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Sinusoidal Signals
Sinusoidal signals are of particular interest in the field of electrical engineering
π£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππ = ππππ cos(2ππ β ππ β π‘π‘ + ππ)
Sinusoidal signals defined by three parameters: Amplitude: ππππ Frequency: ππ or ππ Phase: ππ
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Amplitude
Amplitude of a sinusoid is its peakvoltage, ππππ
Peak-to-peak voltage, ππππππ, is twice the amplitude ππππππ = 2ππππ ππππππ = ππππππππ β ππππππππ
π£π£ π‘π‘ = ππππ οΏ½ sin πππ‘π‘ + ππ = ππππ οΏ½ sin 2πππππ‘π‘ + ππ
ππππ = 170 ππ
ππ ππππ
=34
0ππ
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Frequency
Period (ππ) Duration of one cycle
Frequency (ππ) Number of periods per second
ππ =1ππ
Ordinary frequency, ππ Units: hertz (Hz), sec-1, cycles/sec
Angular frequency, ππ Units: rad/sec
ππ = 2ππππ, ππ = ππ2ππ
ππ = 16 ππππππππ
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Phase
Phase Angular constant in signal expression, ππ
π£π£ π‘π‘ = ππππ sin πππ‘π‘ + ππ Requires a time reference
Interested in relative, not absolute, phase
Here, π£π£1 π‘π‘ leads π£π£2 π‘π‘ π£π£2 π‘π‘ lags π£π£1 π‘π‘
Units: radians Not technically correct, but OK
to express in degrees, e.g.:
π£π£ π‘π‘ = 170 ππ sin 2ππ β 60π»π»π»π» β π‘π‘ + 34Β°
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Sinusoidal Steady-State Analysis
Often interested in the response of linear systems to sinusoidal inputs Voltages and currents in electrical systems Forces, torques, velocities, etc. in mechanical systems
For linear systems excited by a sinusoidal input Output is sinusoidal
Same frequency In general, different amplitude In general, different phase
We can simplify the analysis of linear systems by using phasor representation of sinusoids
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Phasors
Phasor A complex number representing the amplitude and
phase of a sinusoidal signal Frequency is not included Remains constant and is accounted for separately System characteristics (frequency-dependent) evaluated at
the frequency of interest as first step in the analysis
Phasors are complex numbers Before applying phasors to the analysis of electrical
circuits, weβll first review the properties of complex numbers
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Complex Numbers
A complex number can be represented as
π»π» = π₯π₯ + ππππ
π₯π₯: real part (a real number) ππ: imaginary part (a real number) ππ = β1 is the imaginary unit
Complex numbers can be represented three ways: Cartesian form: π»π» = π₯π₯ + ππππ Polar form: π»π» = πππππ Exponential form: π»π» = ππππππππ
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Complex Numbers as Vectors
A complex number can be represented as a vector in the complex plane
Complex plane Real axis β horizontal Imaginary axis β vertical
A vector from the origin to π»π» Real part, π₯π₯ Imaginary part, ππ
π»π» = π₯π₯ + ππππ
Vector has a magnitude, ππ And an angle, ππ
π»π» = πππππ
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Cartesian Form β Polar Form
Cartesian form β Polar formπ»π» = π₯π₯ + ππππ = πππππ
ππ = π»π» = π₯π₯2 + ππ2
ππ = arg π»π» = ππ»π»
ππ = tanβ1πππ₯π₯
Polar form β Cartesian formπ₯π₯ = ππ cos ππ
ππ = ππ sin ππ
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Complex Numbers β Addition/Subtraction
Addition and subtraction of complex numbers Best done in Cartesian form Real parts add/subtract Imaginary parts add/subtract
For example:π»π»1 = π₯π₯1 + ππππ1π»π»2 = π₯π₯2 + ππππ2π»π»1 + π»π»2 = π₯π₯1 + π₯π₯2 + ππ ππ1 + ππ2π»π»1 β π»π»2 = π₯π₯1 β π₯π₯2 + ππ ππ1 β ππ2
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Complex Numbers β Multiplication/Division
Multiplication and division of complex numbers Best done in polar form Magnitudes multiply/divide Angles add/subtract
For example:π»π»1 = ππ1πππ1π»π»2 = ππ2πππ2π»π»1 β π»π»2 = ππ1ππ2π ππ1 + ππ2π»π»1π»π»2
=ππ1ππ2π ππ1 β ππ2
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Complex Conjugate
Conjugate of a complex number Number that results from
negating the imaginary part
π»π» = π₯π₯ + πππππ»π»β = π₯π₯ β ππππ
Or, equivalently, from negating the angle
π»π» = πππππ
π»π»β = πππ β ππ
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Complex Fractions
Multiplying a number by its complex conjugate yields the squared magnitude of that number A real number
π»π» β π»π»β = π₯π₯ + ππππ π₯π₯ β ππππ = π₯π₯2 + ππ2
π»π» β π»π»β = πππππ β πππ β ππ = ππ2πππ β ππ = ππ2
Rationalizing the denominator of a complex fraction: Multiply numerator and denominator by the complex conjugate
of the denominator
π»π» =π₯π₯1 + ππππ1π₯π₯2 + ππππ2
β π₯π₯2 β ππππ2π₯π₯2 β ππππ2
π»π» =π₯π₯1π₯π₯2 + ππ1ππ2π₯π₯22 + ππ22
+ πππ₯π₯2ππ1 β π₯π₯1ππ2π₯π₯22 + ππ22
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Complex Fractions
Fractions or ratios are, of course, simply division Very common form, so worth emphasizing
Magnitude of a ratio of complex numbers
π»π» =π»π»1π»π»2
β π»π» =π»π»1π»π»2
Angle of a ratio of complex numbers
π»π» =π»π»1π»π»2
β ππ»π» = ππ»π»1 β ππ»π»2
Calculators and complex numbers Manipulation of complex numbers by hand is tedious and error-prone Your calculators can perform complex arithmetic They will operate in both Cartesian and polar form, and will convert between
the two Learn to use them β correctly
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Eulerβs Identity
Fundamental to phasor notation is Eulerβs identity:
ππππππππ = cos πππ‘π‘ + ππ sin πππ‘π‘
where ππ is the imaginary unit, and ππ is angular frequency It follows that
cos πππ‘π‘ = π π ππ ππππππππ
sin πππ‘π‘ = πΌπΌππ ππππππππ
and
cos πππ‘π‘ =ππππππππ + ππβππππππ
2
sin πππ‘π‘ =ππππππππ β ππβππππππ
2ππ
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Phasors
Consider a sinusoidal voltageπ£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππ
Using Eulerβs identity, we can represent this as
π£π£ π‘π‘ = π π ππ ππππππππ ππππ+ππ = π π ππ ππππππππππππππππππ
where ππππ represents magnitude ππππππ represents phase ππππππππ represents a sinusoid of frequency ππ
Grouping the first two terms together, we have
π£π£ π‘π‘ = π π ππ ππππππππππ
where ππ is the phasor representation of π£π£ π‘π‘
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Phasors
π£π£ π‘π‘ = π π ππ ππππππππππ
The phasor representation of π£π£ π‘π‘
ππ = ππππππππππ
A representation of magnitude and phase only Time-harmonic portion (ππππππππ) has been dropped
Phasors greatly simplify sinusoidal steady-state analysis Messy trigonometric functions are eliminated Differentiation and integration transformed to algebraic operations
Time-domain representation:
π£π£ π‘π‘ = ππππ sin πππ‘π‘ + ππ
Phasor-domain representation:
ππ = ππππππππππ = πππππππ
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Voltage & Current in the Phasor Domain
We will use phasors to simplify analysis of electrical circuits Need an understanding of electrical component behavior in the phasor domain Relationships between voltage phasors and current phasors for Rs, Ls, and Cs
Resistor Voltage across a resistor given by
π£π£ π‘π‘ = ππ π‘π‘ π π
ππ π‘π‘ = πΌπΌππ cos πππ‘π‘ + ππ
Converting to phasor form
ππ = πΌπΌππππππππ π π
ππ = πππ π ππ =ππR
Ohmβs law in phasor form
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V-I Relationships in the Phasor Domain
Capacitor Current through the capacitor given by
ππ π‘π‘ = πΆπΆπππ£π£πππ‘π‘
ππ π‘π‘ = πΆπΆπππππ‘π‘ ππππ cos πππ‘π‘ + ππ
ππ π‘π‘ = βπππΆπΆππππ sin πππ‘π‘ + ππ
Applying a trig identity:
β sin π΄π΄ = cos π΄π΄ + 90Β°gives
ππ π‘π‘ = πππΆπΆππππ cos πππ‘π‘ + ππ + 90Β°
Converting to phasor form
ππ = πππΆπΆππππππππ ππ+90Β° = πππΆπΆππππππππππππππ90Β°
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V-I Relationships - Capacitor
Current phasor
ππ = πππΆπΆππππππππ ππ+90Β° = πππΆπΆππππππππππππππ90Β°
Voltage phasor is
ππ = ππππππππππ
soππ = πππΆπΆππππππ90Β°
Recognizing that ππππ90Β° = ππ, we have
ππ = πππππΆπΆππ ππ =1πππππΆπΆ
ππ
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V-I Relationships - Inductor
Inductor Voltage across an inductor given by
π£π£ π‘π‘ = πΏπΏπππππππ‘π‘
π£π£ π‘π‘ = πΏπΏπππππ‘π‘
πΌπΌππ cos πππ‘π‘ + ππ
π£π£ π‘π‘ = βπππΏπΏπΌπΌππ sin πππ‘π‘ + ππ = πππΏπΏπΌπΌππ cos πππ‘π‘ + ππ + 90Β°
Converting to phasor form
ππ = πππΏπΏπΌπΌππππππ ππ+90Β° = πππΏπΏπΌπΌππππππππππππ90Β°
Again, recognizing that ππππ90Β° = ππ, gives
ππ = πππππΏπΏππ ππ =1πππππΏπΏ
ππ
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Impedance
For resistors, Ohmβs law gives the ratio of the voltage phasor to the current phasor as
ππππ = π π
π π is, of course, resistance A special case of impedance
Impedance, ππ
ππ =ππππ
The ratio of the voltage phasor to the current phasor for a component or network
Units: ohms (Ξ©) In general, complex-valued
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Impedance
Resistor impedance:
ππ =ππππ
= π π
Capacitor impedance:
ππ =ππππ
=1πππππΆπΆ
Inductor impedance:
ππ =ππππ = πππππΏπΏ
In general, Ohmβs law can be applied to any component or network in the phasor domain
ππ = ππππ ππ =ππππ
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Capacitor Impedance
ππ =1πππππΆπΆ
=1πππΆπΆ
ππβππ90Β°
ππ = ππππ =πππππΆπΆ
ππβππ90Β°
ππ = πππΆπΆππππππ90Β°
In the time domain, this translates toπ£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππ
ππ π‘π‘ = πππππππΆπΆ cos πππ‘π‘ + ππ + 90Β°
Current through a capacitor leads the voltage across a capacitor by 90Β°
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Capacitor Impedance β Phasor Diagram
Phasor diagram for a capacitor Voltage and current
phasors drawn as vectors in the complex plane
Current always leads voltage by 90Β°
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Capacitor Impedance β Time Domain
Current leads voltage by 90Β°
Ξππ = 90Β°
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Capacitor Impedance β Frequency Domain
Capacitor approaches an open circuit at DC
Capacitor approaches a short circuit at very high frequencies
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Inductor Impedance
ππ = πππππΏπΏ = πππΏπΏ ππππ90Β°
ππ = ππππ = πππππΏπΏ ππππ90Β°
ππ =πππππΏπΏ
ππβππ90Β°
In the time domain, this translates toπ£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππ
ππ π‘π‘ =πππππππΏπΏ
cos πππ‘π‘ + ππ β 90Β°
Current through an inductor lags the voltage across an inductor by 90Β°
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Inductor Impedance β Phasor Diagram
Phasor diagram for an inductor Voltage and current
phasors drawn as vectors in the complex plane
Current always lags voltage by 90Β°
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Inductor Impedance β Time Domain
Current lags voltage by 90Β°
Ξππ = 90Β°
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Inductor Impedance β Frequency Domain
Inductor approaches a short circuit at DC
Inductor approaches an open circuit at very high frequencies
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Summary
Capacitor Impedance:
ππππ =1πππππΆπΆ
V-I phase relationship:
Current leads voltage by 90Β°
π£π£ π‘π‘ = ππππ cos πππ‘π‘
ππ π‘π‘ = πππππππΆπΆ cos πππ‘π‘ + 90Β°
Inductor Impedance:
πππΏπΏ = πππππΏπΏ
V-I phase relationship:
Current lags voltage by 90Β°
π£π£ π‘π‘ = ππππ cos πππ‘π‘
ππ π‘π‘ =πππππππΏπΏ
cos πππ‘π‘ β 90Β°
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ELI the ICE Man
Mnemonic for phase relation between current (I) and voltage (E) in inductors (L) and capacitors (C)
E L I the manI C EVoltage
leads current in an inductor
Current leads voltage
in a capacitor
K. Webb ENGR 202
Convert each of the following time-domain signals to phasor form.
π£π£ π‘π‘ = 6ππ β cos 2ππ β 8πππ»π»π»π» β π‘π‘ + 12Β°
ππ π‘π‘ = 200πππ΄π΄ β sin 100 β π‘π‘ β 38Β°
K. Webb ENGR 202
The following current is applied to the capacitor. ππ π‘π‘ = 100πππ΄π΄ β cos 2ππ β 50πππ»π»π»π» β π‘π‘
Find the voltage across the capacitor, π£π£ π‘π‘ .
K. Webb ENGR 202
The following voltage is applied to the inductor. π£π£ π‘π‘ = 4ππ β cos 2ππ β 800π»π»π»π» β π‘π‘
Find the current through the inductor, ππ π‘π‘ .
K. Webb ENGR 202
A test voltage is applied to the input of an electrical network.
π£π£ π‘π‘ = 1ππ β sin 2ππ β 5πππ»π»π»π» β π‘π‘The input current is measured.
ππ π‘π‘ = 268πππ΄π΄ β sin 2ππ β 5πππ»π»π»π» β π‘π‘ β 46Β°What is the circuitβs input impedance, ππππππ?
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Impedance
So far, weβve looked at impedance of individual components Resistors
ππ = π π Purely real
Capacitors
ππ =1πππππΆπΆ
Purely imaginary, purely reactive
Inductorsππ = πππππΏπΏ
Purely imaginary, purely reactive
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Impedance
Also want to be able to characterize the impedance of electrical networks Multiple components Some resistive, some reactive
In general, impedance is a complex value
ππ = π π + ππππwhere
π π is resistance ππ is reactance
So, in ENGR 201 we dealt with impedance all along Resistance is an impedance whose reactance (imaginary part) is
zero A purely real impedance
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Reactance
For capacitor and inductors, impedance is purely reactive Resistive part is zero
ππππ = ππππππ and πππΏπΏ = πππππΏπΏ
where ππππ is capacitive reactance
ππππ = β 1ππππ
and πππΏπΏ is inductive reactance
πππΏπΏ = πππΏπΏ
Note that reactance is a real quantity It is the imaginary part of impedance
Units of reactance: ohms (Ξ©)
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Admittance Admittance, ππ, is the inverse of impedance
ππ =1ππ
= πΊπΊ + ππππwhere
πΊπΊ is conductance β the real partππ is susceptance β the imaginary part
ππ =1
π π + ππππ=
π π π π 2 + ππ2
+ ππβππ
π π 2 + ππ2
Conductance
πΊπΊ =π π
π π 2 + ππ2
Note that πΊπΊ β 1/π π unless ππ = 0 Susceptance
ππ =βππ
π π 2 + ππ2
Units of ππ, πΊπΊ, and ππ: Siemens (S)
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Impedance of Arbitrary Networks
In general, the impedance of arbitrary networks may be both resistive and reactive
ππ = π π 1 + ππππ1
ππ = ππ πππwhere
ππ = π π 12 + ππ12
and
ππ = tanβ1ππ1π π 1
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Impedances in Series
Impedances in series add
ππππππ = ππ1 + ππ2
ππππππ = π π 1 + π π 2 + ππ ππ1 + ππ2
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Impedances in Parallel
Admittances in parallel add
ππππππ = ππ1 + ππ2
ππππππ =1ππππππ
=1ππ1
+1ππ2
β1
ππππππ =ππ1ππ2ππ1 + ππ2
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Sinusoidal Steady-State Analysis β Ex. 1
Determine the current, ππ π‘π‘π£π£π π π‘π‘ = 1 ππ cos 2ππ β 1 πππ»π»π»π» β π‘π‘
First, convert the circuit to the phasor domain Express the source voltage as a phasor
πππ¬π¬ = 1 πππ0Β°
Evaluate impedances at 1 MHz
π π = 10 Ξ©
ππππ =1πππππΆπΆ
= βππ
2ππ β 1 πππ»π»π»π» β 10 ππππ
ππππ = βππ15.9 Ξ©
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Sinusoidal Steady-State Analysis β Ex. 1
The load impedance isππ = π π + ππππππ = 10 β ππ15.9 Ξ©
ππ = 18.8π β 57.8Β° Ξ©
Apply Ohmβs law to calculate the current phasor
ππ =ππππ
=1 πππ0Β°
18.8π β 57.8Β° Ξ©
ππ = 53.2π57.8Β° πππ΄π΄
Finally, convert to the time domain
ππ π‘π‘ = 53.2 πππ΄π΄ cos(2ππ β 1πππ»π»π»π» β π‘π‘ + 57.8Β°)
K. Webb ENGR 202
π£π£π π π‘π‘ = 170 ππ sin 2ππ β 60π»π»π»π» β π‘π‘
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Sinusoidal Steady-State Analysis β Ex. 2
Determine: The impedance, ππππππ, at 60 Hz Voltage across the load, π£π£πΏπΏ π‘π‘ Current delivered to the load, πππΏπΏ π‘π‘
Consider the following circuit, modeling a source driving a load through a transmission line
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Sinusoidal Steady-State Analysis β Ex. 2
First, convert to the phasor domain and evaluate impedances at 60 Hz
The line impedance is
ππππππππππ = π π 1 + πππππΏπΏ1 = 0.5 + ππ1.88 Ξ©
The load impedance is
ππππππππππ = π π 2 + πππππΏπΏ2 ||1πππππΆπΆ = 3 + ππ5.65 Ξ© || β ππ265 Ξ©
ππππππππππ =1
3 + ππ5.65 Ξ© +1
βππ265 Ξ©
β1
= 3.13 + ππ5.74 Ξ©
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Sinusoidal Steady-State Analysis β Ex. 2
The impedance seen by the source isππππππ = ππππππππππ + ππππππππππππππππ = 0.5 + ππ1.88 Ξ© + 3.13 + ππ5.74 Ξ©
ππππππ = 3.63 + ππ7.62 Ξ©
In polar form:ππππππ = 8.44π64.5Β° Ξ©
The impedance driven by the source looks resistive and inductive Resistive: non-zero resistance, πππππππ β Β±90Β° Inductive: positive reactance, positive angle
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Sinusoidal Steady-State Analysis β Ex. 2
πππΏπΏ = ππππππππππππππ
ππππππππππ + ππππππππππ
πππΏπΏ = 170π0Β° ππ3.13 + ππ5.74 Ξ©3.63 + ππ7.62 Ξ©
πππΏπΏ = 170π0Β° ππ6.54π61.4Β° Ξ©8.44π64.5Β° Ξ© = 132π β 3.1Β° ππ
Converting to time-domain form
π£π£πΏπΏ π‘π‘ = 132 ππ sin 2ππ β 60π»π»π»π» β π‘π‘ β 3.1Β°
Apply voltage division to determine the voltage across the load
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Sinusoidal Steady-State Analysis β Ex. 2
πππΏπΏ =πππΏπΏππππππππππ
πππΏπΏ =132π β 3.1Β° ππ6.54π61.4Β° Ξ©
πππΏπΏ = 20.1π β 64.5Β° π΄π΄
In time-domain form:πππΏπΏ π‘π‘ = 20.1 π΄π΄ sin 2ππ β 60π»π»π»π» β π‘π‘ β 64.5Β°
Finally, calculate the current delivered to the load
K. Webb ENGR 202
Determine the input impedance and an equivalent circuit model for the following network at 50 kHz.