source (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 source (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 source (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). Used to model trancitors. The gain is characterised by a transfer impedance which will have units of ohms.
  • Current-controlled current source (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).

These four elements are examples of two-port elements.

Non-linear elements

In reality, all circuit components are non-linear and can only be approximated as linear over a certain range. To describe the passive elements more precisely, their constitutive relation is used instead of simple proportionality. Six constitutive relations can be formed from any two of the circuit variables. From this, there is supposed to be a theoretical fourth passive element since there are only five elements in total (not including the various dependent sources) found in linear network analysis. This additional element is called memristor. It only has any meaning as a time-dependent non-linear element; as a time-independent linear element, it reduces to a regular resistor. Hence, it is not included in linear time-invariant (LTI) circuit models. The constitutive relations of the passive elements are given by;

• Resistance: constitutive relation defined as ${\displaystyle f(V,I)=0}$.
• Capacitance: constitutive relation defined as ${\displaystyle f(V,Q)=0}$.
• Inductance: constitutive relation defined as ${\displaystyle f(\mathrm{\Phi },I)=0}$.
• Memristance: constitutive relation defined as ${\displaystyle f(\mathrm{\Phi },Q)=0}$.

where ${\displaystyle f(x,y)}$ is an arbitrary function of two variables.

In some special cases, the constitutive relation simplifies to a function of one variable. This is the case for all linear elements, but also, for example, an ideal diode, which in circuit theory terms is a non-linear resistor, has a constitutive relation of the form ${\displaystyle V=f(I)}$. Both independent voltage and independent current sources can be considered non-linear resistors under this definition.

The fourth passive element, the memristor, was proposed by Leon Chua in a 1971 paper, but a physical component demonstrating memristance was not created until thirty-seven years later. It was reported on April 30, 2008, that a working memristor had been developed by a team at HP Labs led by scientist R. Stanley Williams. With the advent of the memristor, each pairing of the four variables can now be related.

Two special non-linear elements are sometimes used in analysis but are not the ideal counterpart of any real component:

• Nullator: defined as ${\displaystyle V=I=0}$
• Norator: defined as an element that places no restrictions on voltage and current whatsoever.

These are sometimes used in models of components with more than two terminals: transistors, for instance.

Two-port elements

All the above are two-terminal, or one-port, elements except the dependent sources. Two lossless, passive, linear two-port elements are typically introduced into network analysis. Their constitutive relations in matrix notation are;

Transformer

${\displaystyle \left[\begin{array}{c}{V}_1\\ I_2\end{array}\right]=\left[\begin{array}{cc}0& n\\ -n& 0\end{array}\right]\left[\begin{array}{c}{I}_1\\ V_2\end{array}\right]}$

Gyrator

${\displaystyle \left[\begin{array}{c}{V}_1\\ V_2\end{array}\right]=\left[\begin{array}{cc}0& -r\\ r& 0\end{array}\right]\left[\begin{array}{c}{I}_1\\ I_2\end{array}\right]}$

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 ports is mapped by 1/n. On the other hand, the gyrator 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 just the basic linear elements are necessarily reciprocal, so they 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. The 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.

Examples

The following are examples of representations 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).
• 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 often used when representing semiconductors. For example, on a first degree of approximation, a bipolar transistor may be represented by a variable current source controlled by the input current.