3.4 Analysis of RL and RC Networks
3.4.1 Series RL Network
+
+
VR
R-
Im
Z
jXL = jωL
V
+
L VL
-
Re
0
R
Z = ZR + ZL = R + jXL XL = ωL > 0
2
2
Z = R + XL
−1 ⎛ X L ⎞ φZ = tan ⎜ ⎟
⎝R⎠
V = VR + VL = IR + IjXL = IZ
Im
V
VL
0
Re
I
VR
I
3.4.2 Series RC Network
Im
+
+
R VR
+
C VC
-
R
0 jXC = -j/ωC
Z = ZR + ZC = R + jXC
Re
V
-
Z
XC = -1/ωC < 0
2
Z = R2 + X C
−1 ⎛ X C ⎞ φ Z = tan ⎜
⎟
⎝R⎠
V = VR + VC = IR + IjXC = IZ
Im
0
I
VR
VC
V
Re
3.4.3 Parallel RL Network
I
11
1
=+
Z R jX L jRX L
Z=
R + jX L
RX L
Z=
2
R2 + X L
⎛ XL ⎞ φ Z = 90 − tan ⎜ ⎟
⎝R⎠
+
R
L
V
-
IL = V/jXL
= -jV/ωL
IR = V/R
−1
I = IR + IL
V
V
⎛1
1⎞ V
I= +
=V⎜ +
⎟=
R jX L
⎝ R jX L ⎠ Z
Im
V
IR
0
IL
Re
I
3.4.4 Parallel RC Network
I
11
1
=+
Z R jX C jRX C
Z=
R + jX C
RX C
Z=
2
R2 + X C
+
R
C
V
-
IR = V/R
⎛ XC ⎞ φ Z = −90 − tan ⎜
⎟
⎝R⎠
IC = V/jXC
= jVωC
−1
IC
Im
I
I = I R + IC
⎛1
1⎞V
V
V
=V⎜ +
I= +
⎟=
R jX C
⎝ R jX C ⎠ Z
0
Re
V
IR
3.5 Resonance
3.5.1 Series RLC Network j Z (ω ) = R + jωL −
= R + jX (ω ) ωC 1
X (ω ) = ωL − ωC When reactance = 0 → resonance
1
X (ω0 ) = ω0 L −
=0
ω0 C
1
Resonant frequency ω0 =
LC
I
+
V
-
+
VR
R+
L VL
+
C VC
-
ωL
X=ωL-1/ωC
0
ω0
-1/ωC
ω
A two-terminal network undergoes resonance when the imaginary portion of its impedance (or admittance) becomes zero; or equivalently, that the impedance (or admittance) becomes real.
For series RLC circuit, j Z (ω ) = R + jωL −
= R + jX (ω ) ωC ⎛ ω 2 LC − 1 ⎞
Z (ω ) = R + j ⎜
⎟
⎝ ωC ⎠
At resonance,
Z (ω 0 ) = R
⎛ ω LC − 1 ⎞
2
Z (ω ) = R + ⎜
⎟
⎝ ωC ⎠
2
2
⎛ ω 2 LC − 1 ⎞
−1
∠Z (ω ) = tan ⎜
⎟
⎝ ωRC ⎠
|Z |
R
0
∠Z
ω0
ω
VL
ω < ω0
I
VL
ω > ω0
VR
V
0
VL + VC
VL + VC