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Electric Circuits II Resonance
1
Dr. Firas Obeidat
Dr. Firas Obeidat – Philadelphia University
2
1
• Series Resonance
Table of Contents
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
Resonance is a condition in an RLC circuit in which the capacitive and inductive
reactances are equal in magnitude, thereby resulting in a purely resistive
impedance (voltage and current at the circuit input terminals are in phase).
Resonant circuits (series or parallel) are useful for constructing filters. They
are used in many applications such as selecting the desired stations in radio
and TV receivers.
For the series RLC circuit. The input
impedance in frequency domain is
Resonance results when the imaginary part of the transfer function is zero
Dr. Firas Obeidat – Philadelphia University
4
Series Resonance The value of that satisfies this condition is called the resonant frequency. Thus, the
resonance condition is
Since
or
At resonance:-
1. The impedance is purely resistive, thus Z=R, In other words, the LC series
combination acts like a short circuit, and the entire voltage is across R.
2. The voltage Vs and the current I are in phase, so that the power factor is unity.
3. The magnitude of the transfer function H(𝜔)=Z(𝜔) is minimum.
4. The maximum current for the circuit for an applied voltage Vs since Z is a
minimum value.
5. The inductor voltage and capacitor voltage can be much more than the source
voltage.
Dr. Firas Obeidat – Philadelphia University
5
Series Resonance
At resonance, when θ=0, the voltage across each of the elements in the circuit can
be written as
I =𝑉
𝑚∠θ
𝑅∠0=
𝑉𝑚
𝑅 ∠θ
𝑉𝑅 = 𝐼𝑅∠0𝑜
𝑉𝐿 = 𝐼𝑋𝐿∠90𝑜
𝑉𝐶 = 𝐼𝑋𝐶∠ − 90𝑜
Because 𝑿𝑳 = 𝑿C , The magnitude of VL equals to the magnitude of VC but 180° out
of phase
The average power dissipated by the resistor and the reactive powers of the
inductor and capacitor as follows
𝑃𝑅 =1
2𝐼2𝑅 (w)
𝑄𝐿 =1
2𝐼2𝑋𝐿
(VAR)
𝑄𝐶 =1
2𝐼2𝑋𝐶
(VAR)
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
The frequency response of the circuit’s current magnitude is as in the figure
The magnitude of the current at resonance is
I =𝑉𝑚
𝑅
𝑃 =1
2𝐼2𝑅 =
1
2
𝑉𝑚
2
𝑅
The maximum power dissipated by the series resonant
circuit is given as
The bandwidth, BW, of the resonant circuit is the
difference between the frequencies at which the circuit
delivers half of the maximum power. The frequencies 𝜔1
and 𝜔2 are called the half-power frequencies, the cutoff
frequencies, or the band frequencies.
Dr. Firas Obeidat – Philadelphia University
7
Series Resonance
For the series resonant circuit the power at any frequency is determined as
𝑃 =1
2𝐼2𝑅 =
1
2
𝑉𝑚
|𝑍|
2𝑅 =
1
2
𝑉𝑚
2
𝑅2+ 𝜔𝐿−1
𝜔𝐶2
𝑅
At the half-power frequencies, the power must be
1
2×
1
2
𝑉𝑚2
𝑅=
1
2
𝑉𝑚2
𝑅2 + 𝜔𝐿 −1
𝜔𝐶2
𝑅 → 1
2𝑅=
𝑅
𝑅2 + 𝜔𝐿 −1
𝜔𝐶2
2𝑅2 = 𝑅2 +𝜔2𝐿𝐶 − 1
𝜔𝐶
2
→ 𝑅2 =𝜔2𝐿𝐶 − 1
𝜔𝐶
2
±𝑅 =𝜔2𝐿𝐶−1
𝜔𝐶 → ±𝑅𝜔𝐶 = 𝜔2𝐿𝐶 − 1
−𝑅𝜔𝐶 = 𝜔2𝐿𝐶 − 1 → 𝜔2𝐿𝐶 + 𝑅𝜔𝐶 − 1 =0
The solution of the above equation yields four values for the cutoff frequency.
Only two of these values are positive and have physical significance
+𝑅𝜔𝐶 = 𝜔2𝐿𝐶 − 1 → 𝜔2𝐿𝐶 − 𝑅𝜔𝐶 − 1 =0 To find 𝜔2 take +𝑹𝝎𝑪
To find 𝜔1 take −𝑹𝝎𝑪
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
𝜔1 is called the lower half power frequency and 𝜔2 is called upper half
power frequency.
𝝎𝒐 =𝟏
𝑳𝑪= 𝝎𝟏𝝎𝟐
𝜔1𝜔2 = −𝑅
2𝐿+
𝑅
2𝐿
2
+1
𝐿𝐶
𝑅
2𝐿+
𝑅
2𝐿
2
+1
𝐿𝐶=
1
𝐿𝐶
𝝎𝟏 = −𝑹
𝟐𝑳+
𝑹
𝟐𝑳
𝟐
+𝟏
𝑳𝑪 𝝎𝟐 =
𝑹
𝟐𝑳+
𝑹
𝟐𝑳
𝟐
+𝟏
𝑳𝑪
𝜔1 and 𝜔2 are in general not symmetrical around the resonant frequency, because
the frequency response is not generally symmetrical
The height of the curve is determined by R. The width of the response curve
depends on the bandwidth B, which is defined as the difference between the two
half-power frequencies.
Solving the above two equations gives
Dr. Firas Obeidat – Philadelphia University
9
Series Resonance
The “sharpness” of the resonance in a resonant circuit is measured quantitatively
by the quality factor Q.
𝑸 = 𝟐𝝅Peak energy stored in the circuit
Energy dissipated by the circuit in one period at resonance
𝜔2 − 𝜔1 =𝑅
2𝐿+
𝑅
2𝐿
2+
1
𝐿𝐶− −
𝑅
2𝐿+
𝑅
2𝐿
2+
1
𝐿𝐶 =
𝑅
𝐿
𝑩 = 𝝎𝟐 − 𝝎𝟏 =𝑹
𝑳
The relationship between the bandwidth B and the quality factor Q is given by
𝑸 =𝝎𝒐𝑳
𝑹=
𝟏
𝝎𝒐𝑪𝑹
The quality factor of a resonant circuit is the ratio of its resonant frequency to its
bandwidth.
𝑸 =𝝎𝒐
𝑩
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
A resonant circuit is designed to operate at or near its resonant frequency. It is
said to be a high-Q circuit when its quality factor is equal to or greater than 10.
For high-Q circuits, the approximate expressions for ω1 and ω2 are
The cutoff frequencies can be written in terms of the center frequency and the
bandwidth as:
𝝎𝟏 = −𝑩
𝟐+
𝑩
𝟐
𝟐
+ 𝝎𝒐𝟐 𝝎𝟐 =
𝑩
𝟐+
𝑩
𝟐
𝟐
+ 𝝎𝒐𝟐
The cutoff frequencies can be written in terms of the quality factor and the center
frequency as:
𝝎𝟏 = 𝝎𝒐 −𝟏
𝟐𝑸+ 𝟏 +
𝟏
𝟐𝑸
𝟐
𝝎𝟐 = 𝝎𝒐
𝟏
𝟐𝑸+ 𝟏 +
𝟏
𝟐𝑸
𝟐
𝝎𝟏 ≅ 𝝎𝒐 −𝑩
𝟐 𝝎𝟐 ≅ 𝝎𝒐 +
𝑩
𝟐
Dr. Firas Obeidat – Philadelphia University
11
Series Resonance
Example: In the circuit R=2Ω, L= 1 mH and C=0.4 μF. (a) Find the resonant frequency
and the half-power frequencies. (b) Calculate the quality factor and bandwidth.
(c) Determine the amplitude of the current at 𝜔o, 𝜔1 and 𝜔2.
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
Dr. Firas Obeidat – Philadelphia University
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Series Resonance Example:
a. For the series resonant circuit, find I, VR, VL, and
VC at resonance.
b. What is the Qs of the circuit?
c. If the resonant frequency is 5000 Hz, find the
bandwidth.
d. What is the power dissipated in the circuit at the
half-power frequencies?.
𝑎) 𝑅 = 2Ω
𝐼 =𝑉𝑠
𝑅=
10∠0𝑜
2= 5∠0𝑜
𝐴
𝑣𝑅 = 𝑉𝑠 = 10∠0𝑜A
𝑣𝐿 = 𝑍𝐿𝐼 = 10∠90𝑜 × 5∠0𝑜= 50∠90𝑜
𝑣𝐶 = 𝑍𝐶𝐼 = 10∠ − 90𝑜 × 5∠0𝑜= 50∠ − 90𝑜
𝑏) 𝑄𝑠 =𝜔𝑜𝐿
𝑅=
10Ω
2Ω= 5
𝑐) 𝐵 =𝜔𝑜
𝑄𝑠
=2 × π × 5000
5= 6283.19
𝑑) 𝑃𝐻𝑃𝐹 =1
2𝑃𝑚𝑎𝑥 =
1
2×
1
2𝐼𝑚𝑎𝑥
2R
=1
2×
1
2× 52 × 2 =12.5w
Dr. Firas Obeidat – Philadelphia University
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Series Resonance Example: The bandwidth of a series resonant circuit is 2513.27.
a. If the resonant frequency is 4000 Hz, what is the value of Qs?
b. If R=10 Ω, what is the value of XL at resonance?
c. Find the inductance L and capacitance C of the circuit.
𝑎) 𝑄𝑆 =𝜔
𝑜
𝐵 =
2×π×4000
2513.27 =10
𝑐) 𝐿 =𝑋𝐿
2π𝑓𝑜
=100
2π × 4000𝐻𝑧= 3.98 𝑚𝐻
𝑏) 𝑄𝑆 =𝜔
𝑜𝐿
𝑅 → 𝜔𝑜𝐿 = 𝑋𝐿 = 𝑄𝑆𝑅 =10×10=100Ω
At resonant XL=XC.
𝐶 =1
2π𝑓𝑜𝑋𝐶
=1
2π × 4000𝐻𝑧 × 100Ω= 0.398 μ𝐹
Dr. Firas Obeidat – Philadelphia University
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Series Resonance Example: A band pass circuit able to select inputs within the 1-10 kHz frequency band
(cutoff frequencies). If C=1 μF, find the value of L and R.
𝜔1 = 2𝜋𝑓1 = 2𝜋 × 1000 = 6283.19 𝑟𝑎𝑑/𝑠
𝜔2 = 2𝜋𝑓2 = 2𝜋 × 10000 = 62831.9 𝑟𝑎𝑑/𝑠
𝜔𝑜= 𝜔1𝜔2 = 6283.19 × 62831.9 = 19869.19 𝑟𝑎𝑑/𝑠
𝑓𝑜 = 3162.28 Hz
𝐵 = 𝜔2 − 𝜔1 = 62831.9 − 6283.19 = 56548.71
𝑄 =𝜔𝑜
𝐵=
19869.19
56548.71 = 0.3514
𝑅 =1
𝜔𝑜𝐶𝑄=
1
19869.19 × 1 × 10−6 × 0.3514= 143.24 Ω
𝐿 =1
𝜔𝑜2𝐶
=1
19869.192 × 1 × 10−6 = 2.533 𝑚𝐻
Dr. Firas Obeidat – Philadelphia University
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Series Resonance Example: A series RLC network has R=2 kΩ, L=40 mH and C=1μF. Calculate the
impedance at resonance and at one-fourth, one-half, twice, and four times the resonant
frequency.
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
Dr. Firas Obeidat – Philadelphia University
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Series Resonance
Example: Design a series RLC resonant circuit with R=10Ω, 𝜔o=1000 rad/s and B=20 rad/s. then find circuit’s Q.
Example: Design a series RLC resonant circuit with 𝜔o=40 rad/s and B=10 rad/s.
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