Electrical impedance

Last updated

In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. [1]

Contents

Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of the sinusoidal voltage between its terminals, to the complex representation of the current flowing through it. [2] In general, it depends upon the frequency of the sinusoidal voltage.

Impedance extends the concept of resistance to alternating current (AC) circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude.

Impedance can be represented as a complex number, with the same units as resistance, for which the SI unit is the ohm (Ω). Its symbol is usually Z, and it may be represented by writing its magnitude and phase in the polar form |Z|∠θ. However, Cartesian complex number representation is often more powerful for circuit analysis purposes.

The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law. In multiple port networks, the two-terminal definition of impedance is inadequate, but the complex voltages at the ports and the currents flowing through them are still linearly related by the impedance matrix. [3]

The reciprocal of impedance is admittance, whose SI unit is the siemens, formerly called mho.

Instruments used to measure the electrical impedance are called impedance analyzers.

History

Perhaps the earliest use of complex numbers in circuit analysis was by Johann Victor Wietlisbach in 1879 in analysing the Maxwell bridge. Wietlisbach avoided using differential equations by expressing AC currents and voltages as exponential functions with imaginary exponents (see § Validity of complex representation). Wietlisbach found the required voltage was given by multiplying the current by a complex number (impedance), although he did not identify this as a general parameter in its own right. [4]

The term impedance was coined by Oliver Heaviside in July 1886. [5] [6] Heaviside recognised that the "resistance operator" (impedance) in his operational calculus was a complex number. In 1887 he showed that there was an AC equivalent to Ohm's law. [7]

Arthur Kennelly published an influential paper on impedance in 1893. Kennelly arrived at a complex number representation in a rather more direct way than using imaginary exponential functions. Kennelly followed the graphical representation of impedance (showing resistance, reactance, and impedance as the lengths of the sides of a right angle triangle) developed by John Ambrose Fleming in 1889. Impedances could thus be added vectorially. Kennelly realised that this graphical representation of impedance was directly analogous to graphical representation of complex numbers (Argand diagram). Problems in impedance calculation could thus be approached algebraically with a complex number representation. [8] [9] Later that same year, Kennelly's work was generalised to all AC circuits by Charles Proteus Steinmetz. Steinmetz not only represented impedances by complex numbers but also voltages and currents. Unlike Kennelly, Steinmetz was thus able to express AC equivalents of DC laws such as Ohm's and Kirchhoff's laws. [10] Steinmetz's work was highly influential in spreading the technique amongst engineers. [11]

Introduction

In addition to resistance as seen in DC circuits, impedance in AC circuits includes the effects of the induction of voltages in conductors by the magnetic fields (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part.

Complex impedance

A graphical representation of the complex impedance plane Complex Impedance.svg
A graphical representation of the complex impedance plane

The impedance of a two-terminal circuit element is represented as a complex quantity . The polar form conveniently captures both magnitude and phase characteristics as

where the magnitude represents the ratio of the voltage difference amplitude to the current amplitude, while the argument (commonly given the symbol ) gives the phase difference between voltage and current. is the imaginary unit, and is used instead of in this context to avoid confusion with the symbol for electric current. [12] :21

In Cartesian form, impedance is defined as

where the real part of impedance is the resistance R and the imaginary part is the reactance X.

Where it is needed to add or subtract impedances, the cartesian form is more convenient; but when quantities are multiplied or divided, the calculation becomes simpler if the polar form is used. A circuit calculation, such as finding the total impedance of two impedances in parallel, may require conversion between forms several times during the calculation. Conversion between the forms follows the normal conversion rules of complex numbers.

Complex voltage and current

Generalized impedances in a circuit can be drawn with the same symbol as a resistor (US ANSI or DIN Euro) or with a labeled box. Impedance symbol comparison.svg
Generalized impedances in a circuit can be drawn with the same symbol as a resistor (US ANSI or DIN Euro) or with a labeled box.

To simplify calculations, sinusoidal voltage and current waves are commonly represented as complex-valued functions of time denoted as and . [13] [14]

The impedance of a bipolar circuit is defined as the ratio of these quantities:

Hence, denoting , we have

The magnitude equation is the familiar Ohm's law applied to the voltage and current amplitudes, while the second equation defines the phase relationship.

Validity of complex representation

This representation using complex exponentials may be justified by noting that (by Euler's formula):

The real-valued sinusoidal function representing either voltage or current may be broken into two complex-valued functions. By the principle of superposition, we may analyse the behaviour of the sinusoid on the left-hand side by analysing the behaviour of the two complex terms on the right-hand side. Given the symmetry, we only need to perform the analysis for one right-hand term. The results are identical for the other. At the end of any calculation, we may return to real-valued sinusoids by further noting that

Ohm's law

An AC supply applying a voltage
V
{\displaystyle V}
, across a load
Z
{\displaystyle Z}
, driving a current
I
{\displaystyle I} General AC circuit.svg
An AC supply applying a voltage , across a load , driving a current

The meaning of electrical impedance can be understood by substituting it into Ohm's law. [15] [16] Assuming a two-terminal circuit element with impedance is driven by a sinusoidal voltage or current as above, there holds

The magnitude of the impedance acts just like resistance, giving the drop in voltage amplitude across an impedance for a given current . The phase factor tells us that the current lags the voltage by a phase of (i.e., in the time domain, the current signal is shifted later with respect to the voltage signal).

Just as impedance extends Ohm's law to cover AC circuits, other results from DC circuit analysis, such as voltage division, current division, Thévenin's theorem and Norton's theorem, can also be extended to AC circuits by replacing resistance with impedance.

Phasors

A phasor is represented by a constant complex number, usually expressed in exponential form, representing the complex amplitude (magnitude and phase) of a sinusoidal function of time. Phasors are used by electrical engineers to simplify computations involving sinusoids (such as in AC circuits [12] :53), where they can often reduce a differential equation problem to an algebraic one.

The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by the relative amplitudes and phases of the voltage and current. This is identical to the definition from Ohm's law given above, recognising that the factors of cancel.

Device examples

Resistor

The phase angles in the equations for the impedance of capacitors and inductors indicate that the voltage across a capacitor lags the current through it by a phase of
p
/
2
{\displaystyle \pi /2}
, while the voltage across an inductor leads the current through it by
p
/
2
{\displaystyle \pi /2}
. The identical voltage and current amplitudes indicate that the magnitude of the impedance is equal to one. VI phase.svg
The phase angles in the equations for the impedance of capacitors and inductors indicate that the voltage across a capacitor lags the current through it by a phase of , while the voltage across an inductor leads the current through it by . The identical voltage and current amplitudes indicate that the magnitude of the impedance is equal to one.

The impedance of an ideal resistor is purely real and is called resistive impedance:

In this case, the voltage and current waveforms are proportional and in phase.

Inductor and capacitor

Ideal inductors and capacitors have a purely imaginary reactive impedance:

the impedance of inductors increases as frequency increases;

[lower-alpha 1]

the impedance of capacitors decreases as frequency increases;

In both cases, for an applied sinusoidal voltage, the resulting current is also sinusoidal, but in quadrature, 90 degrees out of phase with the voltage. However, the phases have opposite signs: in an inductor, the current is lagging; in a capacitor the current is leading.

Note the following identities for the imaginary unit and its reciprocal:

Thus the inductor and capacitor impedance equations can be rewritten in polar form:

The magnitude gives the change in voltage amplitude for a given current amplitude through the impedance, while the exponential factors give the phase relationship.

Deriving the device-specific impedances

What follows below is a derivation of impedance for each of the three basic circuit elements: the resistor, the capacitor, and the inductor. Although the idea can be extended to define the relationship between the voltage and current of any arbitrary signal, these derivations assume sinusoidal signals. In fact, this applies to any arbitrary periodic signals, because these can be approximated as a sum of sinusoids through Fourier analysis.

Resistor

For a resistor, there is the relation

which is Ohm's law.

Considering the voltage signal to be

it follows that

This says that the ratio of AC voltage amplitude to alternating current (AC) amplitude across a resistor is , and that the AC voltage leads the current across a resistor by 0 degrees.

This result is commonly expressed as

Capacitor

For a capacitor, there is the relation:

Considering the voltage signal to be

it follows that

and thus, as previously,

Conversely, if the current through the circuit is assumed to be sinusoidal, its complex representation being

then integrating the differential equation

leads to

The Const term represents a fixed potential bias superimposed to the AC sinusoidal potential, that plays no role in AC analysis. For this purpose, this term can be assumed to be 0, hence again the impedance

Inductor

For the inductor, we have the relation (from Faraday's law):

This time, considering the current signal to be:

it follows that:

This result is commonly expressed in polar form as

or, using Euler's formula, as

As in the case of capacitors, it is also possible to derive this formula directly from the complex representations of the voltages and currents, or by assuming a sinusoidal voltage between the two poles of the inductor. In the latter case, integrating the differential equation above leads to a constant term for the current, that represents a fixed DC bias flowing through the inductor. This is set to zero because AC analysis using frequency domain impedance considers one frequency at a time and DC represents a separate frequency of zero hertz in this context.

Generalised s-plane impedance

Impedance defined in terms of can strictly be applied only to circuits that are driven with a steady-state AC signal. The concept of impedance can be extended to a circuit energised with any arbitrary signal by using complex frequency instead of . Complex frequency is given the symbol s and is, in general, a complex number. Signals are expressed in terms of complex frequency by taking the Laplace transform of the time domain expression of the signal. The impedance of the basic circuit elements in this more general notation is as follows:

ElementImpedance expression
Resistor
Inductor
Capacitor

For a DC circuit, this simplifies to s = 0. For a steady-state sinusoidal AC signal s = .

Formal derivation

The impedance of an electrical component is defined as the ratio between the Laplace transforms of the voltage over it and the current through it, i.e.

where is the complex Laplace parameter. As an example, according to the I-V-law of a capacitor, , from which it follows that .

In the phasor regime (steady-state AC, meaning all signals are represented mathematically as simple complex exponentials and oscillating at a common frequency ), impedance can simply be calculated as the voltage-to-current ratio, in which the common time-dependent factor cancels out:

Again, for a capacitor, one gets that , and hence . The phasor domain is sometimes dubbed the frequency domain, although it lacks one of the dimensions of the Laplace parameter. [17] For steady-state AC, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular:

These two relationships hold even after taking the real part of the complex exponentials (see phasors), which is the part of the signal one actually measures in real-life circuits.

Resistance vs reactance

Resistance and reactance together determine the magnitude and phase of the impedance through the following relations:

In many applications, the relative phase of the voltage and current is not critical so only the magnitude of the impedance is significant.

Resistance

Resistance is the real part of impedance; a device with a purely resistive impedance exhibits no phase shift between the voltage and current.

Reactance

Reactance is the imaginary part of the impedance; a component with a finite reactance induces a phase shift between the voltage across it and the current through it.

A purely reactive component is distinguished by the sinusoidal voltage across the component being in quadrature with the sinusoidal current through the component. This implies that the component alternately absorbs energy from the circuit and then returns energy to the circuit. A pure reactance does not dissipate any power.

Capacitive reactance

A capacitor has a purely reactive impedance that is inversely proportional to the signal frequency. A capacitor consists of two conductors separated by an insulator, also known as a dielectric.

The minus sign indicates that the imaginary part of the impedance is negative.

At low frequencies, a capacitor approaches an open circuit so no current flows through it.

A DC voltage applied across a capacitor causes charge to accumulate on one side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero.

Driven by an AC supply, a capacitor accumulates only a limited charge before the potential difference changes sign and the charge dissipates. The higher the frequency, the less charge accumulates and the smaller the opposition to the current.

Inductive reactance

Inductive reactance is proportional to the signal frequency and the inductance .

An inductor consists of a coiled conductor. Faraday's law of electromagnetic induction gives the back emf (voltage opposing current) due to a rate-of-change of magnetic flux density through a current loop.

For an inductor consisting of a coil with loops this gives:

The back-emf is the source of the opposition to current flow. A constant direct current has a zero rate-of-change, and sees an inductor as a short-circuit (it is typically made from a material with a low resistivity). An alternating current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency.

Total reactance

The total reactance is given by

(note that is negative)

so that the total impedance is

Combining impedances

The total impedance of many simple networks of components can be calculated using the rules for combining impedances in series and parallel. The rules are identical to those for combining resistances, except that the numbers in general are complex numbers. The general case, however, requires equivalent impedance transforms in addition to series and parallel.

Series combination

For components connected in series, the current through each circuit element is the same; the total impedance is the sum of the component impedances.

Impedances in series.svg

Or explicitly in real and imaginary terms:

Parallel combination

For components connected in parallel, the voltage across each circuit element is the same; the ratio of currents through any two elements is the inverse ratio of their impedances.

Impedances in parallel.svg

Hence the inverse total impedance is the sum of the inverses of the component impedances:

or, when n = 2:

The equivalent impedance can be calculated in terms of the equivalent series resistance and reactance . [18]

Measurement

The measurement of the impedance of devices and transmission lines is a practical problem in radio technology and other fields. Measurements of impedance may be carried out at one frequency, or the variation of device impedance over a range of frequencies may be of interest. The impedance may be measured or displayed directly in ohms, or other values related to impedance may be displayed; for example, in a radio antenna, the standing wave ratio or reflection coefficient may be more useful than the impedance alone. The measurement of impedance requires the measurement of the magnitude of voltage and current, and the phase difference between them. Impedance is often measured by "bridge" methods, similar to the direct-current Wheatstone bridge; a calibrated reference impedance is adjusted to balance off the effect of the impedance of the device under test. Impedance measurement in power electronic devices may require simultaneous measurement and provision of power to the operating device.

The impedance of a device can be calculated by complex division of the voltage and current. The impedance of the device can be calculated by applying a sinusoidal voltage to the device in series with a resistor, and measuring the voltage across the resistor and across the device. Performing this measurement by sweeping the frequencies of the applied signal provides the impedance phase and magnitude. [19]

The use of an impulse response may be used in combination with the fast Fourier transform (FFT) to rapidly measure the electrical impedance of various electrical devices. [19]

The LCR meter (Inductance (L), Capacitance (C), and Resistance (R)) is a device commonly used to measure the inductance, resistance and capacitance of a component; from these values, the impedance at any frequency can be calculated.

Example

Consider an LC tank circuit. The complex impedance of the circuit is

It is immediately seen that the value of is minimal (actually equal to 0 in this case) whenever

Therefore, the fundamental resonance angular frequency is

Variable impedance

In general, neither impedance nor admittance can vary with time, since they are defined for complex exponentials in which −∞ < t < +∞. If the complex exponential voltage to current ratio changes over time or amplitude, the circuit element cannot be described using the frequency domain. However, many components and systems (e.g., varicaps that are used in radio tuners) may exhibit non-linear or time-varying voltage to current ratios that seem to be linear time-invariant (LTI) for small signals and over small observation windows, so they can be roughly described as-if they had a time-varying impedance. This description is an approximation: Over large signal swings or wide observation windows, the voltage to current relationship will not be LTI and cannot be described by impedance.

See also

Notes

  1. is the imaginary unit; i.e., used in electrical engineering. The character is not used as that is often used for current.

Related Research Articles

<span class="mw-page-title-main">Characteristic impedance</span> Property of an electrical circuit

The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Alternatively, and equivalently, it can be defined as the input impedance of a transmission line when its length is infinite. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm.

<span class="mw-page-title-main">Resonance</span> Tendency to oscillate at certain frequencies

Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration that matches its natural frequency. When this happens, the object or system absorbs energy from the external force and starts vibrating with a larger amplitude. Resonance can occur in various systems, such as mechanical, electrical, or acoustic systems, and it is often desirable in certain applications, such as musical instruments or radio receivers. However, resonance can also be detrimental, leading to excessive vibrations or even structural failure in some cases.

In electrical circuits, reactance is the opposition presented to alternating current by inductance and capacitance. Along with resistance, it is one of two elements of impedance; however, while both elements involve transfer of electrical energy, no dissipation of electrical energy as heat occurs in reactance; instead, the reactance stores energy until a quarter-cycle later when the energy is returned to the circuit. Greater reactance gives smaller current for the same applied voltage.

A resistor–capacitor circuit, or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage or current source and these will produce different responses. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit.

<span class="mw-page-title-main">Voltage divider</span> Linear circuit that produces an output voltage that is a fraction of its input voltage

In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). Voltage division is the result of distributing the input voltage among the components of the divider. A simple example of a voltage divider is two resistors connected in series, with the input voltage applied across the resistor pair and the output voltage emerging from the connection between them.

The Sallen–Key topology is an electronic filter topology used to implement second-order active filters that is particularly valued for its simplicity. It is a degenerate form of a voltage-controlled voltage-source (VCVS) filter topology. It was introduced by R. P. Sallen and E. L. Key of MIT Lincoln Laboratory in 1955.

<span class="mw-page-title-main">LC circuit</span> Electrical "resonator" circuit, consisting of inductive and capacitive elements with no resistance

An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act as an electrical resonator, an electrical analogue of a tuning fork, storing energy oscillating at the circuit's resonant frequency.

A Colpitts oscillator, invented in 1918 by Canadian-American engineer Edwin H. Colpitts, is one of a number of designs for LC oscillators, electronic oscillators that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency. The distinguishing feature of the Colpitts oscillator is that the feedback for the active device is taken from a voltage divider made of two capacitors in series across the inductor.

<span class="mw-page-title-main">Phasor</span> Complex number representing a particular sine wave

In physics and engineering, a phasor is a complex number representing a sinusoidal function whose amplitude, and initial phase are time-invariant and whose angular frequency is fixed. It is related to a more general concept called analytic representation, which decomposes a sinusoid into the product of a complex constant and a factor depending on time and frequency. The complex constant, which depends on amplitude and phase, is known as a phasor, or complex amplitude, and sinor or even complexor.

A resistor–inductor circuit, or RL filter or RL network, is an electric circuit composed of resistors and inductors driven by a voltage or current source. A first-order RL circuit is composed of one resistor and one inductor, either in series driven by a voltage source or in parallel driven by a current source. It is one of the simplest analogue infinite impulse response electronic filters.

<span class="mw-page-title-main">AC power</span> Power in alternating current systems

In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the direction of energy flow. Its SI unit is the watt.

<span class="mw-page-title-main">Current divider</span> Simple linear circuit

In electronics, a current divider is a simple linear circuit that produces an output current (IX) that is a fraction of its input current (IT). Current division refers to the splitting of current between the branches of the divider. The currents in the various branches of such a circuit will always divide in such a way as to minimize the total energy expended.

<span class="mw-page-title-main">Electrical resonance</span> Canceling impedances at a particular frequency

Electrical resonance occurs in an electric circuit at a particular resonant frequency when the impedances or admittances of circuit elements cancel each other. In some circuits, this happens when the impedance between the input and output of the circuit is almost zero and the transfer function is close to one.

Ripple in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform after rectification. Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power.

<span class="mw-page-title-main">Capacitor</span> Passive two-terminal electronic component that stores electrical energy in an electric field

A capacitor is an electronic device that stores electrical energy in an electric field by accumulating electric charges on two closely spaced surfaces that are insulated from each other. It is a passive electronic component with two terminals.

The telegrapher's equations are a set of two coupled, linear equations that predict the voltage and current distributions on a linear electrical transmission line. The equations are important because they allow transmission lines to be analyzed using circuit theory. The equations and their solutions are applicable from 0 Hz to frequencies at which the transmission line structure can support higher order non-TEM modes. The equations can be expressed in both the time domain and the frequency domain. In the time domain the independent variables are distance and time. The resulting time domain equations are partial differential equations of both time and distance. In the frequency domain the independent variables are distance and either frequency, or complex frequency, The frequency domain variables can be taken as the Laplace transform or Fourier transform of the time domain variables or they can be taken to be phasors. The resulting frequency domain equations are ordinary differential equations of distance. An advantage of the frequency domain approach is that differential operators in the time domain become algebraic operations in frequency domain.

In electronics, a differentiator is a circuit designed to produce an output approximately proportional to the rate of change of the input. A true differentiator cannot be physically realized, because it has infinite gain at infinite frequency. A similar effect can be achieved, however, by limiting the gain above some frequency. The differentiator circuit is essentially a high-pass filter. An active differentiator includes some form of amplifier, while a passive differentiator is made only of resistors, capacitors and inductors.

<span class="mw-page-title-main">Zobel network</span>

Zobel networks are a type of filter section based on the image-impedance design principle. They are named after Otto Zobel of Bell Labs, who published a much-referenced paper on image filters in 1923. The distinguishing feature of Zobel networks is that the input impedance is fixed in the design independently of the transfer function. This characteristic is achieved at the expense of a much higher component count compared to other types of filter sections. The impedance would normally be specified to be constant and purely resistive. For this reason, Zobel networks are also known as constant resistance networks. However, any impedance achievable with discrete components is possible.

<span class="mw-page-title-main">Gyrator–capacitor model</span> Model for magnetic circuits

The gyrator–capacitor model - sometimes also the capacitor-permeance model - is a lumped-element model for magnetic circuits, that can be used in place of the more common resistance–reluctance model. The model makes permeance elements analogous to electrical capacitance rather than electrical resistance. Windings are represented as gyrators, interfacing between the electrical circuit and the magnetic model.

<span class="mw-page-title-main">RLC circuit</span> Resistor Inductor Capacitor Circuit

An RLC circuit is an electrical circuit consisting of a resistor (R), an inductor (L), and a capacitor (C), connected in series or in parallel. The name of the circuit is derived from the letters that are used to denote the constituent components of this circuit, where the sequence of the components may vary from RLC.

References

  1. Slurzberg; Osterheld (1950). Essentials of Electricity for Radio and Television. 2nd ed. McGraw-Hill. pp. 360 - 362
  2. Callegaro, L. (2012). Electrical Impedance: Principles, Measurement, and Applications. CRC Press, p. 5
  3. Callegaro, Sec. 1.6
  4. Kline, Ronald R., Steinmetz: Engineer and Socialist, Johns Hopkins University Press, 1992 ISBN   9780801842986, p. 78.
  5. Science, p. 18, 1888[ full citation needed ][ failed verification ]
  6. Oliver Heaviside, The Electrician, p. 212, 23 July 1886, reprinted as Electrical Papers, Volume II, p 64, AMS Bookstore, ISBN   0-8218-3465-7
  7. Kline, p. 79.
  8. Kline, p. 81-2.
  9. Kennelly, Arthur,"Impedance", Transactions of the American Institute of Electrical Engineers, vol. 10, pp. 175–232, 18 April 1893.
  10. Kline, p. 85.
  11. Kline, p. 90-1.
  12. 1 2 Gross, Charles A. (2012). Fundamentals of electrical engineering. Thaddeus Adam Roppel. Boca Raton, FL: CRC Press. ISBN   978-1-4398-9807-9. OCLC   863646311.
  13. Complex impedance, Hyperphysics
  14. Horowitz, Paul; Hill, Winfield (1989). "1". The Art of Electronics. Cambridge University Press. pp.  31–32. ISBN   978-0-521-37095-0.
  15. AC Ohm's law, Hyperphysics
  16. Horowitz, Paul; Hill, Winfield (1989). "1". The Art of Electronics. Cambridge University Press. pp.  32–33. ISBN   978-0-521-37095-0.
  17. Alexander, Charles; Sadiku, Matthew (2006). Fundamentals of Electric Circuits (3, revised ed.). McGraw-Hill. pp. 387–389. ISBN   978-0-07-330115-0.
  18. Parallel Impedance Expressions, Hyperphysics
  19. 1 2 George Lewis Jr.; George K. Lewis Sr. & William Olbricht (August 2008). "Cost-effective broad-band electrical impedance spectroscopy measurement circuit and signal analysis for piezo-materials and ultrasound transducers". Measurement Science and Technology. 19 (10): 105102. Bibcode:2008MeScT..19j5102L. doi:10.1088/0957-0233/19/10/105102. PMC   2600501 . PMID   19081773.