Impedance: jω -method

V. Korenivski, KTH-APhys, 2008

1 General properties
A complex (two-component) number has the form

c = a + jb
√
where j= −1 is the imaginary unit. This form is equivalent to the polar form

p b
Aejϕ , with A= a2 + b2 and ϕ = tan−1 . (1)
a
Think of a rotation of a phasor along a two dimensional circle centered at zero .
1 a and
b (cos ϕ and sin ϕ) then correspond to the x and y projections of the phasor, as shown in
Figure 1. These are real and imaginary components of the complex exponential function

Figure 1: A point on a x−y plane is dened by a complex number, c.

1
Young & Freedman, University Physics, Chapter 31.

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(a unit vector of the complex plane), dened by the Euler relation
2

ejx = cos x + j sin x.

Respectively, e−jx = cos x − j sin x corresponds to a phasor having a negative imaginary

(y ) part.
For a stationary alternating current (ac ) owing through a circuit element, i(t) =
I0 cos ωt 3 , the motion of the current phasor is a continuous rotation with instantaneous
angle ωt. The voltage across the element has generally a phase shift with respect to the
current ,
4 v(t) = V cos(ωt + φ) . The same can be expressed in complex notations as
0

i(t) = Re I0 ejωt = I0 cos ωt


(2)

and n o n o
v(t) = Re V0 ej(ωt+φ) = Re V0 ejφ ejωt = Re V ejωt .

(3)

I0 and V0 above are real current and voltage amplitudes. V = V0 ejφ is complex voltage,
which now has two parts - an amplitude and a phase (as dened by Eq. 1).

2 Ohm's law
Ohm's law for a resistor carrying a direct current (dc ) is

V0 = RI0 ,

where the resistance is real and, therefore, the current and voltage are in phase. For a
circuit carrying an ac, the Ohm's law must be modied to reect the phase shift generally
present between i and v (Eqs. 2,3). This is done by introducing a complex analogue of
the resistance known as the impedance Z, such that the Ohm's law for ac becomes

V = ZI0 .

What is the form of Z for the common circuit elements R, L, and C?

2
Wiki: Euler's formula was proven for the rst time by Roger Cotes in 1714 in the form ln(cos(x) +
i sin(x)) = ix. It was Euler who published the equation in its current form in 1748, basing his proof on
the innite series of both sides being equal. Neither of these men saw the geometrical interpretation
of the formula: the view of complex numbers as points in the complex plane arose only some 50 years
later.
3
Stationary alternating here means that every next period is a repetition of the previous period.
Almost always  ac  means a cos or sin form - well behavied functions under dierentiation, in contrast
to triangle or square waveforms.
4
Following Y&F we choose the phase of the current to be the reference, i. e. φi = 0.

2
3 Resistor
The voltage and current for a resistor of resistance R 5
are in phase , and related for any
instant in time by the Ohm's law

v(t) = Ri(t).

This means that

v = V ejωt = RIejωt or V = RI.
The resistive impedance is therefore

ZR = R.

4 Inductor
For an ideal inductor (R = 0) the current-voltage relation is a consequence of the Fara-
day's law
di
v=L .
dt
Using Eqs. 2 and 3, this yields a linear relation of Ohm-type
6 (again, with subscript 0
dropped for brevity)
V = jωLI.
The inductive impedance is then
ZL = jωL
π
and has amplitude ωL and phase
2 , directed along +y in Fig.1 for R along x. For an
inductor with a non-vanishing resistance of the wire

di
v = Ri + L = i(R + jωL),
dt
so the total impedance of the inductor is

ZL = R + jωL.

This impedance is graphically shown in Fig. 2.

5
Once again, subscript 0 refers to the fact that all phases are referenced to the phase of current. We
will keep this in mind and drop the subscript for brevity of notations.
6 di jωt
Recall that dt .e = jωejωt .

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 Figure 2: Inductive impedance as a vector sum of resistance and reactance.

5 Capacitor
The charge on an ideal capacitor is
q = Cv.
Dierentiating both sides yields

dq dv 1
=i=C = jωCv = v,
dt dt ZC

and therefore V = ZC I with (1 ≡ −j 2 )

1 j
ZC = =− . (4)
jωC ωC
The graphical interpretation of the capacitive impedance is straightforward: it is along
the imaginary  j  axis (y axis in Fig. 1) and is opposite to the inductive impedance due
to the minus sign in Eq. 4.
The total complex impedance of a series R-L-C circuit,

 
1 1
Z = R + jωL + = R + j ωL − ,
jωC ωC

is shown graphically in Fig. 3. The resistance R and reactance X correspond to the real
and imaginary components of the impedance:

R = Re {Z} and X = Im {Z} , Z = R + jX,

where the reactance is

1
X = ωL − .
ωC

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 Figure 3: Impedance of a series R-L-C circuit with |ZC | = 12 |ZL |.

6 ac power
The instantaneous power delivered to a circuit element is
7

p = vi = V cos(ωt + φ)I cos ωt = V I cos φ cos2 ωt − V I sin φ cos ωt sin ωt.

It is convenient to characterize circuits by a time-independent average power. Averaging

1
p, with cos2 ωt = 2 and the second term vanishing, yields

1
Pav = V I cos φ = Vrms Irms cos φ.
2
In complex notations, the average power is expressed as follows :
8
( )
V I∗ V ej(ωt+φ) Ie−jωt V ejφ I
   
VI
Pav = Re = Re = Re = cos φ. (5)
2 2 2 2

The product in the braces (Pav →P for brevity)

1
S = V I ∗ = Vrms Irms
∗ ∗
= ZIrms Irms 2
= ZIrms 2
= (R + jX)Irms = P + jQ
2
is known the complex power, which has an active (P ) and reactive (Q) parts. The modulus
of the complex power is known as apparent power. To summarize, one distinguishes
complex, active, reactive, or apparent ac power:

2
S = P + jQ = ZIrms ,
7
See Y&F Ÿ31.4 for more details.
8
A∗ denotes complex conjugation, which inverts the imaginary part of A.

5
 2
P = Re {S} = RIrms ,
2
Q = Im {S} = XIrms ,
p
2
|S| = P 2 + Q2 = |Z|Irms .
From Eq. 5 it is clear that the actual power dissipated in a circuit is associated with the
active power (P ) since the cos φ (called the power factor ) is zero for the reactive power
component (Q).
Power matching in electric circuits means minimizing Q and, therefore, minimizing
the reactance X. This is because for a given voltage, less current is needed to produce a
given amount of power if the load is purely resistive. For a non-resistive component the
current would ow in the reactive channel without producing heat (power).

7 Kircho's laws
The Kirchho 's circuit laws continue to apply in the ac case, with the following gener-
alizations: dc amplitudes are replaced with complex current amplitudes, In → In ejφn ,
and dc voltages are replaced with complex voltages, V →V jφ
e . Thus, for example, the
voltage between points a and b of a circuit is related to the current by the element's
complex impedance:
Vab = Zab Iab .
The generalized junction rule can be illustrated by a circuit with n elements of dierent
resistance and reactance connected in parallel between points a and b (Vab is xed in this
case). The total current is then a sum of the individual currents, which are phase shifted
with respect to each other as determined by the impedances Zn:
X
i= In cos(ωt + φn ).

Performing trigonometric summations is cumbersome. In complex notations the total

In ejφn . Indeed,
P
current becomes a simple sum of complex numbers, I=
nX o X n o X
Re Iejωt In ejφn ejωt = Re In ej(ωt+φn ) =

= Re in (t) = i(t).

8 Filters
Filters are two-port circuits (signal in and out, Fig. 4) whose impedance is designed
to select (lter) a certain frequency range out of a multi-frequency (broadband) signal.
Filters can be low-pass, high-pass, or ban-pass (band-stop). These terms refer to the
frequencies the lter passes through. Thus, a low-pass lter would transmit only low
frequency signals from the input to the output.
The above three lter categories can be realized using RC, RL, and LC circuits. We
use complex notations below to describe some common lter circuits.

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 Figure 4: A general lter with two ports, input and output, and an RC lter.

8.1 R-C and C-R

A serial RC circuit is shown in Fig.4. Vi denotes a voltage source at the input and Vo a
voltage meter at the output of the
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lter . The series-RC impedance is

j
Z =R− .
ωC
The output voltage - the voltage across the capacitor - is given by the current in the
circuit multiplied by the impedance of the capacitor:
 
Vi Vi j j
Vo = IZC = ZC = − = Vi .
Z R − j/(ωC) ωC j − ωRC

The transmission, or lter coecient then becomes

Vo j
T = = ,
Vi j − ωRC
and its amplitude

s  ∗
Vo j j 1
|TRC | = = =p . (6)
Vi j − ωRC j − ωRC 1 + (ωRC)2

For low frequencies, ω → 0, the signal is fully transmitted, |T | → 1. At high frequencies,

ω → ∞, the signal is completely suppressed, |T | ∝ ω −1 → 0. This is the action of a
low-pass lter, used in electronics to lter out high frequencies.
The cuto frequency of the lter is taken to be at the point where one half of the
power is transmitted,
1
Pcuto (ωc ) = Pmax ,
2
with Pmax in this√case corresponding to P (ω → 0). Since P ∝ V 2 , the cuto frequency
is where |T | = 1/ 2. From Eq. 6, this condition corresponds to ωc RC = ωc τRC = 1, or

1
fc = .
2πRC
9
The input impedance of the voltmeter is assumed to be innite - a excellent approximation in most
cases.

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The cuto frequency dened this way is also known as the -3 dB point
10
Exchanging the R and C in the series RC circuit, with Vo now measured across the
resistor, the circuit's frequency response becomes

s  ∗
R ωRC ωRC ωRC
|TCR | = = =p .
R − j/(ωC) ωRC − j ωRC − j 1 + (ωRC)2

For low frequencies, ω → 0, the signal is suppressed, |T | ∝ ω → 0. At high frequencies,

ω → ∞, the signal is fully transmitted, |T | → 1. This is the action of a high-pass lter,
used in electronics to lter out dc.

Figure 5: T (ω): low- and high-pass ltering using R-C and C-R circuits.

8.2 R-L and L-R

If the capacitor in Fig.4 (right) is replaced with an inductor, then the lter factor becomes

s  ∗
jωL jωL ωL
|TRL | = =p ,
R + jωL R + jωL R2 + (ωL)2

which represents a high-pass lter, similar to the C-R circuit described above (red curve
√
in Fig.5). The dierence is that the cuto condition |T | = 1/ 2 now corresponds to
ωc = R/L, so time constant
L
τRL = .
R
10
A voltage ratio in decibel is dened as
X
XdB = 20 log10 X0
. In our case of the cuto frequency,

20 log10 √12 = −3.01 dB.

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Exchanging the positions of the R and L in the circuit results obviously in a low-pass
lter of the R-C type, with

s  ∗
R R 1
|TLR | = =p .
R + jωL R + jωL 1 + (ωL/R)2

8.3 LC in series

In the RC and RL circuits above only one element (C or L) had a frequency dependent
impedance (reactance, XC or XL ). Therefore the circuit impedance either increased or
decreased with frequency. In LC circuits, on the other hand, one expects a competition
between a rising reactance of the inductor and diminishing reactance of the capacitor, as
the frequency is increased. This should result in a non-monotonic behavior of the lter,
i.e. maxima or minima in current or voltage.

Figure 6: Series and parallel LC resonant circuits.

The left panel of Fig.6 shows a series RLC circuit. The current is given by the voltage
supplied by the source divided by the total impedance of the circuit,

V V
I= = . (7)
Z R + j[ωL − 1/(ωC)]

The current is maximum, I0 = V /R, when ω0 L = 1/(ω0 C), from which the LC resonance
frequency is
1
ω0 = (LC)− 2 .
The quality factor is dened as

p
L/C L
Q≡ = ω0 .
R R
Using this denition, the current of Eq. 7 can be rewritten as

ω0 −1
  
ω
I = I0 1 + jQ − ,
ω0 ω

and its amplitude

"  2 #− 12
I ω ω 0
= 1 + Q2 − . (8)
I0 ω0 ω

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 Figure 7: Normalized current of Eq.8 for a series LC circuit.

This function is plotted in Fig.

√ 7 for Q = 1, 10. The cuto frequencies dened by
T = 1/ 2 correspond to
 
ωc ω0
Q − = ±1,
ω0 ωc
with the left and right cuto

s 2
ωc∓ 1 1
= +1∓ ,
ω0 2Q 2Q

and the relative resonance width (∆ω = ωc+ − ωc− )

∆ω 1
= .
ω Q
The voltages across the individual circuit elements are obtained by multiplying the
total current (Eq. 7) by the respective impedance. For example, for the capacitor

I
VC = IZC = .
jωC

Clearly, the sharply peaked current versus frequency (for high Q) results is a high voltage
across the capacitor only for a narrow band in frequency (ωC is a monotonous function).
This is the principle behind band-pass ltering in great many electronic circuits and
systems we use today.

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8.4 LC in parallel

The analysis of the parallel LC circuit is very similar to the one we have just performed,
and is left as a home task. Consider the circuit layout shown in the right panel of Fig.6.
This layout is identical with your lter layout in the LabVIEW lab.
Your model of the impedance and the quality factor should be adjusted to t your
experimental data. Proceed by recognizing that the parallel connection of the L and
C branches is connected in series with R. Furthermore, the resistance of the inductor
wire (r ) cannot be neglected, and forms a series connection with the inductive reactance.
Your output voltage is measured across C.

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